Madcow Super DX3 Build Thread

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Green Jello

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Hello all. Since April I've had a Madcow Super DX3 kit waiting around for me to finish some house projects. This is my first cardboard/plywood HPR kit and I also purchased the add on altimeter bay for dual deploy. My plan is to build this over the next 6 weeks and have it ready for a maiden voyage at XPRS in Black Rock. There are a couple things I'll need to learn that are slightly different from fiberglass builds, but for the most part this should be strait forward.

The design scheme I think I'm going with is Dupli Color Candy Apple Green from their Paint Shop car paint line put on with my new sprayer, and then black vinyl from Stickershock.
Super DX3 Paint Scheme 2.jpgbsp304.jpg

So here goes......



Index of Posts:
1. Unboxing of Parts
2. Motor Mount Assembly
3. Airfoil Fins 1
4. Airfoil Fins 2
5. Install Switch Band
6. Install Eye Bolt on Motor Mount
7. Install Motor Mount
8. Installing Fins
9. Internal Fin Fillets
10. Rear CR and Engine Retainer
11. Electronics Bay Bulkheads
12. External Fin Fillets
 

Attachments

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Here is the standard "unboxing" of the parts. When I was doing this, my younger son came in and asked me why I dumped my rocket all over the floor.

IMG_3948.jpg
 
MOTOR MOUNT ASSEMBLY

Parts Involved:
- 1x Motor Mount Tube (Kit)
- 3x Centering Rings (Kit)
- 1x Aero Pack 38mm Engine Retainer (Link)


Other Supplies:
- 1" Masking Tape
- Pencil
- Wood Glue
- Gel CA Glue
- Medium Grit Sandpaper



Summary: I decided to do the motor mount slightly different than how the instructions direct you. I wanted to leave off the rear CR, so I could do some internal fillets and make it super strong and ready to crash into the hard Nevada playas. It makes it more difficult, but will be worth it in the end I think.


Step 1 - Remove the Glassline: The Madcow tubes are coated with a glassline that makes them smooth. Since epoxy can't grab onto it well, Madcow recommends that you sand it off. After sanding for a couple minutes, I decided to peel it off instead. It came off really easy and almost in one continuous piece.
IMG_3951.jpg


Step 2 - Sand CRs to Fit: Since I made the motor mount tube slightly smaller by removing the glassline, very little sanding was necessary, but I test fitted them all and made sure they fit, sanding off any rough edges. There is an error or change in the Super DX3 kit. The manual shows that there is only 2 CRs included, but it actually came with 3. I assume this is the way all the kits are now so you can sandwich the fin roots in between them.


Step 3 - Mark Location of Bottom CR: Next I placed the aft end of the motor mount tube into the engine retainer and marked it with a line. Next I slid down the bottom CR on top of the engine retainer.
IMG_3952.jpgIMG_3955.jpgIMG_3954.jpg


Step 4 - Tape Fins in Place and Secure Middle CR: I took some masking tape and attached all three fins around the motor mount tube keeping them as perfectly vertical as possible. Once they were secure, I slid the next CR down on top of their roots and held them with some moderate pressure. While holding them down, I tacked the CR in place with some dots of gel CA. Once the CA was dry and holding the CR in place, I removed all the fins and finally made a small fillet of wood glue to hold the CR more permanently. I didn't use much as I will make a larger epoxy fillet later.
IMG_3957.jpgIMG_3958.jpg


Step 5 - Attach Top CR: I attached the top CR (the one with the eye bolt hole) to the upper end of the motor mount tube 1/2" down from the end as per the manual. I glued it in place with a small amount of wood glue until I do epoxy fillets later.


Step 6 - Epoxy Fillets: Lastly I mixed up some 30 minute epoxy with colloidal silica to thicken it to a paste consistency. I used this to create strong fillets on the top and bottom of the top CR and the top of the middle CR. You do not make fillets on the surfaces that butt against the fin roots or they will interfere with the fins later. I also used some excess epoxy to completely cover the top CR to protect it from ejection blasts.
IMG_3963.jpgIMG_3964.jpgIMG_3965.JPGIMG_4051.jpg
 
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AIRFOIL FINS

Parts Involved:
- 3x 1/4" Plywood Fins (Kit)

Other Supplies:
- 1" Masking Tape (Blue)
- Ruler
- Pencil
- Hobby Knife
- Medium Grit Sandpaper
- Palm Vibrating Sander
- Sanding Block



Summary: I wanted a nice airfoil on the fins, but I didn't like the look of a rounded leading edge and tapered trailing edge on this shape fin, so I decided to go with a chiseled diamond pattern on all 3 sides. I don't have a table saw blade that is good enough for fine detail, so this is the method I came up with.


