Microballoons vs Rocketpoxy for fin fillets?

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JoePfeiffer

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I've molded epoxy fin fillets before using a steel rod wrapped in wax paper; it worked well, but left issues trying to smooth the fillet at the leading and trailing edges. Wanting to improve my results on my current build, I'm using Rocketpoxy and shaping them with fondant balls.

I'm finding measuring the Rocketpoxy difficult; it's thick enough that trying to get a fine weight adjustment is very difficult. I've never tried thickening epoxy with microballoons, but it seems like this would let me measure the epoxy accurately, then add balloons after mixing.

For those who've used both, how do they compare for ease of use?
 

dr wogz

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Never used Rocket-poxy..

But I have used various filler in various epoxies.

My go-to now is West system & their faring filler 407. The West system has a bout a 3 hr working time, so there is no rush! It seems to lay nicely, smooths out, and sands easily. Other will say to use colloidal silica, but I find that tends to be like trying to sand a rock!

I also now tend to use a plastic card; a hotel room key-card. Epoxy don't stick to it, they re free, and you can cut different radii in them. I also find they scrape epoxy away on a fin, instead of getting another 'build up'

Mix up teh epoxy, then add a few scoops of teh filler until it's fairly thick: peanut butter or cake icing... Then dab on. do take care to ensure you put it where you want it to go! (I see some people use a table spoon to slather it on, then spend hours sanding half of it away!) I then draw my key-card over the fillet, first at a shallow angle to work it in, then gradually bring teh key-card to near-vertical to get consistent fillets. And wipe the card after each pass! I find any bit will go here you don't want! and teh little 'seam' that appears can be nudged into or off with the clean edge of teh key-card.

Pull the key-card along the fillet, and draw it off, a the same angle off the tip of the fin. This tends to round it off nicely, but it does take a bit of practice, and requires a fare bit of clean up (again, always have a piece of paper town with you!)

Once cured, I then will sand with a folded over piece of used sand paper. it's also rolled to match the fillet. A few light strokes to blend it all in.

And, a bit of Bondo at the end, to fill in the few holes & divets. This is the most tedious part; filling in / faring in to get a nice tube to fin tip transition. At this point, it's a roiled up piece of sand paper, to about a 1/4" diameter, and a bit of a rolling action when sanding it..

go to post #22, to see see what I'm talking about (and to repeat what I just wrote!)

 

JoePfeiffer

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My go-to now is West system & their faring filler 407. The West system has a bout a 3 hr working time, so there is no rush! It seems to lay nicely, smooths out, and sands easily. Other will say to use colloidal silica, but I find that tends to be like trying to sand a rock!
Thanks, that sounds like what I'll be trying in the future....

Fondant balls are fantastic for shaping fillets -- I got a set with eight different radii from Amazon for $6.59. The cool thing about them is since they're balls, there's no issue with maintaining a consistent angle. The epoxy does stick to them, but just clean with alcohol after use.
 

boatgeek

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My only hesitation with microballoons is that they're not very strong. My go-to is to do two steps--first with wood flour (or the structural filler of your choice) for strength, then a thin skim of microballoons over that for ease of sanding. The structural layer doesn't have to be very smooth since it's getting covered up. I usually take a coarse rat-tail file to it to clear off any big bumps and then lay down the final coat.
 

gtg738w

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I use microballoons a lot for fillets and it is great for most sport applications. It is not a structural filler though. There are other options like silica that are better for areas where you need the strength, but heavier and harder to sand.

 

Mike Haberer

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Rocketpoxy is the bees-knees for external fillets, with fondant balls for shaping. It has fillers built into it, so need for adding and mixing. Long working time. You can continue to work it as it sets for quite a while if you need to make minor adjustments, which I usually do with a finger dipped in isopropyl alcohol.
 

aviserated

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Why not use JBWeld? It's cheap, has long shelf life, strong and lightweight if you use less.
 

Mike Haberer

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If you want good looking fillets you don't want to us JB Weld. It isn't easy to use when forming fillets and heaven forbid if you need to sand them afterwards. It has metal in it after all, not something that sandpaper can deal with.

My post above states the case, long pot life, easy to shape while curing. You also don't want to use less of JB Weld just because it is strong. Rule of thumb is a fillet's radius need to be 4-8% of the root chord. Too small and you won't get it far enough onto the fin and airframe to form a bond that won't break. You also want a smooth radius (aka, easy to work) not for looks but for strength. I took materials science classes for my engineering degree, sharp edges do not bear stresses well, rounded edges can deal with flexure.

I've attached my collection of Rocketpoxy testimonials pulled from TRF.
 

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Steve Shannon

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I use silica. One thing that I do differently than some here is that I measure out the epoxy and hardener, but I don’t mix them. Then I add silica to each, a little at a time until each is at the viscosity I want. Then I mix them together. That way I’m not working against the working time of the epoxy while I’m trying to get the viscosity right.
 

Brian Ditmer

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I've molded epoxy fin fillets before using a steel rod wrapped in wax paper; it worked well, but left issues trying to smooth the fillet at the leading and trailing edges. Wanting to improve my results on my current build, I'm using Rocketpoxy and shaping them with fondant balls.

