There's a lot of debate as to whether it's best to put a liner in there when you clean it. My view has always been: If it's leaking around a seam or other pinhole leaks, put a liner it. If not, don't do the liner. Lots, and lots, and lots of problems are caused with liners that have started to disintegrate over time - usually because of lousy do-it yourselfers who did poor prep work. When they start to degrade they plug up every fuel system component, and are a nightmare to strip out. If it gets into your injection pump, you have major issues. If you use a liner, go thick, and focus on the prep work.  I've bought a couple tractors (and one old Fiat convertible) with degrading liners and swear I'll never buy one again. They usually start to degrade only in one spot where the tank had some rust, and the resto of the liner is still stuck so well it's a nigthmare to strip. The acids you need to get it all out will usually eat through the metal (and your hands/clothes/tools).

For a tank that's dirty and not leaking, my method of choice is:

i) Clean with a degreasing agent: That Purple 'Super-Clean' stuff or the degreaser made by WD40 are good options. Several rinses with a small amount are better than one big rinse. Tumble it around well (some people strap it to a cement mixer), and throw some bolts/chain in there to help strip all the grime off. It's important to do this before step ii), because the rust removers won't work at all if there's grease/grime covering any rust.

ii) Clean with a good rust-removing solution. For light rust, the 'Evap-o-Rust' type products work ok. For heavy rust, I prefer phosphoric acid/naval jelly.. Tumble as per above.

iii) Rinse out with water and dry as best as you can with a blow gun.

iv) Rinse with methyl hydrate to displace any remaining water. Blow dry and let it evaporate out (methyl hydrate will bind to any remaining water and evaporate quickly).

v) Rinse with WD40, to coat the bare metal surfaces and wick its way into any seams to prevent future rust.

vi) Install back on tractor, and keep tank full over winter to prevent future condensation/rust. As long as you keep it full over winter and fill it up a couple of other times a year, there will be enough fuel residue on the tank's internal surface to prevent rusting - especially if it's diesel.