Pure citrus peel oil aka D-Limonene transforms polystyrene (styrofoam®) into a very clear usable sealant that has a great uv resistances and dries very hard and very clear.
This also works with many other types of styrene based plastics.
NOTE ABOUT ACETONE OPTION!
Pure Acetone will reduce styrofoam much faster BUT it makes a milky white goop. Not the clear gel we are making.