Permanently mounted tanks don't need recertification, and are built WAY thicker than you think they are to contain more than double the pressure that the LP can produce. For reference, that max pressure is somewhere around 200 psi if the ambient temperature is at 110 degrees. If you manage to be at Death Valley in August (dear god WHY would you do this?) the LP pressure can get to 260 psi when the temperature hits 130F out.
So the tank will be capable of at least double that for safety - don't go nuts with a grinder, but a fine-grit flap wheel will be perfect to take off the scale, then prime and paint and forget about it. Use a quality enamel paint and give it at least a couple coats, and you are good.
I've never done ANYTHING on the now-21-year-old tank under my older coach. It's just fine hanging there.
03 40' Monaco Diplomat: 1020 watts solar / Victron / Magnesine 2800. TRW steering, 23 cuft Frigidaire, dishwasher, W/D, Magneshade, lots of other mods!