I’ve cleaned the windows on my PRU once after noticing that “proven” power/speed settings weren’t quite making it through 1/4" birch ply (from the same batch). After a quick (but very careful) wipe, burnination was again complete.
The inner surface of lid and the tube do get pretty gunky, so I clean them with a damp lint-free cloth from time to time. Jules is correct, though – vinegar & water would cut through the residual film.