Epoxy Fin Fillets

For a long time I always used the two part epoxy clay, which IMO seems to work well and provides plenty of time to form fillets, but afterward you have to spend some time filing, shaping and sanding the fillets to get that desired smooth and uniform blend we all look for when we make our fillets...well I do at least.

I just started using the regular 15 BSI epoxy and it works pretty good. I understand the procedures and concepts of creating a gulley to allow the epoxy to form using masking tape and then removing the tape and dipping the finger in rubbing alcohol to create a smooth fillet.

My weakness lie in creating the fillet so it is uniform around the leading edge of the fin, where it meets the body tube. Lately I have been just wrapping masking tape around the body tube even with the leading edge of the fin and just ignoring the tip of the fin...

What is the best procedure to use when masking that area so the epoxy doesn't flow off and beyond the tape and you get a nice "point/fillet" in front of the leading edge. It seems if you lay the rocket down on it's side how most of us do it, it would be hard to fill in the that area without it going everywhere...the you would have to come back after the epoxy sets and do the other side and try to fill in the gaps.

Hope I'm explaining myself ...

Try using 5 min epoxy, or use a filler such as micro balloons, blown sillica, splenda.

I also used to use the 2 part clay, but moving up to larger rockets it became a bit expensive.

If you use the 5 min epoxy, let it gel a bit, then use a large craft (popsicle) stick to get a nice shape, then remove the tape. It sets slow enough so you can lay down the filets, and fast enough you can knock out 4 sets in an evening.

I've been using milled fiber glass

Check out Sky Pirate High Power Rocketry. They have a great tutorial on making epoxy filets. They might not be point like you are looking for, but they are smooth and blended.

judo said:

Check out Sky Pirate High Power Rocketry. They have a great tutorial on making epoxy filets. They might not be point like you are looking for, but they are smooth and blended.

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That's exactly what I'm looking for, but not as extensive, but very helpful. I haven't used any West System products and probably won't until I use up what I have or get more involved with bigger rocket projects.

For the external fillets, I simply use my finger. With a latex glove on, I pour epoxy (BSI 5min, usually) into the joint. Then, starting from the aft end, form it with the proper size finger for the desired radius fillet. As I get to the tip, I put more pressure on the fillet to taper it to essentially nothing right at the top. A little bit of sanding and it's done. With a little practice, you get a feel for how much epoxy to pour in the joint to eliminate waste and minimize sanding of excess epoxy.

chadrog said:

For the external fillets, I simply use my finger. With a latex glove on, I pour epoxy (BSI 5min, usually) into the joint. Then, starting from the aft end, form it with the proper size finger for the desired radius fillet. As I get to the tip, I put more pressure on the fillet to taper it to essentially nothing right at the top. A little bit of sanding and it's done. With a little practice, you get a feel for how much epoxy to pour in the joint to eliminate waste and minimize sanding of excess epoxy.

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You are right. I think its a matter of finding what works best for you. I don't think there is a right or wrong way of doing them...I just like to keep the mess to a minimum if you know what I mean?

RocketManDan said:

You are right. I think its a matter of finding what works best for you. I don't think there is a right or wrong way of doing them...I just like to keep the mess to a minimum if you know what I mean?

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Yup. For me it was getting a feel for how much epoxy to pour in at the start.

chadrog said:

Yup. For me it was getting a feel for how much epoxy to pour in at the start.

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I hate to waste stuff... I remember when I first started using the epoxy clay any excess I had left over I would stick to the back of quarters and put them out on the sidewalk in perfect view from the garage. As the neighbors would walk their dogs in the late afternoons I would laugh to myself as they bent over and tried to pick up the quarters...I would throw some loose pennies out there too to make it look like someone lost their change.

RocketManDan said:

I hate to waste stuff... I remember when I first started using the epoxy clay any excess I had left over I would stick to the back of quarters and put them out on the sidewalk in perfect view from the garage. As the neighbors would walk their dogs in the late afternoons I would laugh to myself as they bent over and tried to pick up the quarters...I would throw some loose pennies out there too to make it look like someone lost their change.

