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I have to put a vote in for "De-Solv-It" Graffiti Remover - just don't leave it on more than 15 minutes & don't use hard edge tools - brush off under running water & repeat. It also claims to be "polycarbonate safe" on the pack

Here's some pics of a Cheetah project with (dangerous!) caustic soda :

Cheetah project Tamiyagekken

Worked really well...