Could nail polish damage pure PLA?
NO if you want to smooth your print, gently heat the plastic and with a utensil smooth out the plastic. A hair dryer is a good way of doing this but be careful to only lightly melt the outside of the model.
I'm not sure this fully answers the question. You're offering a good alternative suggestion to using nail polish, but it still doesn't tell us whether "nail polish will damage pure PLA". Does it?
@TomvanderZanden - No. It'll discolor colored PLA, but it should have no effect on "natural" PLA.