I tried something like that, may have been the same thing.  Came in a can, brush it on, and the paper was effectively waterproof (and highly flammable, at least for a while).
I've since been told that Thompson's Water-Seal is much the same thing, more widely available, and cheaper.  Haven't confirmed yet, tho.
Can't answer about writing on the maps, but markers ought to work on plastic overlays same as always.
IMHO, waterproofing the map is just insurance, keep the map in a map folder with clear overlay panel(s) for first-line protection.