I used Kilz latex paint before spray painting on a styrofoam project. Part of it melted. What did I do wrong?

I followed the instructions as the latex paint was to be applied to prevent the styrofoam from melting. After I spray painted, part of the styrofoam still melted. Should this have happened?I used Kilz latex paint before spray painting on a styrofoam project. Part of it melted. What did I do wrong?
It's possible that the Kilz wasn't applied properly to the areas that melted. The spray paint seeped through and caused the styrofoam to melt. Styrofoam does not like solvents.