The film-forming solution I referred to above contains a variety of solvents:
ETHYL ALCOHOL ACETONE ETHYL LACTATE
In hoarding used cameras I come across stains on lenses which are hard to get rid off (maybe polymerized fats from fingerprints). I had to use stronger solvents than usual (cellulose thinner) and rubbing to be successful.
A cleaner-film as the above should be successful with such stains to be an alternative.
My method of cleaning is brushing, soft wiping with soft paper wetted with aqueous cleaner and then wiping with solvent.