I've battled with the same thing for some time now and I have tested almost everything so far and none with a good result. From all I have used, pure Acetone seems to work pretty good and it does not damage the panels, but it is bit hard to use especially because it tends to vaporize pretty quickly. The things I have tested are Acetone, Thinner, oven cleaner, brake fluid, lye and paint stripper (nitro mors clone). Atleast those are what I can remember right now.
Paint stripper was the only thing I noticed to damage the panels so that if it was left on the panel for too long, it left blotches as someone said earlier and from those areas I could scrape the grain off with my finger nail. My guess is that a hot lye tank would be the best option to remove the stubborn paint. Maybe a "gentle" treatment with a pressure washer after that would do the trick.
I did manage to get mine clean but I did lost some of the grain from couple of areas and I'll have to come up with some fix for those. The end treatment was couple of days on a room temperature lye tank and after that an industrial type pressure washer treatment (much higher pressure than in a typical household washer).