I am looking for a foam coating like this that wont eat the foam and is readily available (like a composites store, paint shop, building supply store). Maybe just coat the whole thing in bondo and apply a duratec high build primer? Tying to avoid glassing it.
http://www.easycomposites.co.uk/products/pattern-making/composite-pattern-coat-primer.aspx
Duratec “styrosafe”
The EC primer will attack Styrofoam (expanded polystyrene). As will Bondo as it’s polyester based. You need to use polyurethane foam, or extruded polystyrene. Otherwise if you need to use Styrofoam then just use an epoxy laminating resin to seal it.
Duratec Styroshield is made for the job but it’s an expensive solution.
Yeah i am using the pink stuff (insulation foam).
If it’s extruded polystyrene (the stuff I use is also pink, so it probably is) then it should resist styrene. I’m assuming you’ve tested it and found it gets attacked though?
Just seal it with epoxy first, easiest solution.
I’ve used several of those polyester based “Styrofoam safe” primers and it’s been my experience that they all attack polystyrene based foams to one degree or another. The Duratec product seems to work the best but there is still some loss of surface profile.
We can cut our plugs on our 5 axis router and get very accurate surfaces. Putting Styrosafe or Styrosheild type products on a near perfect pattern and then have to build back up just doesn’t make sense for us.
I’ve gone back to non blushing epoxy and then Duratec sanding primer or Duratec base primer over the epoxy.
First of all, I want to let it be known that I am no expert on the subject (but learning more every day!) so there certainly may be better ways/products to use for this, but this is what I’ve found…
I’ve been looking for something similar, as I am making a pink foam (EPS) plug for a snowmobile project. The foam is working out pretty well, but I have run into some setbacks during testing compatibility of the foam with the tooling gelcoat I will use to lay up the mold. I have read that some people have had luck with just painting their plug with household latex primer, then applying a few coats of wax and a few more of PVA.
I tried this approach on a test piece, and the tooling gelcoat melted the foam over about 80% of the test area. The gelcoat did not eat THROUGH the latex, but apparently became warm enough to melt the foam underneath anyway. In hindsight, I may be partly to blame for this, as I laid the test piece up on my porch to cure, which was in direct sunlight at the time and approximately 110 degrees F. I’m guessing this added to any heat present from the gelcoat, and just became too much for the foam. I took notes on the actual mixing and application of the gelcoat, but should have monitored the temperature of the foam piece while it was curing.
Sorry for rambling on, I’ll get to the point in just a minute! 
I was going to try another test piece with a few more coats of latex followed by a few more coats of wax and PVA, but I came across an epoxy coating made specifically for EPS foam. It’s called Epsilon, from Reynolds Advanced Materials. Ideally I would like to just use a thin primer of some kind, as my plug is made to approximately the size I need, but I’m hoping a thin coat of this epoxy will not change the dimensions too much. Luckily, the Epsilon is available in a trial size so I purchased one recently and was able to put a thin coat on a piece of foam a couple of days ago.
It’s a two-part epoxy and mixed easily, and I used a small foam ‘brush’ to apply it to the EPS foam. I was happy to see that it did not seem to react with the foam at all, just put a nice (mostly) clear, hard coating on it. It’s hard to say how thick it is, but I would guess around .010" (.25mm) or so. I will add a second coat on top of the first, as well as a single coat on the other side so that I can see if one coat is enough to protect the foam when I apply the gelcoat. I am thinking that you could probably use any epoxy as long as it doesn’t ‘attack’ the foam, but I’m not sure how you would know until you try. Obviously epoxy is not the same as primer, but after a little sanding it may allow you to use whatever primer you couldn’t use before.
Here’s the Epsilon if you want to check it out: http://www.reynoldsam.com/product/epsilon/ It also looks like they have a storefront/location in Denver, so it may be worth swinging by and talking with them a bit about what you’re wanting to do.
Regarding using Bondo on the foam, I found out the hard way that it can eat it pretty quickly! I say ‘can’ because I’ve had better luck recently with the Bondo (to fill excessively large voids, etc.), by adding a little more hardener. Or maybe a lot more, depending on how much you’re used to using. At the ratio I’ve been mixing it (visual measuring, unfortunately!), I have about 1-2 minutes MAX before it hardens, but this seems to let the Bondo solidify before the chemicals really have a chance to start breaking down the foam.
There is minimal cratering and the Bondo looks like it’s basically sitting on top of the foam, but has bonded quite strongly. Unfortunately, adding the extra hardener seems to make the Bondo even harder (surprise!) which of course makes the sanding a lot more difficult, unless you’re using power tools.
So I’m not sure if the Epsilon will help you out, but I’m looking forward to completing my own tests of it in the next few days (hopefully), and I’ll try to update this with any new findings.
Just a thought; are you using the recommended MEKP for the Duratec? In my experience, Duratec products seem especially sensitive to using the MEKP that Duratec recommends. They do work with other types of MEKP, but they always seem to develop strange little issues. For example Duratec Sunshield used as an in mould coating tends to be more prone to separating and pre-release using Butanox MEKP compared to the recommended Norox 925. Similarly, the surface primer used with the Butanox MEKP won’t polish up as nicely compared to using it with the Norox 925.
Admittedly I have only used Styroshield once, but for me it worked quite well using the recommended Syrgis 925 MEKP. I didn’t notice any immediate surface defects, however the reason I only used it the once and never again is because the foam underneath is still very brittle and prone to giving way under the primer. Plus for the cost of the Styroshield, I prefer to use better more resistant polyurethane foam and just go straight to using polyester based body fillers.
We use DDM-9 on just about everything. I always ask the rep for recommendations when using a new product. In the case of the Duratec product, I can’t recall what was said but I’d bet DDM-9 was okay by him.
You are right about the brittleness, something I’d forgotten about. As I recall, when we examined the pattern after application it appeared that the Styroshield would dissolve the surface slightly, reverting the polystyrene back to a more solid form along the surface. We didn’t have too much issue with the britteness (a couple of pressure dings from errant fingers) but we may have helped ourselves in that regard by piling on the Base primer making a pretty good crust.
On our next round of pattern making we are also going to use urethane. Great minds think alike. 