I agree with Gavin (Retropin). I would not use a hairdryer unless the spirit method only didnt work.

If i was wanting to know the biggest n best secrets of mylar removal, I would telephone Kenny Priest of the Pinball Warehouse in QLD. They remove all mylar from every pinball they sell, and are probably doing several machines a week, with excellent results. I dont know if Kenny will tell u to jump and keep his secret mylar removal tricks to himself, or if he will gladly divulge his methods.  But he is probably the most experienced mylar man in the country. Otherwise, play it safe and only use the hairdryer if Gavins successful way doesnt work.

OR play it extra safe, get a quote from Gavin or El_Timbo to have the work done professionally. These guys are experienced with airbrushing and all things playfield.