Another thought about your epoxy:
I don't know if/how this would apply to a painted-on finish, but when I made molded fiberglass parts with epoxy resin, it had to have post-cure heat treatment for a certain amount of time at a certain amount of temperature to be stable up to some temperature. Specifically: if treated for 15h at 120-130F, it has an operational temp of 183F. If treated for 10h at 175F, it has an operational temp of 222F.
The resin in question was MGS epoxy. Info on it here:
https://www.cstsales.com/mgs_epoxy.html Even post-cure, temps that it wasn't ready for would cause it to become pliable, and it could even flow again from too much heat. I had made a tail-hugger rear fender for a motorcycle, and some local heating from the exhaust resulted in some drops of epoxy appearing on the swingarm.
It might be worth reviewing the data sheets for the epoxy you used to see if it needs heat treatment to stand up to the conditions in that engine, and also to double-check if it reacts to lacquer thinner and/or propylene glycol. PG is a raw material for some epoxies, but I've never heard of any kind of coolant attacking any kind of plastic or resin.