Proto 2000 stuff will almost always strip cleanly with 91% isopropyl alcohol. 70% will work but takes a lot longer. The alcohol is 100% safe on the plastic - but of course is flammable and has fumes. It's a whole lot kinder to the plastic as well as a whole lot cheaper than commercial strippers.
I haven't tried stripping a Walthers-era P2K model yet, but I've used alcohol to strip Walthers passenger cars with reasonable success. FWIW 91% alcohol will also nicely strip most newer Atlas stuff, as well as Branchline. Not sure about current Athearn production.