Fix-It Clay Epoxy

Just got some in the mail.

I am about to put the fins on an Estes Big Daddy. My plan was to use a bead of wood glue to attach the fin to the motor mount. I was going to put a couple of small fillets along the body tube and fin as well, in the normal fasion. This will be my first time to use Fix-It Clay. It says it bonds to all kinds of surfaces. I am using it because it cures "rock-hard" and because it supposedly can be shaped to give you beautiful fillets.

Question: Seems like it bonds to a lot of stuff. My fear is that it won't bond to the wood glue. Should I be concerned? I always tend to over-do stuff. I thought it would be best to have a wood glue fillet underneath the fix-it fillet for added strength. Is this necessary or is the fix-it clay strong enough on its own? If the fix-it clay will in fact bond with the wood glue, maybe I can go ahead as planned.

Bottom line: I always get a little worried about mixing substances. I just want to make sure I don't mess this rocket up after all the hard work I've put into it at this point.

Thanks!

I wouldn't put the Fix-It epoxy clay over the wood glue. It won't add any strength that way since it won't be bonding anything to anything except wood glue to wood glue. If the wood glue fails, the epoxy clay will fall off. If it doesn't, the epoxy clay didn't add anything except weight, and that at the wrong end. If you want to go over wood glue for a nice finish, I'd just use Elmers Fill-N-Finish over it to make a nice fillet shape and size, let it dry thoroughly, rough sand it into the shape you want, drizzle a few drops of water thin cyanoacrylate adhesive (super glue) over it to harden and strengthen it some, and finish sand it. You'll get a pretty fillet that won't add a lot of weight, and it'll be just as strong as the epoxy clay over wood glue. Stronger in fact, as it won't have that heavy epoxy clay tugging on the wood glue.

If you want to use the epoxy clay and are OK with the weight, rough things up and (after using wood glue to make the butt joint) press/rub the clay into the coarse surface before forming your fillet shape, and apply it directly to the fins and tube with no wood glue underneath.

Thanks for the reply. Actually, I wasn't going to make large glue fillets, just small ones to help tack the fin. My main concern was, not knowing anything about this product, how well it actually works as a substitute for wood glue. Allegedly, it works better.

I've heard both ways on that one, and it depends in part on what you're going for.

The Handbook of Model Rocketry states that the optimal fillet, aerodynamically, has a radius of curvature of between 4% and 8% of the fin root chord length. I usually split the difference and shoot for about 6%. That's a nice look too. But for large fillets that size gets heavy if done with epoxy, and more so with the clay epoxy.

How well the epoxy clay adheres depends a lot on how it's applied and how the surfaces are prepared. The clay is thick, of course, so it's not going to really soak in like a liquid could. Rough up the fin and body tube surfaces, and press or rub the clay in well before smoothing and shaping its surface.

Wood glue is surprisingly strong when applied correctly. (Search on "double glue joint".) Done right, it's stronger than the wood and paper you're fastening, and it's light. What wood glue is bad at is building up a decent fillet. It tends to dry to very little thickness. For small fillets, Aileene's Tacky Glue works well and builds thickness faster. For big fillets, use Fill-N-Finish, Hobbylite, or something similar. For larger rockets, epoxy filled with microballoons sands well and is light (for epoxy anyway). Just be sure to do a stronger fillet underneath, of epoxy with a strengthening filler in it, like milled glass. Leave it rough, or apply the smooth sandable covering layer before the strong small underlying one fully cures. This helps the layers hold onto each other.

Originally posted by Mad Rocketeer
What wood glue is bad at is building up a decent fillet. It tends to dry to very little thickness. For small fillets, Aileene's Tacky Glue works well and builds thickness faster.

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I want to dispell the myth... Wood can build decent fillets, there's just a technique unlike any other adhesive to do it!

The trick is to put it on in several thin layers. All you need is wood glue and a damp paper towel (you don't want to use cloth because you're going to be wiping glue all over it!)

First you put down a bead of wood glue along the fin/body tube joint. Then gently run the tip of your finger along the joint to smooth the glue into a nice smooth arc between the fin & body tube. Wipe the excess glue off your finger with the damp paper towel. Smooth again if necessary. Set aside to dry. Repeat this process 3-6 times until you are happy with the fillet's overall shape. (And I usually do this on a pair of adjacent fin joints.)

The very first layer will take the longest to dry because that is when you are laying down the most glue. The successive layers dry much faster--often in only 2-3 minutes--which allow me to work on the next pair of fillets as soon as I finish. But even the first layer doesn't take more than 10-15 minutes to fully dry. Like any other technique, it just takes practice

And I should mention that the fillets won't be as large as epoxy fillets but less glue = less weight = a more stable rocket.

To this point, I have always built my fillets with wood glue only, following the exact approach above. But, what I don't like about this approach is the fact that wood glue shrinks as it dries. Sometimes you get tiny little holes and imperfections that you can't anticipate when it is wet.

I've got a big tub of fill and finish at home. I have been using it recently with pretty good success... but it also shrinks when it dries. This means the results are not always totally predictable.

What I like about the Fix-It Clay Epoxy is that it while it cures, it supposedly will not shrink. So, the look you get while working with the clay is ultimately the same look you get when it dries.

Anyway, this is my first time to use any kind of Epoxy. I'm trying to learn all the various techniques so I can eventually move into HPR and earn my certifications. Right now, the thought of fiberglassing anything scares the heck out of me. So, I'm taking it one little techinique at a time. This project is for me to learn how to use fix-it clay on my Big Daddy. Anyway, I figured this would be a good time to use it since I plan to fly this rocket on E engines from time to time.

