Epoxy over polyester resin

Hi fellas
I have surplas 600gsm roving and a 5L kit of polyester resin that I bought when i didnt really have much clue about these things. For my project in the boat building category I want to build a small sleek console RIB style. Lots of curves and narrow waist for leg room

Just enough width on the face for the teleflex helm and and 6" gps/sounder screen. Bit like a motocyle or jetski waistline

Anyway, I intend to shape it out of a block of polystyrene and paint that with epoxy to seal and fix the surface. The roving is very conforming and drapeable so shouldnt have any dramas building up a thickness of 3 to 5mm which I hope would be enough. I also have some scrap pieces of 450gsm double bias fabric to use in key areas

So coming to my question, I have heard mentioned that polyester on its own needs a sealed surface to totally cure and that epoxy grabs polyester very well. Will a final coat of epoxy over the laminate do the trick to seal it for full cure?

I intend to remove the polystyrene former afterwards by melting it back with a heat gun, this should give additional body of contracted styrene under the laminates

 

Polyester resin will disolve polystyrene. Theoretically if it is fully coated with epoxy the epoxy will protect the foam, but polyester bonds poorly to cured epoxy so it would have to be put on before the epoxy sets. However epoxy and polyester don't cure together well.

Frankly trying to build an epoxy-polyester-epoxy structure is a terrible idea. The materials don't work well together, and trying to mix them like this is a recipe for disaster. You could try putting gel coat over the foam, then building up the glass with polyester. Then finish with epoxy after the polyester is cured.

Frankly I would just set the polyester back on the shelf and make it completely out of epoxy. It will be stronger, and you won't have to mess with trying to get dissimilar materials to work together.

 

Stumble it is not feasible to buy another epoxy kit, would put way out of budget. The first epoxy coat I listed is just to seal the foam from damage by the polyester. Might even use pva for this layer or kitchen grade shrink wrap

Am I wrong in assuming that consoles like these on commercially made boats are fg/polyester laminates? they do seem to survive the abuse

I can spare about 100ml of mixed epoxy. Once the polyester has cured as much as it will, would scuffing that and applying a thickened fairing mix of epoxy help totally cure the polyester?

 

The problem is that polyester doesn't bond well to anything, except when it can chemically bond to polyester. So the first layer of epoxy and the layer of polyester on top of it will tend to seperate and peel apart. If all you intend the epoxy for is to protect the foam, then it would probably be a better idea to use polyester compatable foam in the first place, or protect the foam with something completely disposable.

As far as disposable options the best bet is just a really heavy coat of car wax like. Johnstone paste wax. Just apply a few good coats, making sure to work it in well.

Most center consoles are just poly in a mold. No core is generally needed in small ones, unless the part is big enough that stiffness is a concern, in which case a foam or balsa core could be used.

For the finish coat you shouldn't need a fairing layer. The last layer is typically CSM which is used to prevent print thru of the woven layers underneath. The CSM generally leaves a smooth enough finish that additional fairing isn't required, and you can just sand to a rough finish and spray on paint to finish.

If you do need a fairing layer then wait until the poly is cured, then add a thickened epoxy mix. I like micro balloons, but any non-structual filler will work fine. Apply, smooth, sand, then paint.

 

Saqa, just wrap the foam with a few layers of glad wrap and go for it with the poly... the final coat needs wax in it so it tacks off. This is sometimes called a flow coat. Flow coat is the same as gel coat except it has "wax in styrene monomer " added. Your fg supplier should know all this...

 

Yeah I would go the flowcoat if I already had some but the minimum that can be bought is a 5L kit which would leave too much left over sitting around and its not cheap either. Thats why m wondering if a coat of epoxy will help the polyester cure fully. Also is there such a thing as waxed polyester? I have used this particular brand laminating resin before and after curing it didnt feel wet or tacky, does that mean I can apply primer to that and paint?

 

Yes you can buy waxed polyester , its often used as a final coat on surfboards... if that's what you have then no need to worry. If it tacks off fully then you obviously have a waxed resin. Painting over it can be tricky as the wax is difficult to get rid of completely and it can cause fish eyes in the paint job.

 

Greg, I am pretty good at shaping a former out of styrofoam. I thought that for my skill level between that and making a mold (never tried) it would work ok. A problem I foresee using a mold is that it would be impossible to pull out the "work" from it with the shape I have in mind. With a styrofoam former I can then hollow the "work" out easily enough using heat or petrol

 

Groper, that same brand sells what they call Etch Primer. I was thinking about using that for the boat and console. I used that on the "flying esky" project and it sticks to the epoxy really well with no sign of blistering or cracking. Also used auto paint on that project. Dont know for sure if that primer is meant for metal only but it seems to have done the trick. Wondering what car bodykit makers use to prime the spoilers?

 

Etch primer is designed to improve adhesion to metals... Usually galvanized steel in particular which is more difficult than non-galvanized. But thats not to say it wont work on other substrates. Regardless, you need to get rid of the wax as best as possible no matter what paint you use... after the resin is fully cured wipe it down thoughroughly with solvent etc, then sand the job ready for paint, but before you paint it - wipe it down again with prepsol type solvents again. Ive had mixed results with it, sometimes i got the paint on no worries and other times i just couldnt get rid of what seemed like silicon contaimination / fisheyes etc...In the end i switched to vinylester and got away from the waxed polyester in what i was doing for consistent results...

 

hmmm hope I gets lucky then. Hey what about priming with some sort of auto body finishing putty? The sprayon stuff?

 

Yeah all that stuff is polyester based so its all doable but wax hinders adhesion of anything so the same rules apply...

 

k, if it doesnt work then I can maybe bribe a workman repairing a resorts boat to flowcoat for me

 

You don't need to buy flowcoat, just buy a small amount of surfacing agent (it has many different regional names), you just add a small amount to any polyester and it's now flow coat. If the current resin you have dries to a none tacky surface then there is no need for any wax or flow coat. DCPD based resin, and blends of it, are far less air inhibited than other types of resin and frequently dry tack free rather quickly.

Epoxy will not bond well to an air inhibited sticky polyester surface, so don’t go that way. You would need to thoroughly remove (by sanding) the tacky surface before applying epoxy over it.

Roving leaves a poor surface to fair, plus it’s very weak without CSM, so you need both for it work. CSM as the first layer, plus in-between each layer of roving, then as the final layer.

 

One little trick for getting polyester resin to NOT dissolve polystyrene foam is to coat the expanded PS with a latex rubber layer. In the UK there are a couple of adhesives, I think Cowgum is one that are latex. This will allow you to use a 'lost wax' type core.

I tend to stick to one system when building stuff ie all polyester or all epoxy it stops a lot of problems especially if gelcoats get involved!. If you need to use polyester as a flow coat get a monomer without wax to thin it but to be honest it is mostly more than thin enough anyway.

If you need release waxes for epoxy use a carnauba based one the higher the % carnauba the better and even be prepared to mist spray PVA Blue over it to get release.