Thanks for the tips so far. this project involves indoor waterslides.The styrefoam being cut by a cam machine out of 4x6 ft. blocks .I already tried Interlux 2000e/2001e two part epoxy bottom primer and vc17 two part bottom paint as a sealer filler but they both eat the foam in minutes.As suggested I am waiting for some gypsum plaster to dry to sand then wax pva and finally gel in an assortment of colours.I also thought of draping or heat shrinking some plastic over the foam to remove the pits . This is all about getting that smooth waterslide finish.