Step 1 - Mark 3/8" Border on Fin Edges: I took a high quality steel ruler and marked off a 3/8" border on all sides (except for root) of the fins with a pencil.


Step 2 - Tape off Border: Next I took some 1" wide masking tape and applied it on the inside of the fin borders. I only had regular tan tape at the time. Blue would make it much easier to see during this process. Use an Xacto knife to cut off excess.
IMG_4047.jpg


Step 3 - Sand Edges: At this point, I took my palm sander and carefully started removing fin material. This process ended up being easier than I thought it was going to be because of the layers in the plywood. When sanding, you can see lines created as you pass through them. They make for handy guides to keep things straight. Once I had the sanding close with my palm sander, I switched to a sanding block to fine tune the look.
IMG_4050.jpg
 
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Questions:
1. What is the best way to glue the switch band to the coupler? It wasn't that difficult when I have done it in the past with epoxy on a fiberglass kit because the excess that gets pushed out can be cleaned up with denatured alcohol. I'm not sure that's a good idea on the cardboard tube.
IMG_3525.jpg


2. Plywood fins....What is the preferred method to make them smooth? I can use the watered down carpenters putty, or I also have Deft sanding sealer. I'm mostly terrified of warping them by getting some moisture in them.
 
I'll be watching this build thread closely since my next rocket will be a Super DX3.

I'm going to try your process for putting an airfoil on the fins, using masking tape to mark the edge of the airfoil instead of just eyeballing it which is hard to get perfect. BTW, there is no reason to put an airfoil on the tip edge of the fins, unless you just like the way it looks.

Plywood fins....What is the preferred method to make them smooth? I can use the watered down carpenters putty, or I also have Deft sanding sealer. I'm mostly terrified of warping them by getting some moisture in them.[/B]

I do 1 or 2 coats of sanding sealer, followed by as many coats of high build primer spray as necessary. It takes more primer to fill the tube spirals than it does to hide woodgrain on the fins.
 
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I'll be watching this build thread closely since my next rocket will be a Super DX3.

I'm going to try your process for putting an airfoil on the fins, using masking tape to mark the edge of the airfoil instead of just eyeballing it which is hard to get perfect. BTW, there is no reason to put an airfoil on the tip edge of the fins, unless you just like the way it looks.

I do 1 or 2 coats of sanding sealer, followed by as many coats of high build primer spray as necessary. It takes more primer to fill the tube spirals than it does to hide woodgrain on the fins.


Yeah, I mostly just did it for looks.

The tube spirals I always fill with Bondo Glasing and Spot putty.
 
Questions:
What is the best way to glue the switch band to the coupler?

I do use that method on my electronics bays. Just keep some Q-tips handy and it is easy enough to clean off the excess before the epoxy begins to harden. I then just use a dremel to sand off anything I didn't get when it was still wet.
 
Questions:
1. What is the best way to glue the switch band to the coupler?


Just use diluted wood glue with water and wipe the excess with a towel. you don’t need any structural strength on that part.
 
Questions:
1. What is the best way to glue the switch band to the coupler? It wasn't that difficult when I have done it in the past with epoxy on a fiberglass kit because the excess that gets pushed out can be cleaned up with denatured alcohol. I'm not sure that's a good idea on the cardboard tube.

+1 to the Q-tip tip (q-tip squared?) Also - you can still use acetone / alcohol on cardboard as well if you want. White glue also works since it is a paper-to-paper bond. The diluted glue is a pretty good idea if you want to go that route.
 
AIRFOIL FINS 2

Parts Involved:
- 3x 1/4" Plywood Fins (Kit)

Other Supplies:
- Medium Grit Sandpaper
- Fine Grit Sandpaper
- Sanding block
- Sanding Sealer
- Carpenter's Wood Putty



Summary: Once I was done getting the airfoil shape exactly as I wanted, it was time to move on to sanding and finishing the fins.


