OK i was glueing a broken piece off of one of my box cars i dropped some modeling glue on the sytrofoam on my layout, didnt think much of it, ( i mean it dries clear doesnt it?) well the next mourning i went down to the layout and found this crater , not the whole way thorugh, in my layout about 1/2 in. in diameter. It looks like the glue desolved through the styrofoam aa bit (about half way down) funny it didnt give any warning on the container about this kind of thing. Has this ever happened too you?
"Styr"ene model glue on "styr"afoam - of course it’s going to melt. Fill the crater and move on. Or, find a small black rock, put it in the crater and model a meteor impact site…
Styrofoam will melt easily not only from the model cement, but from enamel as well as lacquers. In fact, anything solvent base will turn that bit of foam into a thin hard puddle.
I use styrofoam extensively, and after it’s shaped, and before I lay any track or anything else, I give it a coat of latex housepaint. You can get a gallon of any earth-tone you like, or even black for around 12-15 dollars, and it should coat most of your layour for you. Use a wide wall brush to apply it with and let it dry completely. Once it’s dry, I’ve used spray paint for various shadings and colors over it with no ill effects. FWIW, I also use tacky craft cement to glue the foam together, as well as to attach the cork roadbed to the foam. The brand I use is (my spelling may be off here) Aileen’s. I get it at the local craft shop and it comes in a large gold-colored squeeze bottle. Dries clear and fast and sets really well.
Anyway, as long as you remember to seal the foam with acrylic or latex paint, you ought to be fine.
Respects
I was using a piece of scrap styrofoam to hold some trees I was painting - some electric green bottle brush evergreens. I was toneing them down with a coat of Floquil Pullman Green. The foam looked OK when I left them, but the next day when I checked the surface was all pitted and melted.
The stuff works good for a lot of things, but is reactive to a lot of others. Because it is a blown material, there is not much solid to a given volume so a little solvent material will go a long way in destroying the foam structure. Like ezielinski said, fill the holes up, move on and add the experience to your knowledge base.[swg]
Didnt realy think that foam was that sensitive, Well you learn something new everyday 
some days are more painful then others.
Wow im going to quote myself on that phrase and add it too my sig. 
Every modelrailroader should have a bottle of Aileens handy!!
Ayuh… I swear by the stuff. best tacky glue I have EVER used, bar none. Works well with foam as well as foamcore.
Respects,