epoxy compatability

I know that putting epoxy over another layer when it's gelled gives a stronger, chemical, bond than waiting till it's fully cured and sanding.
If I'm using two different epoxies, i.e.- laminating with US composite medium over parts assembled with Rocketpoxy, will there be a chemical incompatability?
Should I wait till the first is fully cured?
Thanks

It's hard to say without knowing the formulations. Most of the epoxy systems are just variations on basic formulations so two different systems may well be chemically compatible. (You can even see some typical molecules in the Wikipedia article on epoxy.)

Either with the same or a different epoxy system, if the base layer cures, make sure to wash of the amine blush with soap and water before sanding. (Sanding is important to make a rough surface for the next application to stick to.)

pythonrock said:

I know that putting epoxy over another layer when it's gelled gives a stronger, chemical, bond than waiting till it's fully cured and sanding.
If I'm using two different epoxies, i.e.- laminating with US composite medium over parts assembled with Rocketpoxy, will there be a chemical incompatability?
Should I wait till the first is fully cured?
Thanks

Click to expand...

Dan, I'm doubting if there would be any benefit, i.e., chemical compatibility, especially with RocketPoxy which is nearly a putty/filler type of formulation. Most of the app notes I have read regarding set vs cure time application apply to multiple lamination steps with the same epoxy. Personally, I would just allow the proper cure time and sand before application.

Im just guessing although I have never used it that Rocketpoxy is a fairly fast cure formula. The rule of thumb with epoxy is
the longer the cure the stronger the bond. I have used fast cure epoxies in small amounts for tacking fins followed by slow cure epoxies like Aeropoxy structural adhesive with great results.

glenbo said:

Im just guessing although I have never used it that Rocketpoxy is a fairly fast cure formula. The rule of thumb with epoxy is
the longer the cure the stronger the bond. I have used fast cure epoxies in small amounts for tacking fins followed by slow cure epoxies like Aeropoxy structural adhesive with great results.

Click to expand...

Nope... ya guessed a wee bit off glenbo...lol.
If you need to compare it to something....JB Weld would be close in pot life and cure time.

I 've been through several gallons of it , since it's release and was part of the Beta test group. It goes through 3 phases:

1. first mixed, is very "loose" and has pot life of 30-60 minutes based on ambient temp of 65-75 & great for gluing stuff together. Does not even get warm in mix container.

2.If using for fillets, spread on section of cardboard about 1/4in thick and let sit for 15-45 minutes, based on temp again. If your mixing at 70...45 min at 85 -90 only 10-15 minutes. By spreading out like this, all air bubbles will rise to surface and escape, making for flawless fillets. To test for readiness, pull some epoxy "up" with stick, making small peaks. If they stay put it's ready. If the "peaks" fall down and spread out, not ready yet.
Place in V-grooves and pull fillet smooth. You can do all 6 fillets [on 3 fin] at once this way, & they hold their shape.

3. after epoxy stiffens up to "Clay" consistency it still can be used for bonding things or filling gaps.


Takes 24 hrs for full cure and ready for sanding. [give or take, based on temp again]

As you can see, the pot life is very long, ready to handle is several hrs, full cure 24.
Definitely not a fast cure epoxy.

I have used RocketPoxy for attaching fins to MM tube, then after only 2-3 hrs so it "sets", injected West 105 right over the RocketPoxy, for internal fillets.
Did small external fillets with RP, once again after just setting several hours, went right back over them with West filled with colloidal silica [406 light weight filler].
Done this on several 6in rockets that fly on big L's & M's. The fins are still on...lol [even after one landed on the highway and broke a 3/16th fin, but the fillets survived just fine, no cracks.]



Edit: why did I do this? [use 2 epoxies] I have done many projects, with "just RP' but was in the middle of one and knew I was going to run out. So I decided to test a "theory" and also see how well West would color black, using the supplied black dye that comes with RP.
I liked the combo and continue to use it for various reason. By the way West does accept the black dye beautifully.

Thanks all,
I was concerned that one epoxy might actually inhibit the cure of the other, but that doesn't sound like it's too likely. The overlap would be small so I think I will do like Jim - let it set and go over it as needed.

If you are really concerned about a set structure being adhered to by another layer you could consider applying peel ply to the first structure

glenbo said:

Im just guessing although I have never used it that Rocketpoxy is a fairly fast cure formula. The rule of thumb with epoxy is
the longer the cure the stronger the bond. I have used fast cure epoxies in small amounts for tacking fins followed by slow cure epoxies like Aeropoxy structural adhesive with great results.

Click to expand...

Would you please send me an email at my sales address...
Or use the contact form on my site....
I don't know why,, but I can not send or receive PM's on TRF...

Thanks man.....

Teddy

pythonrock said:

I know that putting epoxy over another layer when it's gelled gives a stronger, chemical, bond than waiting till it's fully cured and sanding.
If I'm using two different epoxies, i.e.- laminating with US composite medium over parts assembled with Rocketpoxy, will there be a chemical incompatability?
Should I wait till the first is fully cured?
Thanks

Click to expand...

The second word in "rocket scientist" is "scientist." Try it. Lay out some of the US Composites on a piece of scrap, wait until it is in the gelled state you want, then lay the Rocketpoxy on top of it. See what happens. Assuming it doesn't burst into flame, disintegrate, or emit toxic gas, wait 24 hours and then beat the heck out of it and see if it survives. Then report your results.

Lemme, post one observation I've noted with some epoxies. Sometimes after they've set for awhile say in a fin fillet and start to "gel", one can hit it with a heat gun and the epoxy will start to liquify again. When this behavior occurs, I'm convinced it will
soak into the substrate more and result in a stronger bond. I've noticed this on rocket post-mortems on joints treated in this fashion.

Now, some epoxies if one hits them with a heat gun, they'll sort of "kick off" or accelerate curing and not "liquify" again first. One has to test by trial and error.

The OOP Powerpoxy Weld would liquify nicely and soak into substrate. Proline epoxy 4500 will "re-liquify" nicely and is becoming one of my favorites. Cotronics 4525B or IP will have a tendency to set up as opposed to "re-liqufy".

When the epoxy "liquifaction" would occur, it has a tendency to really soak into plywood. In the rocket post mortems, I could not crack off the fin fillets with my fingers and had to use a screw driver to pry the epoxy off the fin root. Now that's a strong bond!. Kurt