Step 1 - Peel Off Masking Tape: Remove all the masking guides from the 3 fins. If you have no fingernails like me, this can be more difficult than should be. (I eat those things like they were Skittles)


Step 2 - Sand Fins and Fill Imperfections: Next I took some medium grit sandpaper and got all of the surfaces smooth except the root edges. I also used some carpenter's wood putty to fill two small holes that were in the leading edges. I also made sure the patterns on the edges all matched in size.
IMG_4052.jpg


Step 3 - Sanding Sealer: Next I gave both sides a good coating of Deft Lacquer Sanding Sealer and let dry. Once they were dry, I sanded all of the surfaces with some 150 grit sandpaper. Then I cleaned off the fins and sprayed them again with the sanding sealer. Once they were dry again, I sanded all the surfaces with some 220 grit sandpaper until they were smooth all over. At this point, there is still a tiny bit of wood grain left in the texture, but it will be taken care of in the primer stage. During this process I left the root edges rough since they will be inside the rocket and it will help with epoxy adhesion.
IMG_4053.jpg
 
Install Switch Band:

Parts Involved:
- 1x Coupler Tube (Kit)
- 1x Switch Band (Option)


Other Supplies:
- Wood Glue
- Masking Tape
- Pencil
- Measuring Tape
- Moist Paper Towels



Summary: The switch band is an option for this kit that I purchased from Madcow along with the altimeter bay upgrade. Installation is very easy as long as you take some steps to avoid glue getting where it shouldn't.


Step 1 - Mark Location for Switch Band: At this point you will notice there is a second error in the manual and they used to ship this kit with a 6" coupler. My kit came with a 8" coupler and I assume this is the way they all are now. The manual tells you to have leave 4 1/2" inches exposed and you are going to glue the remaining 1 1/2" inches into the forward body tube. Since I am building this as dual deploy, I want the switch band to be in the midpoint of the coupler. The switch band is actually 1 1/4" wide (not 1" as advertised), so to center it on the 8" coupler, you need to measure 3 3/8" from each end. I traced the switch band with a pencil after getting it centered.
Manual Error 1.jpgIMG_4056.jpg


Step 2 - Tape Off Coupler: Next I applied some 1" masking tape along the line I traced on one of the ends. This will serve as protection from getting excess glue on the coupler.
IMG_4054.jpg


Step 3 - Glue Band to Coupler: Now I applied wood glue all around in the area where the switch band will go and smoothed it out with my finger. Once that was done, I slid the switch band down on top of the glue in between the two lines and twisted it a bit to spread the glue. This has to be done quickly since wood glue has a tendency to set and catch fast.
IMG_4055.jpg


Step 4 - Clean Excess Glue and Remove Tape: Finally, I took some damp paper towels and cleaned off any excess wood glue that was pushed forward. Once that was done, I removed the tape and let it dry.
IMG_4056b.jpg
 
Questions:
1. With my fiberglass kits, I used plastic rivets to hold the body tube to the coupler. Do these work just as well on cardboard tubes? Do the holes get worn out and sloppy over time? It it best to coat the holes in CA or something?

2. I'm going to fill the body tube spirals with Bondo Glasing and Spot Putty as usual. Does this stuff stick to the glassline, or should it all be removed? I would like to only sand off the glassline around the fins where the fillets will be if I can get away with it.
 
Questions:
1. With my fiberglass kits, I used plastic rivets to hold the body tube to the coupler. Do these work just as well on cardboard tubes? Do the holes get worn out and sloppy over time? It it best to coat the holes in CA or something?

It does work well for cardboard but you nailed it with needing to reinforce with something. CA works well - just wick it in there and sand down the ridge that pops up on both sides.

I am getting ahead in your build but you might want to do all of these at once: If you're doing shear pins you'll also want to reinforce those with CA as well. I would also recommend using some kind of metal strip embedded in the nose cone shoulder to cut the pins cleanly. This will help prevent distortion of the shear pin hole in the cardboard airframe. I used a small strip of brass from Lowe's "hobby" drawer in the hardware aisle. Works well.
 
