JB. Weld vs Regular epoxy vs 2-part for fillets

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Mlee

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What is the conventional rocketry wisdom for or against JB Weld for fin fillets? Also, is the Apogee's "Epoxy clay" any different than the 2-part epoxy putty you can buy at Lowes/Osh/Depot, etc?
 
JB Weld is relatively expensive per volume even compared to the aircraft grade epoxies like Aeropoxy and others that are available from rocketry vendors. I'd argue that it would only be beneficial for bonding motor tubes for heat resistance, but personally I just use regular epoxy and have not found a reason to do otherwise. My favorite method is to get West Systems colloidal silica and thicken epoxy so that it has the consistency of peanut butter for fillets, instead of using epoxy clay.
 
I use JB Weld sometimes for Fillets, but mostly because it is so easy to sand. I don't have a problem with how much it costs because I buy it from a Store I used to work at and get a discount.
 
JB weld is overkill for rockets. Even for mach busting HPR there are better choices.

For most of my LPR in need of epoxy fillets, I add micro-balloons to aid in running and later sanding. For MPR cardboard tube/wooden fins I use epoxy with wood flour (looks and spreads lime peanut butter). Fiberglass/Quantum tubing with G10 fins, epoxy with milled glass.

Jerome :)
 
I use Aeropoxy Light. It is a 2 part epoxy that is very easy to sand. I would not use Aeropoxy Light for surface mount fins it is great for TTW fins. I use JB for surface mount fins.
 
I used JB Weld for fillets on a FSI Black Brant. The were still intact after the FSI F100 exploded....
 
MarineTex. It is smoother than these others. "Flows" better when doweling the fillet, and stops moving just right. Comes in a rainbow of colors from white to black and nothing in between.
 
JB weld is a structural epoxy. It is designed for high stress applications. Used as fillet is simply a waste of money as well as adding unnecessary mass due to its density. There are plenty of other two part epoxies much better suited for fillet work. Aeropoxy light, Devcon in several strengths and so on.
 
I'm starting to discover that JB Weld is actually a "normal" epoxy that has a denser hardener and uses iron powder in its formulation. The prime benefit is it's thermal resistance (to about 500 degrees F). It was designed for repairs on engines and engine blocks. And you guys are right about the comparative cost. It's not cheap, though those little $6.99 tubes fool you into thinking it's cheap.

Here is what I find funny about all this epoxy discussion: have ANY of you had a MMT or fin fail due to the epoxy not holding? Because I've been using off-the-shelf epoxy from Lowes and Home Depot on Mid-power rockets (up to G75s) and I've never had a failure like that. It is my opinion - based on those experiences- that the cardboard or balsa or fiber glass would fail long before even the cheap epoxy. Now, in the L3 world, well maybe. But I wonder if we're all over-buying complex epoxy formulations when even the standard stuff would be fine?

we'd have to do an experiment where we build 2 of the exact same rockets; one with cheap epoxy and one with one of the premium aerospace epoxies. Then launch them both on the same engines, increasing in power until one fails structurally. Then note if that rocket had "inferior" epoxy.
 
On one of my current builds, Everything is done with JB Weld, but there are no Fillets needed, because the Fins are Gusseted. I'll harden the Gussets with Super Thin CA when they are all done and cured. The Fins go TTW to the MMT, and there is JB Weld there, then the body tube to fin is JB weld, and finally the Gusset Details are JB Welded with as little as possible, but pressed into place and the excess wiped away.

Better than Fillets 001.jpg
 
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I'm starting to discover that JB Weld is actually a "normal" epoxy that has a denser hardener and uses iron powder in its formulation. The prime benefit is it's thermal resistance (to about 500 degrees F). It was designed for repairs on engines and engine blocks. And you guys are right about the comparative cost. It's not cheap, though those little $6.99 tubes fool you into thinking it's cheap.

Here is what I find funny about all this epoxy discussion: have ANY of you had a MMT or fin fail due to the epoxy not holding? Because I've been using off-the-shelf epoxy from Lowes and Home Depot on Mid-power rockets (up to G75s) and I've never had a failure like that. It is my opinion - based on those experiences- that the cardboard or balsa or fiber glass would fail long before even the cheap epoxy. Now, in the L3 world, well maybe. But I wonder if we're all over-buying complex epoxy formulations when even the standard stuff would be fine?

we'd have to do an experiment where we build 2 of the exact same rockets; one with cheap epoxy and one with one of the premium aerospace epoxies. Then launch them both on the same engines, increasing in power until one fails structurally. Then note if that rocket had "inferior" epoxy.


That and all the advice over the years on not to use 5 minute epoxy because it gets brittle and breaks. I have never seen that happen, maybe someone else has. My 22 year old Conformation rocket is held together with 5 minute and I'm using it this spring to get my 1st level again on a I357 eventually getting 2nd with a J350. I bet it holds together ;)
 
I buy the 10oz professional JB weld for $12 with prime shipping on Amazon. I only use JB weld on motor retainers and motor tubes of fiberglass kits. fillets for fiberglass I use rocketpoxy lately. for cardboard and wood rockets I use 15 or 30 minute epoxy. fillets on those get 30 minute bob smith thickened with milled fiberglass or colloidal silica. I would agree that regular 30 minute epoxy is plenty strong for most builds. We really overbuild, especially with fiberglass.
 
There is no reason to use JB Weld for anything on any rocket that does not involve heat transfer. In fact in many structural applications, JB Weld is actually sub par.
 
Jerome, where in So. Cal are you? I'm in South Orange County.
 
I've been using devcon 5minuite and 2-ton (30min) epoxy for decades. Have Never had a single failure on any model fin or clustered motor mount in all that time. I have some models that used Devcon 5 minute that are pushing 30years old with 30-50fights. the epoxy has yellowed a bit but is still flexable and holding fast.
 
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