Kelly Anderson
Senior Member
member

Kelly,
Most polyurethanes do not eat styrofoam,(test a bit first, formulas vary) For a metal object where you want a smooth look to the finished piece I would spray it on either with a HVLP sprayer ($17 at harbor freight) or a regular paint sprayer (not spray paint, the propellant eats the foam, not the poly but same effects). Once there is a protective barrier on the foam, a silver spray paint will be fine.
Instead of polyurethane you could coat it with Elmer's glue anything that will give a protective barrier to the foam, but spraying the polyurethane will give a smoother look than a brushed on barrier. Most of the time the blue and pink foams have a thin layer of plastic on one side and that works as a barrier, but you will still have the other side and edges to deal with so you might as well do the whole thing. spray paint is evil for foam.
good luck,
Allen Hopps