When to apply Epoxy of different brand?

[scherzoja]

How long should I wait to apply epoxy of one brand over recently-applied epoxy of a different brand (thumbnail-hard, wait a week)? Vendor "complete cure time" windows vary, depending on temperature (1 - 6 days or more), so those instructions are not all that helpful.

I applied epoxy to a deck patch using the fast-cure of one brand so I could get it sealed before the rain hits tonight. All I did was paint it on to seal the surface. Once the rain clears in a day or two, I will lay in the fiberglass using normal-cure of a different brand of epoxy.

In this case (Raka and Progressive), I did a test mix of one hardener and the other resin to confirm that the hardener of one brand will not completely cure resin of the other brand (both 2:1 ratio).

If different brands don't cure others, then there will be no chemical bond between layer, right?

Thanks

[Spot]

Since the project has been out in the rain and sitting a couple days, I would treat it as 'old work' and sand the surface of the old epoxy before applying the new, using the matching hardener for the resin since you know they work better as a set. If your temps are lower than 70 F (20C) would will probably have slower cure times regardless. Most low/non-blush epoxies will specify a recoat time in which you do not have to sand before the next application. This is usually a day or so.

As far as chemical bond goes, I think it may or may not happen depending whether the old cured fully these last two days or not. I do not think it is fully based on exact hardener chemistry.

[tkeithlu]

I like Spot's answer.

I consulted the admiral, who happens to be a chemist. She pointed to the question of whether the hardening is purely curing, it's exothermic reaction, or also includes drying, by which she doesn't just mean water - if you can smell it while it's curing something is being released in the process. If it's drying it needs a free surface to release all those nasty hydrocarbons into our breathing gas.

Without consulting her I would say that a chemical bond isn't really going to be important; sand it and establish a mechanical bond.

[scherzoja]

Thanks for the answers y'all. I went the Spot's advice and applied 'fast cure' from the same vendor on a day when temps were cold enough (3 days later) to give me enough pot time to lay in the 6 or 8 layers of cloth for the patch.

Maybe I'll get some chemical bond, but at least I might have avoided some chemistry incompatibility between different epoxy chemistry. It's just a deck patch / windlass reinforcement, so the force is all sheer and torque with windlass bolts compressing the deck/patch together. Not as critical as a below-the-waterline through hull removal/patch.