I'm on Shovelhead USA quite a bit and sealing tanks has been a very hot topic over the past year or so. Everyone has an opinion and theirs is the best.
All I can say is that I reviewed a LOT of them and I went with POR-15's tank sealer kit for my Boardtrack's new gas and oil tanks. Even though they're newly welded up by me and don't pressure test with any leaks, I chose to eliminate any future leaks.
The stuff works well and sticks like its part of the metal. I've not filled them yet, but others on the other site used POR-15's kits and like them as well.
So do your research as there are many out there. Another one that got good reviews was Caswell tank kits.
POR-15:
https://www.por15.com/POR-15-Fuel-Tank-Repair-KitCaswell:
http://www.caswellplating.com/epoxy-gas-tank-sealer.htmlBoth have different size kits based on tank size.
I had to buy a kit for a 20 gallon tank and it came with 1 quart of sealer. The reason was that I have a couple undercuts, inside boxes, on my tanks for motor mounts and heads. So to get enough fluid to flow over the inside box corners, I needed more fluid.
I had three tanks to coat and I started with the one with the most inside boxing. Then I drained it back out into the can and reused it in the next gas tank and finally into the oil tank. When done, I had about 3/4 of the quart still in the can. The interesting thing is that they said to fill the can up with water over the POR-15 and let it set overnight and then throw it away. I poured in some water and it didn't mix, obviously, but the next night after work the can was solid as a rock. Into the trash it went.
I saw some samples of the POR-15 at a local paint supply and they had it on the outside of a piece of exhaust tubing. They had hit it with hammers and clamped it in a vice with pads and the POR-15 would not chip or flake off. That's what kind of sold me on that product even when Caswell was also well liked.
Good luck with your new tank and sealing it.