I have used the POR-15 tank sealer, and it can't be beat. It even does a good job of sealing pin holes. I volunteer at a railroad museum, and have applied it to three fuel tanks there, in addition to my roto-tiller at home. We have used 3 methods for cleaning, the POR method, tumbled a thick walled tank with 1/2" nuts in it, and had the last one boiled out at a radiator shop. Tumbling or the POR method work best, the boiling at a radiator shop left a coating of flux in the tank that was terrible to get out. Our next one will be a 40 gallon air tank.
