This is a pretty common discussion on here. Here were my thoughts to a similar post not liking ago. In this case the OP was also considering a liner (I don't recommend unless you have pinhole leaks). I know the tank is a pain to remove as it's integral to the hood. But I'd still advocate for doing it.
The general consensus seems to be if you're tank's leaking, RedKote or the POR-15 liners are best. But, if it's not leaking (or if you were able to fix the leak via other methods) and it's just rusty/dirty, you're far, far better off to clean/strip and give it a few rinses in WD40 to coat the bare surfaces. Nothing wrong with a liner if the prep work's done right and the tank is properly stripped/cleaned before applying it. But there are oodles of horror stories of liners not put in properly that start to disintegrate and plug up the fuel system after several years. Prep work is key, regardless of brand.
I have an old Fiat convertible I'm still running off a boat tank in the back seat because the PO did a lousy job prepping for the liner. It started to dissolve, and once they start to dissolve it's a huge pain to try and get it all stripped out. I've had to strip a few liners out of tractor tanks after they've started to degrade and plug up the fuel system. It's a nightmare to get them fully stripped, and thus I've procrastinated on doing the Fiat. In the future I'll probably avoid buying any tractor or vehicle that someone else has put a liner in, because there's a good chance they didn't do the prep work properly and I'll just end up fighting it in a few years. The acids and chemicals you need to strip a poorly-done liner are not friendly to anything, and it's a long, tedious process. Again: If it's not leaking, don't put a liner in it.
To clean, strip, and prep for a liner (or to clean a rusty tank even if you're not using a liner) there's a big difference in what works best, depending on whether the tank is full of rust, grime, or (more likely) both. For pure rust, it's hard to beat a good acid. Hydrochloric/muriatic seems to be a popular choice, but I prefer phosphoric (naval jelly) as it converts any remaining bare metal into iron 3 oxide, which is more stable and prevents further corrosion (compared to regular rust, which is primarily iron 2 oxide).
If, however, there's any grime/sludge in there, neither acids nor any off-the-shelf rust-remover will touch it. I've found the best stuff for this is a high-strength degreaser. There's a degreaser that comes in purple jugs called 'super clean' that works wonders, but I'm sure other degreasers will do just as well. If there's a combination of rust and sludge, the rust removers and acids won't work until the sludge is removed first.
My method is probably not perfect, but I've had good luck with it: First, pressure wash as well as you can to remove all loose/chunkular bits. After pressure washing, drain the water and use the degreaser: Pour some in, swirl it around, let sit for 15 minutes (swirling occasionally), drain, repeat about 3-4 times. Remember that huge volumes of product aren't required. More rinses with smaller amounts are far better than one big rinse with a pile of product. One 4 quart jug of the purple stuff is more than enough for one tank.
After draining the degreaser for the last time, air dry as best you can with a blow gun and do an acid wash. Similar process to the degreaser, unless the tank's incredibly rusty, in which case I strap the tank to my cement mixer with the acid and a handful of bolts/length of chain inside and let it tumble itself clean. Make sure whatever you put in there to tumble is magnetic, because it makes it a lot easier to remove if you can use a magnetic pickup tool. I once foolishly put some stainless bolts in a tank that had a really awkward inset filler cap, and it took ages to shake them out.
Once done the acid wash, I rinse well with water and quickly air dry again with a blow gun as thoroughly as possible. It could be argued it's also best to neutralize the acid with a baking soda wash. But given that any acid you use will be aqueous/water-based, you should get it diluted and cleaned out sufficiently with decent rinses.
If you're not using a liner, I then do a rinse with WD40. It displaces any remaining moisture. It also coats any bare metal to prevent rusting, and penetrates its way into any crevices/seams. If you're using a liner, obviously avoid the WD40 and go to the etching compound step of the liner.
The best way to prevent future rusting is to keep it full, but if for any reason I'm not planning on keeping a tank full (and I'm not using a liner), I'll give it one last rinse with two-stroke oil to further coat any bare metal.
I've somewhat lost patience for the store-bought rust removers ('Evap-O-Rust', or similar products). They work ok...ish... for light surface rust. But for any heavier rust they aren't nearly as effective as acids and a good tumble with bolts/chain. And just to re-emphasize: The rust removers won't do anything to touch grime/gunk, and can't access the rust if there's grime caked on top. The grime has to be cleaned off first using a degreaser.
