Making easy epoxy fillets

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Well-Known Member
Jan 25, 2009
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I just finished (ok, yesterday) making some great looking fillets. Using a combination of techniques from around the web and magazine articles, I used these tools and supplies to make these fillets.
  • Finishing (Thin) Epoxy
  • Microballoon filler
  • Plastic Spoon
  • Masking Tape
  • Rubbing Alcohol
  • Latex gloves

Then I followed these steps to make the fillets.

  1. Put on some protective gloves. I used latex gloves.
  2. At the front end of the fins, run a piece of masking tape right at the tips, all the way across.
  3. On the fin and on the body, run lengths of masking tape to define the borders of your fillets. I approximated a 1/4", but I didn't measure it. Make sure the tape is straight!
  4. Mix up appropriate amount of epoxy for fillets. On my fillets, which were about 9" long, I used about 10mL of epoxy for two
  5. Mix in enough Microballoons to make the epoxy thick. Referencing the West System epoxy users guide, I ended up with a thickness between ketchup and mayo.
  6. Make sure all this is mixed in THOROUGHLY!
  7. Now pour a nice, thick bead of the epoxy/microballoon mix into the valley of the soon-to-be fillet. Don't worry about getting excess on the masking tape.
  8. Using the plastic spoon's backside (not the scoop side) run it from the forward part of the fillet to the back. Use a gentle angle on the spoon so that you don't scoop too much epoxy out. Don't worry if you do. Just quickly move it to the front of the fillet and let it drip back into the fillet, and run it back through at a shallower angle.
  9. Using the rubbing alcohol and your finger, DRIP a little bit of the alcohol onto the fillet, then gently rub the spoon over the fillet again, this time at almost a flat angle. This step thins the top bit of epoxy out and gives a glass-smooth finish.
  10. Now let the epoxy cure for at least 90 minutes, then you can gently remove the masking tape from the edge. This will allow the thickened epoxy, set but still flexible, to fill in that gap you see.
  11. That's one way to do it. You also can leave the masking tape on for eight hours and then just remove the tape. If you do it this way, you'll likely be left with a bit of masking tape stuck under the fillet. To remedy this, just use your hobby knife to cut straight down the fillet and pry the masking tape up.
  12. From this point, if you are happy with the fillet, leave it alone. If not, you can sand the fillet easily with some 220 grit to start.

    I hope this will help some people when they start making fillets.

Very well done Jason !

This is something that should be created as a tutorial with images and the like to finish up a sweet tutorial. For me, 2 weeks ago I'd not have known what microballoons are, but your process makes it easy and effective - and it sounds like you've mastered an age-old question.

Thanks for posting this. 8)

Any chance you have some pics of a before and after ?
Let me tell you what Silverleaf. I'll be building my 29mm Deuce soon, and I will most definately be using this technique when I do. I'll ask one of my friends if I can borrow their video camera and make a movie showing the steps. I'll also take plenty of pictures then, since I'll have my camera back by then, hopefully.

I've been using a similar technique, but instead of using a spoon, I use popsicle sticks. I put some pencil graphite on the edges of the popsicle stick curve and drag that along the joint to mark where the tape should go. It makes it so you can peel up the tape without fear of leaving a "ridge" on the edge of the epoxy. I then use that same curved end of the popsicle stick to shape the fillet. Works VERY well also. I'm going to be trying it with some of the Apogee Components Fixit epoxy clay sometime this week.

Here's another twist to add...

In my quest to minimize "wait" time during rocket builds, I've found that 90-second (yes, *seconds*, not minutes) is a great boon to building fillets.

I mix enough for two fillets - I do them with the rocket in a horizontal position and do the "top" two fillets, wait for it to set, mix another small batch, do the next two joints, etc...

The 90-second stuff I bought at Home Depot comes in the "dual-syringe" dispenser, so it's easy to the get critical proportions right for small quantities. I don't use microballons as this stuff seems thick enough to apply as long as the fin joints you're working on form "troughs" - no drips or runs that way.

Of course, I only do LPR stuff, so YMMV as to whether this material is up to the task of HPR fillets. I had a parachute fail to fully open on a RedMax clone, and it landed fin first on asphalt. The trailing tips of two fins were dinged up pretty good, but the joints were unharmed.

BTW, all the aboved mentioned taping, smoothing techniques work with this stuff just fine. I use an alcohol dipped (gloved) finger and get glass smooth fillets.
Hey Jetra, thanks to your post my fillets are coming out pretty nicely these days!!

I have been using G5 5-minute epoxy and no thickeners. I think I will start using microballons because that makes it easier to sand to trim it up. I still gotta work on the taping and edges...
Very nicely done Jason!
that last item can't be stressed enough, rubber gloves well help prevent becoming alergic to epoxy resin, which builds up in the body over time and exposure. Very good method!
I'm going to chime in with my personal variation of epoxy fillets...

It's pretty much the same thing except that I used milled fiber (also referred to as chopped fiber), which is essentially short threads of fiberglass. While it acts as a thickening agent (like microballoons) turning 30-minute epoxy into the consistency of ketchup, it also (claims to) triples the strength of the epoxy! The only downside is that once it cures, it better look exactly how you want it--'cause you aren't going to sand it to shape! It's too hard :eek: Thankfully the slower curing epoxy gives you plenty of time to smooth it out with a spoon and/or rubbing alcohol to the shape you want.

