You might need a new blade if it's the one that came with the machine but more than likely the machine still isn't set up right. A narrow blade will...
My opinion is the repairs can only be done properly if the sashes are taken out. Trying to do it in place risks breaking the glass and that's a deal...
I have no experience with SW's Water White CV, or any solvent based finish, sorry. GF's Enduro Clear Poly is resistant to short term exposure to most...