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THAT'S AWESOME!! I used to do that to a co-worker every morning - he never realized he was being had for my amusement. I glued a nickel down at one point, and he chiseled it off! Back on track... I never had much waste epoxy, I usually line up a couple different things to do at once. Minimizing the sanding was the goal for me. I've pretty much got it down to a science now, my last project (ADD Express) required less than ten minutes of fillet sanding.

chadrog said:

THAT'S AWESOME!! I used to do that to a co-worker every morning - he never realized he was being had for my amusement. I glued a nickel down at one point, and he chiseled it off! Back on track... I never had much waste epoxy, I usually line up a couple different things to do at once. Minimizing the sanding was the goal for me. I've pretty much got it down to a science now, my last project (ADD Express) required less than ten minutes of fillet sanding.

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You definitely have it down. I like using the clay too, but I always want to try my best to proficient at different techniques. Clay takes a lot of time shaping it just right.

RocketManDan said:

You definitely have it down. I like using the clay too, but I always want to try my best to proficient at different techniques. Clay takes a lot of time shaping it just right.

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That fact is exactly why I've stayed away from it - I'm too impatient.

I guess I'm a bit lazy. I tape off the fins mix up some cheap 30 minute stuff and lay it in the fillet with a wooden skewer. When I think I have enough epoxy in the fillet I use the sharp end of the skewer to evenly distribute the epoxy, pop any potential bubbles and pull a little epoxy around the leading edge. Once that's done I pull the tape off and let the 30 minute stuff self level. As long as I'm careful about the amount of epoxy I lay in I get nice looking fillets. I really hate to sand, like I said, a bit lazy.

I use BSI 30 Minute epoxy, sometimes with microballoons mixed in, and sometimes straight. Adding in the solids makes it somewhat easier to handle, but not that much. Whether adulterated or not, I don't have any problems forming smooth fillets with the "pokey 'poxy," even though it is thinner and runnier than the 5 Minute or 15 Minute types, and it takes longer to set up. It (and Aeropoxy 6209) are the only epoxies that I have ever used for fillets. I don't have much trouble keeping it in place. I mostly just mask the side of the fin and the side of the tube so that I don't smear more epoxy on those areas when I smooth it with my finger. I dip my (nitrile-gloved) fingertip in a small bowl of 91% isopropyl alcohol immediately before doing the smoothing procedure, so perhaps (just a guess) the action of the quick-drying alcohol creates surface tension that helps to keep it in place. I smooth down and taper the upper and lower ends with my finger at the same time.

I usually do two or three smoothing maneuvers in the first minute or two (the first one is never thorough enough), wiping off the excess with a paper towel. As I said, this seems to keep the epoxy in place. An initial leak or drip does happen once in awhile, and I just wipe those off with a paper towel that is damp with isopropyl. The tape comes off right away as soon as I am done with the smoothing. As you might expect, I create just one fillet at a time and wait until it is pretty firm before starting on the next. No big deal; I just leave it and go do something else during each fillet's cure time. I can get all of them done over the course of an evening. Recently I have been holding a high intensity shop light over the fillet for several minutes to speed it up. This also seems to cause the epoxy to ultimately cure harder than if I let it cure at ambient temperature.

I like to use the slow-curing epoxy because of its great self-leveling ability, which is enhanced by the comparatively thin viscosity and leisurely set-up time. I can get glass-smooth fillets with it, with no subsequent sanding needed.

RocketManDan said:

What is the best procedure to use when masking that area so the epoxy doesn't flow off and beyond the tape and you get a nice "point/fillet" in front of the leading edge. It seems if you lay the rocket down on it's side how most of us do it, it would be hard to fill in the that area without it going everywhere...the you would have to come back after the epoxy sets and do the other side and try to fill in the gaps.

Hope I'm explaining myself ...

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First mask off the area(s), including in front of and behind the fin, then make a dam out of modeling clay or wax. Set the rocket so that the V between body tube and fin is level, then you can mix up and dab in epoxy of just about any curing time. The thinner and slower the mix, the more it will self-level. As epoxy sets up, pull off masking. Gotta do this proceedure 2x per fin...