Using Polyurethane on Balsa

Discussion in 'Techniques' started by markschnell, Aug 15, 2010.

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  1. Aug 15, 2010 #1

    markschnell

    markschnell

    markschnell

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    Will a polyurethane finish work as a sealer for balsa fins? I'm just thinking of using what I've got around the house. I would put on two or three coats, sanding between coats before I painted the fins.

    I have plenty of Minwax oil-based polyurethane and wondered if that might be a good option. I've got both wipe on poly and brush on varieties.
     
  2. Aug 15, 2010 #2

    ctl73

    ctl73

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    Hello to all this is my first post but I have been reading and learning for weeks. My first rocket is in a tree and I am learning how hard it is to paint a rocket to my standards. I was just wondering the same thing I got some stuff called aro glass sanding sealer and it works well but will polyurethane work I am painting with rust-oleum ultra cover rattle cans. This forum has been great and look forward to your feed back. I will post some pics later this week to make you all laugh at my paint growing paint. Thanks Chris
     
  3. Aug 16, 2010 #3

    luke strawwalker

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    On the polyurethane sealer, I'm not sure... it's worth a try on some scrap balsa first to see how you like it. Please be sure to post your results here! Hopefully some others with more experience with this stuff will post and let us know their experience with it as well.

    For the second post, yes, many modellers SWEAR by sanding sealers and prefer using it. From what I've read, the stuff smells positively awful and requires several coats and sandings to get good results.

    Other modellers, myself included, prefer to use regular Elmer's Wood Filler, which has been thinned to about the consistency of mustard (or maybe a touch thinner) with a few drops of water. I myself have a few baby food jars that I put a small dollop of wood filler into and put a few drops of water in with it, and mix it up with a paintbrush. The thinned filler can then be easily painted right onto the fins and allowed to dry, and is easily sanded down smooth with 220 grit, usually followed by 400-600 grit sandpaper, which then leaves the rocket ready to be primered and sanded again for painting. You can get fins looking almost like plastic with this method. Beware though, very thin fins can be warped from the extra water in the thinned filler, though I personally haven't had a problem with this. One can also apply thin liquid CA (superglue) to the fins to seal them (and strengthen them) before applying the filler. Do this outside though, and stand upwind/crosswind, because the fumes can be rather noxious, and don't glue the fins to your hands! Best to knock off the "hardened fuzzies" that the CA raises on the fins with some 220 grit sandpaper before you fill them. I've done this a few times and it makes nice somewhat stronger fins...

    Another trick for making slick fins is, simply glue a layer of regular printer paper directly to the fin. Sand the fin to your desired shape (airfoil, wedge, rounded, whatever) and apply a THIN layer of white glue to one side, press it down onto a piece of printer paper, and the apply another thin layer to the topside of the fin, roll it over the leading edge of the fin, and press it down firmly onto the other half of the piece of paper (I cut the paper to about double the size of the fin with about 1-2 inches all the way around before I start). Use the round tip and barrel of a Sharpie marker as a rolling pin and burnishing tool to work all the excess glue out from between the paper and the surface of the wood fin. Start at the leading edge of the fin and work back to the trailing edge and to both root and tip edges from the center. Set them aside to dry overnight, underneath some heavy books if your worried about possible warpage (large or thin fins). Cut them out from the excess paper with scissors to within about 1/4 inch of the edge of the fins, and then using a SHARP hobby knife, shave the paper off down flush to the edges of the balsa fin core, around the tip, trailing, and root edges of the fin. The leading edge is already done for you if you burnished the fin right gluing the paper on, and the once the fin is trimmed, draw it across a sheet of sandpaper a couple times along the root, trailing, and tip edges to remove any stray 'hairs' of paper, and they're ready to glue on the rocket with no further filling required. Couple coats of primer and they'll look like plastic fins.

    In all fairness, some folks who paper their fins prefer to forego printer paper and white glue, and use Avery Label Paper purchased from Office Depot and such stores. Some folks have good luck and swear by it, which is all well and good. Whatever works best for you is what you should do!

    Good luck and hope this helps! OL JR :)
     
  4. Aug 16, 2010 #4

    luke strawwalker

    luke strawwalker

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    We've discussed painting techniques in several threads recently... might try a search to see what you can find for helpful pointers...

