Actually recently I removed spray paint from the side of my cab's tolex and I used "Krud Kutter Graffiti Remover" from Wal-mart. It is water-based and to be honest I didn't think it would work at all, but it did! You just spray it on let is sit a little and then I scrubbed it away with a tooth brush...
I found it in the aisle with the paint thinner, etc.
From
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The ultimate spray paint graffiti remover.
A safe alternative to hazardous, solvent-based graffiti removers.
Removes:
acrylic, acrylic enamel, aerosol paint, enamel, high gloss, oil lacquers, primers, sealers, semi-gloss, Speed-E-Namels, stain killers, super enamels, urethane enamels, and water-based enamels.
Effective on most hard, soft, and porous surfaces:
aluminum, brick, cinder block, concrete, fiberglass, formica, freeway signs, glass, metals & alloys, plastic, plexiglass, street signs, slumpstone, split rock, stainless steel, stucco, tile, terrazzo, vinyl, and more...
Earth Friendly:
Water-Based . Biodegradable . Non-Flammable . Non-Toxic . Non-Abrasive
Available in: 32oz., 1gal., 5gal., 55gal. drum