I'm finding measuring the Rocketpoxy difficult; it's thick enough that trying to get a fine weight adjustment is very difficult. I've never tried thickening epoxy with microballoons, but it seems like this would let me measure the epoxy accurately, then add balloons after mixing.

For those who've used both, how do they compare for ease of use?
take at look at the rocketpoxy fillet video at https://www.badassrocketry.com/badassvideos
 

Mike Haberer

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I've molded epoxy fin fillets before using a steel rod wrapped in wax paper; it worked well, but left issues trying to smooth the fillet at the leading and trailing edges. Wanting to improve my results on my current build, I'm using Rocketpoxy and shaping them with fondant balls.

I'm finding measuring the Rocketpoxy difficult; it's thick enough that trying to get a fine weight adjustment is very difficult. I've never tried thickening epoxy with microballoons, but it seems like this would let me measure the epoxy accurately, then add balloons after mixing.

For those who've used both, how do they compare for ease of use?
I bought a small scale that measures down to .1 gram and small paper cups. When I turn on the scale with the cup on it, it zeros out the cup weight. I add the epoxy first, because it is thick, using a popsicle stick. I then use a second popsicle stick (or the opposite end) to add the hardener, which flows. You dribble the hardener in until you get a 1:1 weight mix. If you go over a bit you can extract a bit of hardener easily. I get a perfect mix every time.
 

OverTheTop

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JB Weld is generally too heavy for such use.

I use West Systems 105/206 with Qcell microballoons for fillets. Nice to work with and sands easily. If you listen when you are sanding you can tell when you are not on the fillets ;) .
 

CalebJ

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I'm really looking forward to experimenting with the various epoxy flavors that are more tailored to rocketry. But I've got a fair bit of JB Weld to burn through first. One of these days...
 

blackjack2564

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AS per instructions..."NEVER add anything" to Rocketpoxy. It will weaken or degrade it. Comes with all additives in proper amounts already in it.

It goes through 3 stages during cure.
1.pretty gooey ,can be used as normal adhesive....first few minutes.
2. let sit as described below for fillets.
3. use like epoxy clay after it gets stiff.

Since it's 1 to 1 mix...just lay out 2 equal lines on section of card board... mix like you would Bondo....spead out and let sit for several minutes so air bubbles can rise to surface.
For fillets:
Depending on ambient temp...let sit for 15 min [80 or above] to an hour [ 65 ish]
Dip mix stick in, and pull up quickly making a pointy mound....when that stays put, no longer settles back into pile. You are ready to make perfect fillets that stay put!.

At this stage I can do all 6 on 3 fin rocket and they stay put...no sagging.

If you are having shaping issues at ends, let cure and use round files to get taper...easy...peasy, Well iff you know how to use files to shape...lol

I did beta testing with it...search my threads for complete "how to" with the stuff and info from John who makes it. Biggest issue with RP is folks trying to add fillers to it, which is big NO...NO.
 
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OZRoc

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Never used Rocketpoxy but have used West Systems and Q-cells as OTT above. These days for large fillets I go for a high strength epoxy glue (West Systems or Techniblue). When mixed they form a gel that is easy to apply and form due to a long drying time. And as Jim extols you can do all fillets at one sitting.
Cheers
 

REK

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To those of you who have not tried rocketpoxy, “TRY IT” you can purchase these from Apogee Rockets and a few other vendors. It is great stuff and if you follow the instructions that blackjack2564 mentioned above, you will have great structual looking fillets.
 

Pepe Le Pew

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From my homebuilding days I use cotton flock with the epoxy. it is an aircraft grade, approved structural adhesive/filler. Obviously I use this on larger rockets 2" and bigger for the smaller stuff any of the fillers mentioned above work fine.
 

Mike Haberer

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Rocketpoxy and ONLY Rocketpoxy for external fillets; it's stupid simple. Aeropoxy or whatever your favorite structural epoxy is for anything internal to the airframe.
 

JoePfeiffer

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From my homebuilding days I use cotton flock with the epoxy. it is an aircraft grade, approved structural adhesive/filler. Obviously I use this on larger rockets 2" and bigger for the smaller stuff any of the fillers mentioned above work fine.
First I've ever heard of it! How does its behavior differ from microballoons?
 

G_T

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Microballoons are used to lighten epoxy. There are multiple kinds with different strengths - glass, phenolic, and believe it or not, metal - not quite so micro - for when you need stronger. I've not used the metal ones and rarely come across them. I'm not sure they save appreciable weight anyway, but they are likely stronger than the bulk epoxy would be. But having never used it and not researched it, I'm speculating.

Fumed silica is used to make epoxy thixotropic (keep it from flowing).

Cotton Flock is a fiber reinforcement to make the epoxy less prone to cracking and to slow crack propagation. It also thickens the epoxy. I've used it a lot. Beware the dust though - ref Brown Lung. Don't breathe the dust!

Cotton Flocking plus Fumed Silica is a blend I used a lot.

If I needed structural, MT-13 was on the list to consider.

Gerald
 
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