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Install Eye Bolt on Motor Mount:

Parts Involved:
- 1x 1/4"-20 Eye Bolt (Kit)
- 1x 1/4" Washer
- 1x 1/4"-20 Nut
- 5 Minute Epoxy


Other Supplies:
- 11mm Wrench
- Dremel Tool with Cutting Wheel



Summary: I decided not to build this with a kevlar Y-mount like I have with all my other fiberglass builds and simply use the eye bolt method as the kit was designed. I did, however, replace the kit eye bolt with my own forged one to increase the strength. I will be using a quick link to attach the cord rather than tying a knot. Before you decide to go this route, make sure your arm will fit down in the body tube and have room to work.


Step 1 - Install Eye Bolt: First install the eye bolt, washer and nut and get it snug by hand. This way you can tell how much excess length there is to the bolt. You can either mark the excess with a marker, or simply cut it while installed on the centering ring.


Step 2 - Cut off Excess: Using my Dremel tool and a cutoff wheel, I removed the excess bolt and smoothed out the end. This is really only for the purpose of saving weight.
IMG_4060.jpgIMG_4062.jpg


Step 3 - Tighten Bolt and Epoxy in Place: Next I tightened the nut down with an open ended wrench, keeping the eye perpendicular to the center of the ring. Once the bolt is very tight, I coated the nut and washer end with some epoxy to seal it in place.
Manual Eye Bolt.jpgIMG_4065.jpg

 
I would also recommend using some kind of metal strip embedded in the nose cone shoulder to cut the pins cleanly. This will help prevent distortion of the shear pin hole in the cardboard airframe. I used a small strip of brass from Lowe's "hobby" drawer in the hardware aisle. Works well.

Interesting. Do you have any pics of this? I'm not completely sure I understand what you are getting at.
 
Install Motor Mount:

Parts Involved:
- 1x Motor Mount and CR Unit (Kit)

Other Supplies:
- Epoxy
- 24" Wood Dowel
- Sharpie Marker



Summary: Time to install the motor mount into the bottom body tube. I didn't get many shots of this process because I was working quickly with epoxy. I only glued the top CR to the body tube since everything will be held securely later by the fins.


Step 1 - Mark Location of Top CR: First, I slid the motor mount into it's exact place where the bottom edge of the middle CR was perfectly at the top of the fin slots. Once this was in place, I took a Sharpie marker and traced the location of the top CR onto the inside of the body tube. This was going to serve as a guide for the epoxy.


Step 2 - Epoxy Inside of Tube: Next, I mixed up some quick 5 minute epoxy and with a long wooden dowel I applied it to the inside of the body tube about a half inch below the line I drew earlier with the Sharpie.


Step 3 - Push Motor Mount into Place: Now I slid the motor mount into the lower body tube up from the bottom. As is neared it's final position it pushed through the epoxy. I twisted it slightly to help spread it around. Once I had it in place, I made a fillet on top of the top CR with the remaining epoxy. I tried to use the dowel at first, but I found it was easier to just pour it down in there and use gravity to slide it around.
IMG_4076.jpg


 
Install Fins:

Parts Involved:
- 3x Plywood Fins (Kit)

Other Supplies:
- Wood Glue
- Styrofoam Poster Board
- Fin Guide (Payloadbay.com)
- Hobby Knife
- Aluminum Angle Stock
- Pencil
- Medium Grit Sandpaper



Summary: In this step I made a fin guide and tacked the fins in place with wood glue.


Step 1 - Make Fin Guide: I always use the fin guide tool found here. Do to the size of the rocket, I had to print it out at work on 11x17 paper. Simply tape the paper guide on top of a piece of styrofoam poster board as a template and cut it out with a hobby knife. The process can be time consuming, but I think they are the greatest way to ensure strait fins every time.
IMG_4078.jpgIMG_4079.jpg


Step 2 - Extend Fin Slot Lines on Body Tube: Next, I wanted to draw some guidelines on the body tube to ensure the long fin roots were in line. You can do this in a door jam, but a piece of aluminum angle works far better. Simply line up the guide and extend the fin slot lines with a pencil about 12" upward onto the tube. Once that is done, take some medium grit sandpaper and rough up the glassline surface between the lines where the fin will attach. I also used this time to draw a guideline at the midpoint between two fins for the rail guides later.
IMG_4077.jpg


