Epoxy resin dissolving tooling gelcoat - help

I spent a significant amount of time prepping a part for mold-making (wax + PVA). I then applied tooling gelcoat from Fiberglass Supply Depot and let it cure for 12 hours at ~60°F. The gelcoat felt hard, but I could still leave a faint fingernail impression.

Tooling gelcoat: Tooling Gelcoat Red Gallon https://fiberglasssupplydepot.com/Tooling-Gelcoat.html

Next, I started laying up fiberglass cloth using epoxy from US Composites (medium hardener). As soon as the epoxy hit the gelcoat, the gelcoat seemed to start dissolving.

Epoxy: Epoxy : Epoxy Resins and Hardeners https://www.uscomposites.com/epoxy.html

Total area is ~10 sq ft, and epoxy was applied over ~3 sq ft so far. There are a couple spots (roughly 1/4" x 2") where the gelcoat dissolved enough that a darker shade of the original part is visible. I don’t think the PVA layer is affected, but I can’t be 100% sure.

Is there a way to save this project? Are any of these ideas reasonable?

• Maybe the gelcoat wasn’t fully cured? Should I wait another 24 hours before continuing?
  
  
• I read about tie-coats, but I’m not sure if that applies here—and I’m not finding anything for sale (maybe I’m using the wrong term).
  
  
• Alternative plan: sand back as much epoxy as possible and continue using vinyl ester + fiberglass (but I’m not sure I can sand off all the epoxy cleanly).
  

 

My understanding is that tooling gelcoat is for making molds, not for making parts.

Tooling Gel Coat is a scratch resistant gel coat for making molds where gloss, hardness, and craze resistance and minimum distortion are key. The life of a polyester mold may be extended considerably by the use of tooling gel.

Since the wax is under the tooling gelcoat, hopefully the current part will strip from the mold.

I have never molded a part with polyester gelcoat under epoxy. I would have tried polyester or vinylester first.

 

You are correct: I am making a mold from a part. (prepared a part with wax and PVA and applied tooling gelcoat which will end up the top surface of the mold). And, yes, in retrospect, I would have used polyester or vinylester...

I am just surprised that Epoxy reacted with the gelcoat like this. I have used the same epoxy with another gelcoat (west marine: Amazon.com https://www.amazon.com/Marine-Coat-One-MEKP-925-Catalyst/dp/B08L447NL4) with no problems.

the question is whether I can somehow save the project with one of the options listed in the original post or to strip the current layer of tooling gelcoat and start all over.

 

Epoxy does not dissolve gc. GC, otoh, can dissolve epoxy. In either case, the two are not compatible for these and other reasons.

Pull the part and restart with only esters..

 

It is entirely possible to buy epoxy tooling gelcoat.It isn't often publicised and it works well with subsequent layers of epoxy.I take it that there is a compelling reason to make epoxy tooling,rather than just a notion that epoxy is the "best" thing?Coupling materials exist and work,but they are seldom used as resin suppliers tend to steer customers in the direction of compatible materials.Normally,I would expect epoxy to adhere to polyester or vinylester,but wouldn't have high hopes for the reverse occurring and it is possible that you may wind up with a mould that could be used as long as the surface quality isn't of paramount importance.For a small part,intended for a painted finish,it might be possible to persist with the laminate that has been started.If the troublesome areas are only 2" X 1/4" a gelcoat repair isn't a huge undertaking but it may have a short lifespan before detaching itself.Alternatively,you peel it now and start again with compatible materials.

 

Not clear to me if it's truly dissolving or just a bit of pigment coming out of the air inhibited surface. I'd suggest as others have to start over, if you're set on epoxy for the structural laminate then back up the gelcoat with some vinyl into some surface tissue/veil x 2 x layers, once set that will take a scruff up for the epoxy. The tissue can be a pain, make sure it's "shiney" side up and use a decent brush to tamp/stipple your resin in. Tissue should be a part of any tooling layup, at least it was where I trained over 40 years ago.
All the best from Jeff.

 

If you can leave a fingernail impression, the gelcoat was not cured. There is likely a chemical reaction between the epoxy and the not fully cured polyester. I had seen that issue on a repair.

 

Polyester and epoxy are incompatible in an uncured state, and epoxy will not work when used with gel coat in that way.

There are epoxy compatible gel coats, so if you have a real need and desire to use epoxy, you need to use the correct gel coat.

 

Thank you, all. Yes, the issue was the undercured gel-coat. As I mentioned, I have done this before with no problem and it is widely known that Epoxy will adhere to PVA (not the other way around).

After waiting another 24 hours, the process went very well.

 

Adhere to PVA?

Epoxy does not bond well to polyester gel coat when used like that!! Again, you can buy epoxy compatible gel coat, but you need to indicate that's what you want when buying it. I'm not sure where you are getting your information.