Blue foam headaches

Hi guys, I’m after a bit of wisdom from you model makers out there.

I am in the process of making a plug for a slipcast mold and I am hitting a stumbling block with blue foam and epoxy resin.

I’ll explain my process:
1 - shape the model in blue foam
2 - paint on one layer of epoxy resin
3 - once that is dry, spray a coat of primer
4 - sand primer and repeat
5 - sand through the primer/resin to the foam
6 - curse
7 - paint on another coat of epoxy resin

My problem is, when I paint on the next coat of epoxy, it doesn’t dry properly and melts/drips off the model, resulting in a completely useless block of foam, primer and epoxy.

Can anyone tell me why when I apply epoxy resin on top of primer, it reacts badly? (Is the surface too smooth? Is there some reaction happening that I’m unaware of?)

I’ve just ruined my second attempt and I’m not in a hurry to ruin a third one!
Many thanks

I’m likely not the right person to ask for his, and have many friends who are much better, but I’ll take a stab.

Most likely reason you’re having issues trying to epoxy over primer is the type of primer/epoxy you’re using. I’m almost certain there’s just a chemical reaction happening, either between the mid-cure primer(such as water or alcohol), or the cured primer acting as a third catalyst. Layering resins and paints can be relatively frustrating, because there’s so much happening at the chemical level.

Depending on the scale of models you’re doing, it might be worth switching over to some type of polyurethane foam and applying primer directly to that? If that’s not possible, I’d grab some samples and try and hunt down a prototype/autobody/foam shop to get local expertise. Very likely you could solve your issue by switching to a certain type of resin or a certain type of primer(such as 2-part catalyst cure primer).

Best of luck,

I’d recommend just putting on three consecutive coats of epoxy resin (after each cures thoroughly). Primer is for getting a surface ready to paint, whereas epoxy is structural, (used in fiberglass layups) and is meant to stick to itself in multiple coats. With no primer between the coats of epoxy, you shouldn’t have problems with it sticking. Once you have the built up thickness of three epoxy coats, you should be able to sand it smooth before going through.

You might also want to try something like Bondo or water putty instead of epoxy. They’re thicker, so you’ll be able to mush them onto the surface more easily.

One way to make sanding through less likely is to spray a coat of a contrasting color paint over the primer, and sand till that contrasting layer is all gone. It’ll stay in the pits of the surface till the pits are all sanded away. Then once you’re down to primer or epoxy in an area, stop sanding.

Good luck!

Try this on a scrap piece of blue-foam to see if it helps your process:

1 - shape the foam
2 - dip or THOUROGHLY paint with latex house paint (this should seal the foam from whatever coatings come next)
3 - filler\epoxy \ spackle etc… for smoothing
4 -paint
5 - sand\finish. (don’t sand past the latex or…)

I’ve found that dipping a couple times in the latex paint really seals it up.

Styrene foam reacts to everything - I hated modeling with it, but it is cheap and fast.

Thanks for the input everyone
I’ve managed to get through to a completed model (thank god).
I took a little bit more time and applied two coats of epoxy and one coat of red primer before going ahead with my grey primer. Giving a more substantial base to work from and less chance of sanding through.

I don’t know what exactly the issue was, but it was almost certainly a chemical reaction between the semi-sured primer and the epoxy.

Oh well, lesson learned! And many thanks again for the tips.

I concur with jGray. Latex paint is a necessary step.

I remember my first experiments with polyester resin and blue foam. I bet the results are still stuck to a stairway at ASU hehe.

mmm … if the resin you are using is melting your foam, it isn’t epoxy. In all likelihood it’s polyester.

Epoxy will bond to damned near anything but it absolutely doesn’t like to have anything between it and more epoxy. Why are you priming between applications of resin; to visualize the surface?

Here’s a great how-to resource → EpoxyWorks

and some old threads…

for others (and there are a lot) on your URL line type → blue foam (there must be a space between the “m” (in com) and “blue”