Step 3 - Glue Fins into Place: Now I coated the all of the fin root and tab surfaces with wood glue except the rear edge that will touch the bottom CR. Once this is done, slide the fin into place and move the guide to hold it in. This is just tacking the fin into position and will be held stronger later on with internal fillets. I had to hold one of the fins by hand until it dried because the wood was slightly warped.
IMG_4080.jpgIMG_4081.jpg


 
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Internal Fin Fillets:

Parts Involved:
- Lower Body Tube and Fin Can (Kit)

Other Supplies:
- Epoxy
- Collodial Silica
- Wood Stir Sticks
- 6" of 3/8" Sharkbite Tubing



Summary: In this step I made some internal epoxy fillets to lock the fins in place for strength. Many people think this is overkill, but I think the trade off of a little more weight for strength is worth it, especially when I fly exclusively in the desert playas of Nevada.


Step 1 - Mix Epoxy: I mixed up some more 30 minute epoxy with colloidal silica to make it into a smooth paste. This makes it so I could do all 12 internal fillets at once and not have to wait for each set to cure.
IMG_3963.jpg


Step 2 - Smear Epoxy into Fillet Areas: Taking a wooden stir stick I packed epoxy into the 12 fillet areas as well as I could. This is something where looks don't count as it will be sealed up and never seen.
IMG_4081b.jpg


Step 3 - Pull Fillets with Tool: Using a piece of 3/8" Sharkbite tubing, I pulled the fillets smooth. Again, it's not pretty, but functional.
IMG_4082.JPGIMG_4083.jpg


 
Rear CR and Engine Retainer:

Parts Involved:
- Lower Body Tube and Fin Can (Kit)
- 1x Plywood CR (Kit)
- Aero Pack 38mm Retainer (Link)


Other Supplies:
- Wood Glue
- J.B. Weld Epoxy
- Wood Stir Sticks



Summary: In this step I installed the rear CR and the Aero Pack engine retainer after letting the internal fillets cure.


Step 1 - Install Rear CR: I used wood glue to install the rear CR. I simply applied it to all surfaces on the fin roots, body tube and motor tube. Once the glue was on, I pressed the CR firmly in place and stood the rocket aft down so gravity would help make fillets while the glue dried. This step can be tricky because if you want to dry fit the CR first, you aren't going to be able to get it out easily.
IMG_4084.JPGIMG_4085.JPG


Step 2 - Mix J.B. Weld Epoxy: Mix up equal amounts of the J.B. Weld tubes until the paste is evenly gray.
IMG_4086.JPGIMG_4087.jpg


Step 3 - Install Aero Pack Retainer: Using a wooden stir stick, I coated the outside of the exposed motor mount tube as well as the inside of the retainer with J.B. Weld. Once both surfaces were coated, I slid the retainer over the epoxy twisting to spread it around. Next I used my finger to clean up the excess and make a smooth fillet around the base. Then I set it aside to sure for a few hours.
IMG_4088.JPGIMG_4089.JPG


 
I must say, your build threads are very well organized. Very nice clean build you've got going!:) I'm watching.
 
Electronics Bay Bulkheads:

Parts Involved:
- 2x Coupler Bulkheads (Kit)
- 2x Body Tube Bulkheads (Kit)
- 2x 1/4"-20 Eye Bolts (Kit)
- 2x 1/4"-20 Nuts (Kit)
- 4x 1/4" Washers (Kit)
- 4x 1/4" Fender Washers (Hardware Store)
- 2x 1/4"-20 Wingnuts (Hardware Store)


Other Supplies:
- Wood Glue
- Drill or Drill Press
- 1/4" Drill Bit
- Thin CA Glue
- Medium Grit Sandpaper


Summary: In this step I built the bulkheads for the electronic bay/coupler. This involves parts that came with both the main kit and the electronics bay upgrade.


