If it's not rusted, you can. If it is, I highly recommend Caswell's. I've used several different polymer types of sealants over the years. I don't use them anymore. Caswell's is a two-part epoxy. You can thin it with alcohol, or acetone, and do a couple of tanks at a time. The stuff is good, and the tank doesn't have to be perfect for it to work, unlike the polymer types which require perfect prep and etching.
I soak rusty tanks in my trusty muratic acid bucket. (It's a big bucket- six or seven gallons) After a day, I dump in a handful of sheet metal screws and a half cup of acetone. I rattle them around manually, dump, one more rinse with acetone. Then I seal, then I paint. Not the best way maybe, but it's what I do.