Acrylics & foam??

[foto-phreak.livejournal.com]

I bought some pre-cut foam shapes and painted them with some acrylics that I already had at home. I had noticed in the same section, there was special paint for the foam, but I didn't want to spend $4 bucks on each color. Now I'm wondering how I should seal these? I'm making antenna balls...and here in Seattle, you have to have everything weatherproofed! Would a polyurethane spray be enough to waterproof it? I'd hate for the paint to run all over my friend's car!

It is EPS (expandable polystyrene). I checked the website (plus many others) and only found tips as to what type of paint to use...not how to seal it for the weather :( but these are specifically made to be antenna toppers, so you'd think there'd be some instructions for sealing

http://plasteelcorp.com/Merchant2/merchant.mvc?Screen=CTGY&Store_Code=PCO&Category_Code=FCS