Stripper.

Lost in the replies was this tidbit:

grimreaper said

try disturbing the surface of the paint with coarse sandpaper prior to application.

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Here in California, we haven't been able to get good stripper for years. I've found the "green" stuff (some kinda citric acid based crap) actually works pretty good. BUT only if I scuff the surface with 60 grit or coarser sandpaper first.