Remove tap from tank, plug hole with plug. Pour in lots of white vinegar, slosh, slosh. Leave in for 24 hours or so, sloshing occasionally. Talk to a local marine diesel wallah to see what he uses to line tanks - there's an epoxy stuff that's brilliant but I can't recall the name. Drain the tank, save the acid for the next one. Wash out with water, then again maybe with a little baking soda added. Get it nice and clean and then dry with heat. Remove plug and coat threads with wax. Wax plug and reinstall. Pour in epoxy mix. Epoxy zinc is nice but ordinary WEST resin will do fine. Rotate tank so everything's covered. The bottom seam is most important. Invert tank to get excess out - if it won't come out, never mind.