Fillets: Fix-it clay epoxy vs. 5 min (or other) "liquid" epoxy

Has anyone ever experimented as far as which type of epoxy (clay vs. "liquid") provides a *stronger* fillet? If I use the word "better" as I proceed with this thread, I mean "stronger". I know there are inherent differences on the ability to finish the two, I'm more concerned with strength.

Though I don't know whether just straight liquid epoxy is stronger than the clay, I would be willing to bet that if you put some strengtheners in the liquid stuff (milled fibers, silica, etc.) that the liquid stuff would then be stronger. I don't know how well you would be able to mix strengtheners into the clay, though, so for truly strong fillets, use liquid with some strengtheners.
Reed

i havent used fix-it clay epoxy, but ive used ACE's version of the same thing.
to give you an idea of how fast it dries, and how hard it gets (strong), listen to this.
fist off, i have used it on fin fillets, and it's very easy to get beautiful, smooth fillets, and id be willing to bet that if you roughed up the surface a bit with sandpaper before applying it, that you could rip the body apart before the clay would crack.
to test the strength, i kneated up a small batch of it (another plus, its very easy to mix) and pressed it over a 1/4" elastic shock cord scrap piece. Then i pressed that onto my workbench and let it dry for only 30 minutes. When i came back, it was hard as a rock. i tried to pull on the shock cord to see how well it bonds to rocketry related stuff. I pulled harder and harder, and harder, and harder, and then SNAP, the shock cord tore in front of where it was attached to with clay, it hit my hand and hurt, but it just goes to show how tough this stuff is. then i used a spackel knife to pry it off the workbench. oncei got it off, i tried to crack it with a hammer, and i COULDNT, the most i could damage it was poking tiny holes with a screwdriver. so that just about sums up clay epoxy.
as for liquid epoxy, well, its hard to get a *perfect* batch, and unless you do get a *perfect* batch, id be willing to bet on the clay epoxy all the way.
thats just my $0.02.

HMMMmmm...

I have a bit of experience using the Fix-It clay for fillets. ( )

There is no question (in my mind) regarding the finished strength of the clay itself.

I guess I'm more concerned with the bond between the epoxy (either clay or "liquid") and the surface underneath (the fin, the airframe tube, the bit of glue that squished out from the joint between the fin and the airframe tube, etc.)

Does sandpaper really cure any bonding issues?

I agree with reed. For the strongest fillets use liquid epoxy with milled fibers added for strength and body. The liquids will penetrate better for a stronger bond between the fin and body tube. And yes sanding the surface will rough up the surfaces and provide a better bond, especially on "glassed" or coated tubes. Glues dont like smooth "glassy" surfaces. The epoxy clays are strong but do not bond as well in this application.

I tried JB weld clay, didnt work to well, hard to smooth it out, I had to cover it with some thick epoxy to make it look "reasonable" but I think if I started off with thick or even runny epoxy with milled fiber I would have gotten a much better fillet. But if you know what your doing it may work for you.