BTW, BSD (& I believe Binder Design) sell a small bottle of the stuff for ~$5. I used it on my BSD Thor & Apache but still have over 3/4 of the container left! It lasts forever...
I've found a real simple technique that so far has held up well (including a Javelin that flew into the ground horizontally, under power - more on that in another thread :mad: ).
I just use 15 min. epoxy thinned out with a couple drops of rubbing alcohol (70%). Mask off the area around the fillet, then slowly pour it along the length of the fin. Let it set up for a few minutes then pull the tape before it dries. So far I've gotten perfect fillets without having to smooth or sand them.
Jason, awesome review. I bet this method saves you quite a bit of time when it comes to finishing. I also imagine that if the masking tape leaves a ridge, you could always use FnF or Microfill to get the fine detail. Well done. (I take it this is for the Big Red rebuild?)
OK Chilly & Jason, how do you guys mask the airframe at the ends of the fillets? just a simple piece of tape across the tube & the fillet tape? or something more elaborate? I'm looking for an "easy" way to get "perfect" fairing of the fillet ends ... thanks!

What I've done before is used a Dremel tool to sand it down a little bit. It'll usually make a nice fairing. If you go a little far, you can easily use some Fill n' Finish and fill it in till it's perfect.

More later...

I agree. I make mine a little bit long, use a masking tape dam, and then dremel it down. It's pretty tough with the 15-minute epoxy without any sort of microballoon filler, but the dremel gets it done eventually. If you are going to use this approach, I must advise you to invest the few bucks in a dust mask so you don't end up inhaling all the epoxy dust.

sounds like the Dremel really is a great tool, I'll have to get one.

Can you guys send my wife a quick note and tell her it's OK for me to buy a Dremel? Father's Day isn't that far off. I am still in trouble for the drill press & sander a couple weeks ago :) wait until she sees the bill for the J motors for L2 in a couple weeks!!!
Dear Mrs. CLS,

Please buy your husband a Dremel tool for Father's Day. It will save him a lot of time, and while you're out buying it from YOUR account, you can buy yourself a nice something out of his account of equal value to the Dremel and then you can call yourselves even!



:p :p :D :rolleyes:
excellent about next time you do some photos so we numbskulls can follow you step by step?
a pic is worth a 1000 words....
Originally posted by wwattles
. I'm going to be trying it with some of the Apogee Components Fixit epoxy clay sometime this week.


Well, I tried it. I'm probably not going to try it again on anything less than a large MPR. Having done the liquid epoxy tricks mentioned in this thread with absolutely amazing results, the epoxy clay left MUCH to be desired. It turned out rather lumpy, smeared all over my rocket, and in general was more hassle than doing the masking tape and 5-minute epoxy that I know and love!


For us first timers... I'n nervous enough just looking ahead...LOL
While reading it somewhat makes sense... nothing would help more that seeing how the tape application looks before adding the epoxy...Thanks

I am in the midst of building my upscale Fliskits Flea right now. I will be doing these kind of epoxy fillets on it just as soon as I can. Probably by middle of next week I'll have pics of all the steps!

Thanks for the interest in this technique!


Sorry about the wait guys. School work has been a pain, along with work, and I've been just a bit too tired to actually sit down and do this...but now I can! So, now I am going to document the steps with the pix that I've taken. They're not the best pix in the world, but they'll do. I hope this helps ya'll make purdy rockets from now on!

OK. First Step!

Put on some gloves! I'm using latex gloves, but I've been told that Nitrile (sp?) gloves may work better. IT IS IMPORTANT to put on gloves! Keeps ya from getting an allergy to the epoxy! ICK!

Second Step:

Get all your stuff. Here I have: A rocket (duh!), 20 min Finishing (thin) Epoxy, Microballoons, Mixing cup, Mixing Stick, Plastic Spoon (not in pic), Masking Tape, a box, and some papers. You may ask what the box and papers are for. Box: Keep the rocket level as you apply the fillets. Papers: To keep your better half from yelling at you/protect your work surface.

Third Step:

Mask off your fin areas with the masking tape. I feel it is best and not overkill to use an approximate 1/4" fillet. Just measure it by's not that big of a deal.

Fourth Step:

Measure out and mix your epoxy. For these fillets on my 29mm Big Daddy mod, I used 5mL and had epoxy left over. I feel that using a popsicle stick is the best way to mix these small amounts of epoxy. You need to scrape the sides of the cup and the bottom to make sure you get all of it mixed well.

Fifth Step:

Add microballoons. These things are a pain in the you-know-where sometimes, and they will blow around a lot, so make sure when you add them to the cup, the nozzle of the container is close to the epoxy. I added enough microballoons to get a mixture the consistency of thick ketchup, but not quite mayo.


Sixth Step:

After you've mixed the balloons in well, pour them somewhat thickly into the fin-body joint. Don't really worry about maing a mess on the tape and making the fillet look ugly...we're getting to that part.

Seventh step:

Now take your plastic spoon and at a slight angle, run it along the fillet. Don't put it a such a steep angle that it scrapes all of the epoxy out, but enough that it scrapes SOME out, and makes it smooth. See the picture for a comparison between a "first-scrape" fillet and a "no-scrape" fillet.

Here's a quick pic of both the fillets on a "first-scrape" basis. See that drip? It's thick enough that it won't go far, but scoop it up with your spoon.

Eighth (and final) Step!

Now you are going to get some rubbing alcohol and drip some into the fillets. As this spreads out along the fillet, it'll make a nice, smooth surface. Not too much though! Just dip your finger into the alcohol and drip it along the fillet.

Now, here is how you are going to leave the rocket for at least two hours. You need to let the epoxy set into place. After about an hour, go ahead and try to peel the tape off. If it starts pulling up your fillet, put it back down. You can always trim the tape away with a kinfe later on.

Wasn't that easy? Just rinse and repeat for all your other fillets!

:D :D :D


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