If these are fiberglass tanks, you need to coat them with something, even if they DON'T leak now. Do a little research on Ethanol blended gas (NOT the E85 stuff, just the 10% Ethanol gas) and fiberglass tanks in boats. (Boats since fiberglass tanks were very common in larger boats.) The Ethanol takes out part of the resin, softening the tank to the point that a screwdriver or similar non-sharpened item can be easily pushed thru and gas will permeate thru and then leak. Worse still, the resin comes out of solution with heat, so in boats it ends up in intake manifolds and on intake valves. More than a few engines wrecked this way. It won't do any less harm to these motors, that stuff will be going thru the bottom end, contaminating bearings and blocking oiling holes, so guys, seal up those tanks. (There ARE some resins that were less affected and not affected, but at least in the marine applications, the vast majority of tanks had to be replaced, my parent's boat included, at a cost of many thousands.)
All you need to do is clean out the tank, apply sealant to the inside, and you're set.
Sure beats replacing the tank and motor.