After much research, I made a plaster plug using Ultracal 30, let it sit for a week in 90 degree weather, then tried to seal it with epoxy laminating resin, brushed on. The resin didn’t like the plaster and the result was a lumpy surface. Little areas opened up, exposing bare plaster, looked kind of like cellulite and it took a long time to cure. The leftover epoxy in the mixing cup gelled at the normal time, about 15 mintues, but the epoxy on the plaster took about 2 hours. The plan was to spray it with 2k auto paint after sealing it, then sand and polish. Any thoughts as to why the epoxy reacted like that? The weather that day was about 85 degrees. The same kind of epoxy I’ve used many times.
Could be excess moisture in the mold that caused the resin to react to.
The longer gel time may or may not be related to possible excess moisture on the applied surface, since you have a greater volume in the mixing cup.
What type of resin are you using? A pic may help.
Using Tap 314 resin and 109 clear medium hardener
What other products are people using to seal plaster plugs before painting?
Shellac…
I have never done a plaster plug before but I imagine any epoxy paint primer would be good too or any two pack primer?
Then sand and spray with two part top coat.
Resin coating with epoxies can be tricky at times, it flows and runs easily and can be sensitive to other materials with fish eyes etc not to mention being really hard to sand and an even bigger sanding problem when on top of a really soft material like plaster.
Yeh, I don’t think there was any reaction between the epoxy and the plaster. As Fasta said, laminating resins are prone to fish-eyeing like you have described when used as a coating like that. Generally you would do 3-4 coats, just letting each coat tack off before applying the next. This would then give you a thick enough coating to cover up those separation issues, but would require a good amount of sanding and you would lose any fine details you’ve got in the plaster.
Definitely a spray on primer would be the best choice. If you don’t have the facilities to spray then you may get away with using a brush on primer as at least it would be much easier to sand than a laminating epoxy.
Thanks to everyone for your help. I’ll do some testing with Shellac and some primers. I do have spray equipment, just not very good skills yet. Anything in particular I should look for with a primer? I have the Duratec Surface Primer, but my first attempt at spraying that resulted in bad orange peel and a slow cure.
I have had this happen if either the Duratec or the MEKP is past it’s shelf life. Also ensure you apply multiple thinner coats initially, once you have a few thin coats and allow them to tack or dry, you can lay it down pretty heavy.
I gave up on using plaster for anything plug and mold related a while ago because there were so many issues like this to solve.
If you are going to use it, there are products sold in art shops specifically made to seal plaster and clay parts.
It’s often a good idea to dry them in the oven before applying anything.
If you are going to use a spray primer, the trick is thin passes and letting each thin layer dry before adding the next. Add too much and it will run and fisheye like the resin.
If you want to add a resin surface coat, I suggest using a super thick gel-like resin. Any time I tried to use a thin resin to coat something that isn’t flat, I got fish eyes and run off etc. The epoxy black surface coat from ACP composites should work well if the plug is dried properly.
If it isn’t for a high temp mold, I would suggest recasting it in a more user friendly material. Epoxy putty, for example, holds paints, primers and coatings a lot better.
So, to be clear, you have used Shellac to seal the plaster, then applied 2k auto paint over the shellac with no problems?
Shellac will work fine with waxes over the top and it’s ready to mould off. This is the old school way of decades ago. Good thing with shellac is that even if it sticks to your new mould it then just dissolves off with methylated spirits easily.
Okay, maybe Shellac isn’t my best option. I’m looking to seal the plaster with something that will take 2k auto paint well so I can wet sand and polish for a flawless finish. And I don’t want to lose any detail.
Realistically, you’re better off not using plaster. It can be used successfully, but honestly if you want flawless results then it is better to use materials that suit the process. With plaster, it is very likely you will have some fixing up to do, and from there you are chasing your tail.
I used plaster because I’m copying a plastic part. When I made a PE mold from the original plastic part, the exotherm cracked the plastic part so much that the surface finish of the mold was poor. I tried to repair the gelcoat, but gave up and decided to make a plaster copy of the part so I could fix the surface and make a better mold.
That sort of sounds more like the gelcoat reacted with the plastic, but even so - there’s every chance the exotherm will affect the plaster as well.
It’s hard to recommend a better approach without knowing what the object looks like. If you managed to cast it in plaster then I would suggest that casting it in polyester or polyurethane resin would be better. Hell, you may even be able to use a polyurethane foam and then just finish it in the conventional way using body filler and automotive paints etc.
It depends a bit how much you have invested in the plaster plug. I’m not saying it can’t be done and you shouldn’t do it, because I do know that plaster can work. But I also know that plaster isn’t the most straightforward material, and it can have its own problems that need a whole other set of expertise. It may be a little bit of a complex way to achieve what you are doing.
The parts I’m copying are door panel inserts. It is a very hard, clear plastic (with a silver coating on the back that shows through). Acrylic I think. I may as well continue and see if the plaster plug works. I thought Ultracal and Hydrastone plaster were used all the time for plugs where a PE mold would be taken from it. I’m doing the left and right pieces separately so I may try the foam when I do the other side. Thank you very much for your help.
Should I only use epoxy based 2k primers and paints on the plaster? Seems like I could use urethane based 2k as long as I also use PVA before adding PE Gelcoat for the mold. Is this correct?