Covering the styfofoam heads with the hardner and then filling in the cracks is very time consuming. The hardner is like plaster (maybe that's all it is). So glooping it on the heads and then sanding it off ultimately makes thin spots over the foam.
So last night I was looking at the bus stop robot heads and considered the many hours needed to get them into a state where they could be painted and look good. And then I impulsively I went out and purchased a can of gelcoat to experiment. I wanted to see what gelcoat would do over the plaster and if it would fill in a lot of the imperfections. I reasoned I could paint a little gelcoat on the backside of one of the heads.
Well, by acting on my impulse and not thinking this through, I ended up with a couple ugly holes that were melted in the back of the head where the plaster was thin or a crack where the gelcoat could leak through to melt the styrofoam. Kind of a mess but the head is still usable. What I should have done is painted the heads with a latex paint first. I think that may have sealed it better so the gelcoat wouldn't react to the styrofoam. Ahhhhh yes another lesson in building.