I have access to painted metal garage doors ( panels) and wondered if I can make them safe to use as a raised garden bed. What should I use to paint the inside to make it safe for growing vegetables? I know in water tanks they use a bitumen based paint, to seal the inside.
One option is to have them lightly sandblasted on the soil side to remove all the paint first, if you are concerned it may have lead or other leach-able toxins in it and the metal the doors are made of can handle exposure to the air and moisture. You could also line the resulting bed with pond liner or other inert or water/food safe plastic to keep the soil inside from contacting the metal and the paint at all. I don't think I'd use bitumen, as it's basically a type of road tar and is made from petrochemicals that are not necessarily meant to be food - or plant - safe.
I always recommend painting the insides of anything dodgy with a product called Mulseal (a bitumen/latex compound) (official name is Fosroc® Mulseal DP) and available at least in much of the English-speaking world, just wash the doors first with soapy water, lightly sand to help the Mulseal bind and away you go - I normally use Mulseal to paint concrete edges where I might want to plant erica and Rhododendrons which are very sensitive to lime.