How can I clean insulation foam ("Great Stuff" from Home Depot) off sheet metal ?
Moisture-cured polyurethane foam (e.g. Dow Great Stuff), once cured, is just about impossible to remove by solvents or chemicals. Your only solution is mechanical removal (sanding, cutting, sandblasting, scraping, wire wheel, explosives, etc). Your choice of which depends on the substrate, the environs, and whether you're angling for a Darwin Award.