Hi I am starting a small switching layout and using a foam board as the base board with 1/2 plywood under the foam board for support. I would like to know do I need to use a primer on the foam or can I just paint a brown latex directly to the board?
Latex or acrylic paints will not attack styrofoam. Solvent-containing oil based paints probably will. You should be fine with the latex.
We used latex paint on our club layout: two inch thick pink foam board. It worked fineāno primer required.