Epoxy isn't needed for E power, though learning on the Big Daddy for later use with L1 isn't a bad idea.

Yep. Wood glue does get its little pockmarks as it dries, but successive layering can handle most of that. Thinner layers pockmark less, so eventually you'll get it filled in and smooth. Keyword is "eventually".

Most of my rockets are all wood glue fillets, and it's true that it will do the trick, but wood glue dries pretty flat per layer, so it takes a lot of layers to get it the way I like 'em. Just a couple of layers is plenty strong, but I want to actually see some curvature in a fillet, and that's slow with wood glue.

For small rockets, I've found that Aileene's Tacky Glue is nice. I do a layer or two of wood glue for the strength (though Aileene's is strong enough for most purposes), then I use the Aileene's to get the thickness I want. Just a quick layer or two and I'm done. It doesn't seem to pockmark nearly as much either.

Fill-N-Finish is for the bigger stuff. Big Daddy could use it. Epoxy clay is good too, but I wouldn't apply it over a wood glue fillet. Rough the fin up some, right down by the root where the fillet will cover it, and similarly for the tube area that will be covered. Then rub the clay in and shape it. Should make a pretty fillet that will hold fine.

As you build more rockets, try new things each time. Then use the techniques and materials that appeal to you.

Just watch the weight, and keep it stable.

Thanks! Actually, stability is a concern. If you've noticed, I have another discussion thread going right now on nose weight. The reason is that I've already added some kevlar around the motor mount tube. It is tied off and secured by wood glue. I may have added an ounce or so in weight by doing this. I also plan to use a nylon parachute over the plastic version. There is some added weight here. I also plan to use a swivel for the first time. My thought is, surely I'll need some nose weight. The thing is... I'm just not sure how much.

Originally posted by Mad Rocketeer
Yep. Wood glue does get its little pockmarks as it dries, but successive layering can handle most of that. Thinner layers pockmark less, so eventually you'll get it filled in and smooth. Keyword is "eventually".

Most of my rockets are all wood glue fillets, and it's true that it will do the trick, but wood glue dries pretty flat per layer, so it takes a lot of layers to get it the way I like 'em. Just a couple of layers is plenty strong, but I want to actually see some curvature in a fillet, and that's slow with wood glue.

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DING DING DING DING!!! WE HAVE A WINNER! That is exactly what I meant...

Originally posted by Mad Rocketeer
For small rockets, I've found that Aileene's Tacky Glue is nice.

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IMHO, you can't get any better than Aileene's TG for building LPRs.

I may try Aileene's next time... but this time I'm gonna play with putty!

Hi Cougar93,
I finished building a Big Daddy a few weeks ago and used epoxy mixed with colloidal silica for the fin fillets (the smoothest fillets I've made). In total, the fillets weighed 8 grams (I weighed it before and after applying them). I've also upgraded to a nylon 'chute and improved on the shock cord design. I think it's very unlikely (although I can't be certain) that you will have added an ounce by using a kevlar cord attached with wood glue. I'm going to put about an ounce of nose weight in to counteract the added tail weight of epoxy over wood glue (and also the 24mm RMS motor I'm hoping to fly it on one day!). Rocksim says this should put the CG where it would be for a stock BD on an Estes motor. Anyway, best of luck with the rest of the build.
Edward

Thanks Edward. Actually, I think the one, single heaviest item I'm putting on this baby is the swivel. Going to the fishing supply at Academy this weekend to see if I can find a good, small, ball-bearing swivel.

Originally posted by EdwardH
Hi Cougar93,
I finished building a Big Daddy a few weeks ago and used epoxy mixed with colloidal silica for the fin fillets (the smoothest fillets I've made). In total, the fillets weighed 8 grams (I weighed it before and after applying them). I've also upgraded to a nylon 'chute and improved on the shock cord design. I think it's very unlikely (although I can't be certain) that you will have added an ounce by using a kevlar cord attached with wood glue. I'm going to put about an ounce of nose weight in to counteract the added tail weight of epoxy over wood glue (and also the 24mm RMS motor I'm hoping to fly it on one day!). Rocksim says this should put the CG where it would be for a stock BD on an Estes motor. Anyway, best of luck with the rest of the build.
Edward

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You don't want to put epoxy over wood glue. The wood glue fills the pores in the wood, so the epoxy can make only a surface adhesion wherever the wood glue is. Further, the epoxy is holding onto the wood glue there, not the wood, so all that has to happen to peel it off is for the wood glue to tear there. Make a wood glue fillet, let it cure, then dig in with a fingernail to see how strong or weak it is. A wood glue joint is strong (stronger than the wood, generally), but layered on wood glue, as in a fillet, isn't all that strong by itself. It would be better to do the fin's butt joint in wood glue or unfilled epoxy, then apply unfilled epoxy in a thin film to the fillet area, let it cure until tacky so it has time to soak in some and is thin enough to do so, then place the filled epoxy fillet over that. Be sure to rough up the fillet area before applying anything. This will give you a stronger bond at about the same weight.

Hi Mad Rocketeer,
you are quite correct - I hadn't made it clear but I used unfilled epoxy for the initial joint and a small fillet and applied the final fillet (with the epoxy/silica mix) before that had fully cured. Also very good advice about roughing up the areas to be glued - I don't think Estes mentioned it in the instructions. Cheers,
Edward