Step 1 - Glue Bulkheads Together: After sanding and dry fitting the parts, I covered one side of a larger bulkhead with wood glue making sure not to get too close to the edges. Next I slid a 1/4"-20 hex bolt (or eye bolt) through a large fender washer and then through the center hole of the large bulkhead. Then I slid the smaller bulkhead down onto the glued surface and secured it with another fender washer and wingnut. The hardware here isn't necessary, but it does an excellent job of holding the pieces tightly together while the glue dries. Do this for both sets.
IMG_4066.jpgIMG_4067.jpgIMG_4068.jpgIMG_4069.jpg


Step 2 - Drill Holes for Threaded Rod: Since the electronics bay is an option for this kit, the holes aren't there on the smaller bulkheads. Once the glue is dry on each set, I took out all the hardware and drilled the holes the rest of the way through with my drill press and a 1/4" bit.
IMG_4073.jpg


Step 3 - Assemble Bulkheads with Eye Bolts: Once the holes were drilled, I installed the eye bolts, nuts and washers that were supplied with the kit though the center hole. The fender washers could be used but it's just excess weight for no real reason.
IMG_4074.jpg


Step 4 - Coat Coupler Ends with CA: I took some thin CA and coated the inside of the coupler ends down about 1/2" and let it dry. Once dry, I sanded it smooth with some medium grit sandpaper and installed the bulkheads.
IMG_4071.jpgIMG_4075.jpg

 
External Fin Fillets:

Parts Involved:
- Fin Can Body Tube Section (Kit)

Other Supplies:
- 30 Minute Epoxy
- Phenolic Microballoons
- Popscicle Sticks
- 2" Length of 1/2" PVC
- Denatured Alcohol
- Medium Grit Sandpaper
- Masking Tape
- Pencil


Summary: Making good fin fillets is probably the most technical and difficult part of building any high power rocket. In this step I made fin fillets one at a time over the span of 2 days.


Step 1 - Mark Fillet Area: After deciding that 1/2" PVC made the correct size fillet for this rocket, I took a short piece of it and colored it with a pencil very heavily. (Many people suggest a black marker for this, but I found a pencil works far better. Then I rubbed the PVC up and down the fillet area transferring the black onto the body tube and fins. This gives you guidelines to where you put the masking tape.
IMG_4095.jpgIMG_4096.jpg


Step 2 - Mask off Fillet Area and Sand: Next I took some 1/2" masking and taped off the entire area using the black lines as a guide. Once that was done, I used some medium grit sandpaper to score up the areas where the epoxy will touch for better adhesion.
IMG_4091.jpgIMG_4092.jpg


Step 3 - Mix and Apply Epoxy: Now I mixed up a 40 ml batch of my U.S. Composites medium epoxy and added phenolic microballoons. The balloons greatly increase the volume without adding hardly any weight at all. You simply keep adding small amounts and stirring until you get to the consistency of a paste that will hold it's shape. Once you are satisfied with the texture, take a popsicle stick (or similar) and generously pack the epoxy into the fillet areas. I didn't get pics of this since it's a stressful step and I forgot so here is one from my Wildman Jr. build.
IMG_3632.jpgIMG_3633.jpg


Step 4 - Smooth Fillets with Tool: Now for the tricky part. Dip the PVC tool in denatured alcohol, shake off drips and then "pull" it through the epoxy at about a 30 degree angle trying to complete each side in one movement. There are a million opinions on this, but I personally like to pull one side at a time from back to front. Since the epoxy is like a sandy paste you can gently fix problem areas (usually on the front and back) with your fingers.
IMG_4093.jpg


Step 5 - Remove Masking Tape and Repeat: Allow the epoxy to set up for an hour or so (can depend on epoxy type) and then gently pull off the tape. DO NOT forget about this or you will have major problems once the epoxy is fully hardened. Let cure overnight. I like to do one set at a time, but you can do all three at once if time is crucial.
 
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you should use west colloidal 406, micro balloon remove structural strength
 
you should use west colloidal 406, micro balloon remove structural strength

That is true, but I wasn't worried on this one with how strong it is already. The internal fillets are all with West 406. The external ones are more cosmetic than anything.

Is there a trick to getting the 406 perfectly smooth? Seems like I can't get all the lumps out.
 
witch epoxy you are using ? It smouth very well with Aeropoxy, for wood and cardboard aeropoxy is the best, it as a great penetration
 
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