    Painting is a skill that CAN be learned!

    later! OL JR :)
     
  5. Aug 16, 2010 #5

    Marc_G

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    I'll chime in with my own experiences so far.

    I'm a BAR as of this past Memorial Day and have spent the summer getting back into things. Here's what I've tried:

    Elmers Carpenters Wood Filler. The kind in the tube doesn't really need thinning, just spread it on. I got some warping but not bad and generally unwarps as it dries. Lots of dust from this method but nothing toxic. Reasonable smoothing but not much strengthening.

    CA: Highly toxic of course but the CA went on easy, soaked in, dried, no warping, lots of strengthening. Took a bit of work with 150 grit to smooth it out but painted beautifully. My experiment used I think crazy glue in a bottle with brush.

    Minwax water based polycrylic: Ick. Megawarp. Didn't strengthen much either. Save this stuff for furniture.

    The lots-o-primer method. Couple heavy coats of automotive filler primer (Rustoleum, from WalMart). Do the whole sheet of fins at once as a sheet. Sand back to nearly bare wood, shoot again, lighter sand. Done. Glue fins. For me this seems to work well and is maybe my prefered method. Only a little fin strengthening though. But easy and fins look nice. I cheat and use an orbital palm sander. :blush:

    Haven't tried urethane yet nor "Minwax Wood Hardener."

    Marc
     
  6. Aug 16, 2010 #6

    Micromeister

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    It totally depends on your (and your household occupants) tolerance for smells.

    I'm lucky enough to have permanent paintbooths in my basement BUT!!!! that doesn't always clear ALL the smell from some applications.

    Balsa sealer and Balsa Filler are to different things; Sealer is just that a sealer that takes a lot of coats and time to Fill any grain. Balsa Filler has heavier solids filling grain much quicker. Both are Lacquer thinner based products that smell to high heaven. I like the Filler but the better 2/3rd does not so It's resevered for very special projects, almost never used anymore.

    Brush/roll on Polyurethane clear is not a filler either. While it doesn't smell nearly as bad as the balsa fillers. It is a clear polyurethane finish... Ment for high foot traffic wood floor areas...it does not sand with a hoot! so I'd not suggest it for filling gain.

    Miniwax Wood hardener is another very high smelling wood product that really does harden the Sapwood parts of most any wood. Given enough coatings it would i'm sure fill some balsa grain but the resulting weight and hardness to sanding would likely make it not worth the smell or trouble. It really does harden wood though and is a great product for reparing damaged wood or stiffening very light weight balsa. I would not use it as a filler.

    Elmers or however it's being packaged these days "Fill n Finish" is a fine product of those who don't mind taking the time to thin it out, trowel it on, wait for it to dry and then sand it off, and repeat. It is by far the least smelly of all the options, but it's also WAY to slow, and from the times I have used it, really no better then my current preferred seam and balsa grain filling method "Cheapy Auto grey spray Primer".

    Cheapy sandable auto primer. I'm buying .89 cent a 12oz can Walmart rattlecan spray Auto primer. It's used in two to three unsanded coats, let dry 10 minutes then two to three additional coats. This process is repeated one more time making 3 sets of 2-3 coats over the entire model than allowing the model to set drying for 2 days. Once it's NOSE Dry ("With nose on or right next to the model if you can smell it...it is NOT dry".) Once dry, sand with 120, 240, to 360 dry sandpaper. Checking often for remaining seamlines or grain. If after sanding I've reached bare wood or cardboard with grain or seamlines remaining, an additional repeat 2-3 sets of 3 coats and sand again to a babies butt smooth 360grit finish. Then base color coat, and on to finishing.
    This method works great on any size model requiring one or 1-1/2 cans if the model is up to BT-80 & about 40" long. A bit more on larger models but not much, only rarely have I used over 2 cans on a single model. As mentioned the drawback to the method is it does sometime allow the other members of the family know your spray painting in the dungon and you need a way to control the overspray (such as a paint booth):)
    Hope this helps a little.
     
