Epoxy Technique

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Jul 16, 2004
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When I look at "in progress" pictures of other people's rickets, I see beautiful, even, smooth lines of epoxy. My epoxy-ing tends to look more like my 5-year-old got into the honey.

Does anyone have some general tips for aplication of epoxy?

Are you talking about fillets? I usually just get it real warm (ie, thin) and pour the fillets, and let them sit. If your using west system epoxy, you could almost thicken it to the perfect consistency.

But yeah, I think thats the trick. Don't shape your epoxy, let it do it itself.
If you're dealing with epoxy thin enough to flow, then just keep the joint horizontal until it sets. Many common epoxies, however, are a bit too thick for that trick, especially the stuff sold in hardware stores in the little tubes. There, dip a latex-gloved finger into rubbing alcohol and drag it along the fillet to smooth it out. Using masking tape to keep it off unwanted areas is also helpful.
I use silica filler in epoxy to do my fillets. The consistency should be like peanut butter.
Patience, Patience, Patience....

I second the latex glove usage. I use them anytime I'm working with epoxy. I also try to let gravity do the work filleting only a couple at a time. Make sure you also have paper towels and rubbing alcohol around.

I use milled fiber, do you mind describing what your substance looks like?

I use colloidal silica too. I think it's 407 from west system. It's VERY powdery, almost fluffy. It weigs next to nothing. It's like baking soda puffed up and powdered more. Sorry for the odd description, but it's all I got. :D
I bet I have the same stuff then just different name. I would probably describe the stuff I got the same way.
I purchased mine from Binder Design and it's sold as the name I mentioned. I'd like to add I just build one of there kits and it kick some serious ___!
In the directions it called for it so I got me some.
Thanks for replying so fast.

latex will work for minimal exposure but for larger jobs many people suggest Nitrile gloves .. epoxy will penetrate latex
I had chores so I couldn't reply immediately. The poder is like fine white chalk powder. And yes, latex gloves are horrible for epoxy. Once the gloves get any on them the inside of the gloves get sticky and need to be replaced often
Vinyl gloves are supposed to be better for epoxy, less stuff gets through.

I take 15 min. epoxy and thin it with rubbing alcohol until it pours fairly easy, and mask the BT and fins with blue tape. Slowly pour it out of a paper cup along the root, and let it cure for an hour or so (pull the tape off first so it doesn't become a permanent feature). Keep the rocket horizontal with the fin turned off to one side so the fillet sits in a "valley". So far I've gotten very smooth, strong fillets.
The gravity method works fine. As far as the finger-in-glove technique, I have had any luck other than ruining nice fillets. West should be mixed undiluted and the ends of the fins dammed via copious amounts of masking tape.

The way I've been doing them lately is 3:2 404 filler:mixed resin (105/205). Ultra gap filling, ultra sandability, ultra shapeability (I've been going contour crazy as of late). I apply it with a craft stick or tongue depressor (depending on size of rocket/fin/fillet).

Sands real easy for me, too, by using the front edge of my DeWalt sander and contours via my Porter-Cable Detail Sander.

Don't recall if you can see the last depicted rocket I did but here's a link anyhow:
https://www.metrarocketclub.org/RIP/Forsaken/forsaken012.jpg . I've been digging that contoured look since. Got a lot of compliments on this rocket when I debuted it. Nice feeling for a guy with 21 thumbs :eek:

Here's another link:

My dad's shi^^y CMOS camera was all I had around but you get the idea...

Mind you, I'm not the handiest craftsman by any stretch, so I do a lot of meticulous sanding and filling. It was worth it. I was real happy with how it came out. Too bad I buried it at 600MPH. Fillets held up, though...:D

Thanks for all the great tips! I'll try some of this out and post pictures of the messes I make in the process... :)

This is somthing that I've spent alot of time with. I never liked how they look before. Fin straight into a body tube. Or a bunch
of epoxy. The thought to my self was. Why add so much. The tubes are only so strong even with glass. And the same goes for the plywood fins. Or even G-10 for that matter. After so much the fin joint becomes stronger than what its holding together. For the best looking fin filltes. I found that if you just run a small epoxy joint. Then I use whats called fiber glassing jelly. Made by bond.
I get it at wall mart now. And the spreaders used for it. Then you can cut them into the shape that you want. When mixing this up I found that if you get it to a brown color. This will give you the time to work with it. If it's a red. Then you've added to much cure. If it's a green. Then you don't have enough. I'll just mix enough up at one time to do one fin. You use this spreader. As you would use your finger tip. Like alot of people explained already. Just start at the top and work your way down. Let it set up for about 5min. And cut the extra off with a knife. If you wait to long, You'll
have learned the hard way. Lots of sanding to be done. It takes alittle time. But this is what works best for me. And I think it's strong enough. I've seen where the bondo has pulled fins apart. And fiber glass off of body tubes. Here's a few pictures just to let you see what can be done with fiber glassing jelly. The frist is a 3" booster Thats been glassed after the bondo was done. Sure makes for a nice fin joint.
Those fillets are just great when you're not glassing. You see, composites don't like being shaped around hard curves. They tend to break easier. Big fin fillets are used to act like cross supports at joins. Triangles are stronger than squares Just like a trellis. I have fillets on one rocket that are huge to maximize this fact and minimize the curve for the fibreglass. A rocket going mach needs all the help it can get!:D
GL-P, clarify a few things for me, will you? I don't understand:

You: You see, composites don't like being shaped around hard curves. They tend to break easier.
Me: 1st sentence: why? 2nd sentence: Based onnnn...?

