fwiw,
I know 2 professional restorer's for the MB's( business's of restoring W113's, 300 SE 6.3's, Gullwings, 190SL's, 300SL's, etc,) mask them off with the tape themselves, then give them to the painter. It IS time consuming. Patience required.
I've asked them if there were a stencil, and if so why not use them... they both said it's not precise enough. Of course, these guys are selling their perfection in restorations to those who want to afford to pay for it so there's some latitude for the rest of us.
I don't know if they ever use an exacto knife to trim though... I rather doubt it, but will ask next time I see one of them. I would think there's always the good possibility of scratching the chrome layer though when using the knife to trim.
Longtooth
67 250SL US #113-043-10-002163
'02 SL500 Sport