    Last edited: Aug 16, 2010
  7. Aug 17, 2010 #7

    markschnell

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    Thanks Micro, your experience is very helpful, as usual.:D
     
  8. Aug 17, 2010 #8

    ctl73

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    Wow that was great thanks for all the input. I know I need to do more searching but I am surfing this forum on an iPod touch and the search function does not work with it. I will have to get out my laptop at some point. Thanks for all the advice now it's back to watching paint dry or run who knows what will happen next. I need to be more patient.
    Chris
     
  9. Aug 17, 2010 #9

    TheAviator

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    I find that Bondo works quite well and puts a fairly rock hard surface on fins and nose cones. It's a pain to work with sometimes because it sets up so fast, but it works like a charm. Smell, IMHO, is "mildly annoying, but ignorable," though you should work with it outside, really.
     
  10. Aug 17, 2010 #10

    Trident

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    I've not found any balsa fillercoat locally, and not bothered yet to find via the internet. I do use SIG Sanding Sealer, since it is carried by our Hobby Lobby stores, and I use a 40% off coupon.

    Smells great! (Translates to: Smells Awful!). But it reminds me of my youth, when using Pactra or AeroGloss Sanding Sealer was pretty common.

    My routine now is to seal balsa during the week -- a little each night. So the need to do a lot more coats using sanding sealer rather than balsa filler is not a big deal to me. I can get sealing done in time for weekend priming and painting. And if there is no wind, I can prime late on a Thursday or Friday evening, and have the nice warm weekend days to paint. Winter time can be a real drag, and maybe I'll do a really complex build this Winter to pass the time. :)

    I am currently filling fins while in the sheet, and will then prime them, and then pop them out when done. So far, this seems to me to be the way to go for laser-cut fins, and it is not a method I've tried before, since I tend to clone, or built older kits that have no laser cut fins. I'd suggest trying several methods, and see what works for you. Everybody finds their own favorite method, and there are plenty to try!
     
    Last edited: Aug 17, 2010
  11. Aug 17, 2010 #11

    Marc_G

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    One more thing:

    For scratch build projects I use basswood instead of balsa because it takes a lot less effort to fill and is stronger per thickness.

    Marc
     
  12. Aug 18, 2010 #12

    Boosterdude

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    I use two to three coats of Minwax Polyurethane to seal the wood and provide a hard base. After that I brush on a coat of filler, and prime. I really haven't read or seen a technique that works faster or better.
     
  13. Aug 23, 2010 #13

    SCIGS30

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    Micro, You can you post a picture of the Walmart Primer you are using? The ColorPlace Primer at my Walmart is .99 cents but it has the recoat within 1 hour or after 7 days. I use the Old Krylon Formula paints, so can they go over the Walmart Enamel Primer?
     
  14. Aug 23, 2010 #14

    SCIGS30

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    Here is my long post on how I am now filling my balsa fins for all the new Estes kits.
    1. I keep all the parts in the balsa sheet and sand smooth.
    2. Shoot a heavy coat of Plastikote sandable primer, weight 20 to 30 min and sand with 220 grit.
    3. Repeat this process 1 or 2 more times until no grain is showing. Last sanding job is with 320 for smooth fins.
    4. Punch out balsa parts, sand edges and seal with balsa filler, sand and hit the edges with primer. This is a fast process and I am ready to build in the same day. With my vintage kits, I still use Aerogloss balsa sealer 3 to 4 coats and that also produces a smooth finish. I have tried building the entire model first then filling with primer. For me that was harder since sanding was a pain in the rear. Primer filler method and Balsa Sealer both work great for me.

    1. Sanded both sides smooth with 220 grit sandpaper

    [​IMG]

    2. Spray a heavy coat of sandable primer into the balsa grain.
    [​IMG]

    3. After 20 to 30 min, sand with 220 grit until smooth. Grain will still be showing. Then spray primer again.
    [​IMG]
    [​IMG]

    After second coat of primer everything is pretty smooth. The process went so fast I decided to hit a touch up primer coat.
    You can see up the light how all the grain is filled when compared to the rest of the balsa sheet.

    [​IMG]
    [​IMG]
    [​IMG]

    I then sanded the LE round and added balsa filler to the edges.
    [​IMG]
    [​IMG]
    [​IMG]
    Phoenix also used the primer method
    [​IMG]
    The Centuri Screaming Eagle used 4 coats of balsa sealer. Brush on sealer, after last fin, repeat. Wait 20 to 30 min and sand with 220 to 320. Repeat 2 to 3 times until smooth. For some reason Balsa filler does not come out as smooth for me as the sealer.
    [​IMG]
     

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