Get a biggish lump of crumpled aluminium foil (takeaway food containers, maybe) and rub it with that, with a little vegetable oil.
Any oil will do, linseed best, sunflower ok, castor probably worst.
You can use the top/bottom of an aluminium can first as a scraper, and/or wire brush.
If you can be bothered to break out the power tools, you can also power-abrade with aluminium, but there's a bit more risk of going through if its very marginal.
If it has time to set, it probably won't wash off unless it gets significantly mixed with engine oil while you're applying it. Not too big a deal to re-apply annually anyway, assuming access is OK
PRO: Non-toxic, AND you don't have to buy anything. (I don't like buying things.)
CON: Non-toxic, BUT you don't have to buy anything. (Some people like buying things.)