High-build primer for styrofoam prop painting

[RowBoatCop]

High all, hate to make a thread for a specific question but the search function didn't give any clear answers, so I'm hoping someone here has an idea. I'll be painting some styrofoam pieces (specifically extruded polystyrene, not the usual dense foam people use for prop building) & I want to get a smooth-as-possible finish. Obviously polystyrene is slightly pitted & non-uniform in places so I'd like to eliminate this if possible. My plan is to first coat the pieces with a polystyrene sealer to prevent anything eating into the material, then my plan was to use this high-build primer from a local auto-parts store;

http://www.halfords.ie/motoring/paints-body-repair/primer/halfords-high-build-primer-500ml

I'm hoping the high build will give me that smooth finish I'm looking for, but not having used it previously I've no idea if it'll give me the desired effect, anyone have experience with this kind of thing? Also, I'd assume this kind of primer would add some hardness to the surface, is this correct? & If there are any other solutions that would work better please feel free to let me know.

Any insight would be a big help, cheers!

 

[doppelganger]

The problem with high build primer in a can is that it isnt very high build. Its thinned down so as the thinner evaporates it shrinks back. It will add a skin but wont add any strength to the piece. 2k high build is much thicker but dangerous without the right breathing equipment. Have you thought about using fibreglass resin mixed with a bit of gelcoat to make it a bit more thixotropic to hold to the surface without running (or just use straight gelcoat). This will add strength and will be sandable for a nice smooth finish. But as you say... seal the surface first or it will eat all your hard work.