henry said:
if it's got pinholes in it why not soft solder or braze them shut - sealing is just a bodge, even if it's a reasonably good one!
Henry has a very good point. The tank is stripped and if you are going to do a procedure that would be detrimental to paint then now is the time, IMO. Once that is done you will still need to coat the inside of the tank to prevent further corrosion.
If you do or don't then you have a couple of reasonable options. wdietz186 has suggested the best tank sealer that I know but I think this is best for plastic or resin tanks. It is still great for steel but I think there is a good, user friendly option, Redkote.
Redkote is an acetone based sealer. You can thin it down and apply it in thinner coats. Being thinner you can improve the chances of it getting into tiny pinholes and sealing along the seams. It can also be disslolved in the future by MEKP (from memory). The problem with Kreem or POR 15 is that if you don't get it right first time it is hard to rectify and the degreasers and acids needed to etch the steel can scar any paintowrk if contacted. POR 15 can be dissolved and it needs some form of paint stripper with "methyl ethyl chloride?" from memory again. Google it to be sure. If you read the Caswell's information page it will tell you what you need to remove Kreem or POR 15.
If money isn't an object, get it mechanically sealed and chemically coated. Henry and wdietz186's suggestions in combination. From what I can tell, Brad's suggestion is an epoxy product that appears similar, if not the same, as Caswells. Apologies to him due to my lack of familiarity with the brand if that is the case.
You have number of options and you will receive a number of suggestions. You now have mine.
Cheers
Rod, who has a pinhole in his Alpino tank that needs to be fixed when I get my hands on it. Its painted so I'll probably redkote it to ensure it penetrates into the pinhole without too much risk to the existing paint. I do have some Caswells here so I may change my mind once I see the tank.