Can Petroleum Jelly be used as a PCB resist?
After cleaning it with Acetone, I spread two different thicknesses of petroleum jelly on my copper-clad board (about 1 mm and 3 mm thick), and scraped away jelly with a toothpick along where I wanted to etch. As an additional experiment, in an otherwise empty section of the board, I wiped off most of the jelly with my finger, leaving just a very thin layer.

After a few minutes the etching was finished. The etch was pretty good; everywhere the jelly covered was protected, and although some of the thinner lines had some copper left they were well-defined enough to be easy to clean up with an exacto-knife (I would recommend using something slightly broader than a toothpick to do the scraping, and to spread the jelly thinner so that there is less chance of residue flowing into other lines). The jelly acted as an equally good resist at all thicknesses, including where I'd wiped most of it off.