Leave it bare. This 1976 CB500T tank looked acceptable and then came up clean once it was actually looked at. A jug of phosphoric acid was already in the shop, bought as insurance. The decision was not to use it, and not to pour a sealer in. Fifty years is not a reason to coat a tank that is not rusting.
There is no product link on this page. The things a search offers you, tank sealer and rust remover, are the purchases this tank did not need.
What a clean tank gets
| What you see | Do | Do not |
|---|---|---|
| Bright metal, no flakes in a drained sample | Leave the coating off | Seal it so it stays that way |
| Surface stain that wipes, no pits | Fresh fuel, and a look at the petcock screen | An acid bath as maintenance |
| Scale, pinholes, or rust in the petcock | Then it is a rust job, and a liner is a last step after the rust is gone | Sealer poured onto loose rust |
A liner fails by letting go in sheets. Those sheets lodge in the petcock and in the carb inlets, and the idle that follows looks like a jetting problem. Pulling sealer back out of a tank is a worse afternoon than the rust it was meant to stop. If the metal is clean, the prevention is fuel in the tank and the cap on, not a coating.
Acid is the same kind of extra. It is for rust you can see, used with the paint protected, and neutralized when the rust is gone. Wiping it around a clean tank, or leaving it in overnight because the bottle was open, etches the steel you were trying to save and stains the paint if it creeps a seam. The bottle for this bike went unused.
The petcock has a screen and a sediment bowl. Those had not been opened on this bike when the tank decision was made. Look there before you add an inline filter the factory did not fit. A filter can be a later part, after you have seen what the bowl actually caught. It is not a substitute for leaving a clean tank alone.