Starting an old engine ....

Absolutely drop the tank and give it a good clean. My car had sat for ten years, and when I dropped my tank to clean, man you won't believe the garbage. The fuel filter is only gonna catch so much and the rest ends up in the bowls in the carb.

If the tank looks to be in good shape, drop it and pour some kind of non flammable cleaner in it, not a lot enough to cover the bottom and let it set for a while. I put some tumbling stones in mine and my wife and I gave it a good shaking to loosen the gunk and then poured it all out. I then flushed it real well with water and let it dry. I reconditioned the outside of the tank as best I could and popped it back in. Also blow the lines from front to rear that's the opposite direction of normal flow and will catch most build up if any and force it out.

They do make a product to coat the inside, I didn't do it because I intend to replace the tank eventually.