Re: removing decals from transom
Chuck, if the area that was under the decals stands a bit high, a polishing wheel or compound may level it off and match the color. Odds are the discoloring is a chemical reaction to the decals, OR a difference in the gelcoat being UV-protected under them. If the wheel doesn't make you happy, the ultimate answer is to repaint just the transom, once those spots have been levelled out.
Or you apply a nice shiny new white vinyl "plaque" or other nameplate over that area and hide all the sins.
Oxalix acid is a good rust remover and yellowish stains are often iron stains, but I doubt that would have been the problem under the decals. "Bartenders Friend" "Zud" and similar products sold in the grocery use the same ingredient for the same purpose, certainly worth a try for the price.