You: Big fin fillets are used to act like cross supports at joins.
Me: Fin fillets serve an aerodynamic purpose. Big fillets don't seem to do this; they just seem to increase surface area. Also, if they're "too big", you wind up with two more anomalies:

-the joint becomes stronger than the individual attached parts.
-the flightward-facing fillet area needs to be tapered and contoured in relation to the height of the fillet. Hence, more "necessary" surface area (for the proper aerodynamic component) and "unnecessary" weight (weight is not our friend).

You: A rocket going mach needs all the help it can get!
Me: A rocket needs little help with "going mach" other than a zero AOA (assuming proper construction and pursuant building technique(s)). Period.

After about M 0.8 and with increasing velocity, EVERY structural component - airframe, nose cone, fins, airframe joints, fillets - starts to become air-dammed or dirty air generators. Hence, one needs to minimize surface area.

Sorry it took awhile. I had trouble getting to the site.

1. What I said was what I heard was true for carbon cloth. little to no filleting at sharp curves prevent epoxy from throughly getting into the cracks.

2. Fillets also prevent side to side oscillation. As long as the tube doesn't warp, then fillets can keep fins that are reasonably sized straight and reduce side to side flexing from badly aligned fins or a turn in the flight path for whatever reason.

3. Fins produce as you say "dirty air generators" no matter what anyone can do. All rockets vibrate at transonic speeds. It's a known fact. Larger fillets which are though a little bigger in surface area and heavier spread the load better and therefore prevent side to side oscillations which can really kill minimum diameter rockets which aren't built in a perfect world, by perfect rocketeers and are flown in perfect conditions.
1. Agree

2. Disagree. Fillets will do little other than PIVOT the root chord (aside from giving an aerodynamic advantage). If a fillet - regardless of size - resists stresses, there's no guarantee a fin won't split OUTSIDE a fillet. Seen it more than once *(see below).

3. "Fins produce as you say "dirty air generators" no matter what anyone can do." Disagree. A properly airfoiled fin should split air and return it to its approximate state. NOTE: I said "approximate".

"All rockets vibrate at transonic speeds. It's a known fact." I've never heard that stated before.

"Larger fillets which are though a little bigger in surface area and heavier spread the load better and therefore prevent side to side oscillations"

Fillets merely increase the area exposed to whatever stress. Since they're rigid, it will TRANSFER the load.

Fillets won't prevent oscillations; properly-sized and -affixed fins prevent oscillations.

"...side to side oscillations which can really kill minimum diameter rockets"

Oscillations can kill any rocket.

*One of our club members flew his stock EZI-65 on an AT K550. About 2 seconds into the burn, all three fins came off the rocket. On tape, the first fin that came out had sheared clean away from the MMT (bad epoxy joint). The second and third fins were torn almost parallel to the fin fillet, about 10-20mm's outside the external joints. The fillets were entirely intact. The second and third fin took a hard stress load - far away from a 0° AOA. Their fillets were stronger than the materials they were in contact with. Also, not the purpose of a proper fillet.

Not giving you a hard time, just agreeing to disagree.
At least we've gotten one thing cleared up.:D

2. A properly scuffed fin should hold glue better. Epoxy will outlast a fin but scuffing fins helps a lot on the fin and on the tube.

3.The problem is that it's very hard to create perfect airfoils in such a way as you describe. I would kill to be perfect at filleting! Also fins are almost never perfectly aligned.

I agree in a sense. They will transfer a load but a proper airframe that is stiff should not flex of crumple. A reasonably sized fillet will be better than a small fillet which doesn't spread the load as much.

I have a program that shows the Cd compared to velocity. It's called Aerolab. In all rockets there is a highly defined jump in drag at transonic speeds. This is when the rocket passes through the sound barrier. I've never taken official classes on this but it's obvious in the program and some other groups on supersonic drag that vibrations from breaking the sound barrier cause this. I asked a professor in aerospace about this. I've heard that you know you're going mach after the vibrations stop. Heard this from a personal account from Chuck Yeager.

I've heard that EZI-65 really can't more than an I. I've heard on R.M.R that they can and will self destruct on a J.

I once went overboard on a 24mm rocket. I used epoxy for big fin fillets. The rocket popped a streamer at apogee but did a reverse glide which is when the fins lead in a backwards glide. The rocket landed but ended up stripping a fin off cleanly. Fillet and fin came off in one piece. The cause: the fins were big for a rocket this size and the tube was an estes tube which has a glassin layer. Big mistake!

Well, this is what forums are for!