For protecting/priming foam:
I have used -
*craft paint
* paint can primer
* acrylic gesso (from the craft store - basically a primer)
For working with foam
* various brands hot wire cutters - very awesome.
* bread knife.
* big blade utility knife
* high grit sanding sponges
* various scraping tools
* heat gun (be careful of fumes)
* soldering iron
Oh, and this ....thing! (I could not remember what it was called). For doing negative surfaces (i.e. cutting INTO something, rather than cutting something off...) this tool is VERY handy.
http://www.harborfreight.com/curved-multi-rasp-66641.html
I do enough terrain (for our group, and I also do large project commissions) that I even built my own hot wire cutting table, although to be honest for this simple trench work you don't really need one (its handy for making buildings, lots of walls, bridges, etc.).
best of luck!