We've had this foam left over from VBS for several years! It is insulation foam that you purchase at your home improvement store. I finally decided I'd try using it to make some fun things for my classroom. First though...I needed to purchase a hot knife at Harbor Freight (never forget your coupons!). It was only $19.99...plus I had a coupon for 20% off!
I've used hot knives in the past. Each one is different though...and you will need to practice a bit with an extra piece of foam to make sure it isn't too hot (you don't want to totally melt the foam!).
I used regular acrylic/craft paint on the surface with no gesso. You may want to paint a coat of Mod Podge on top to help protect it once you are finished.