Hi GTSRider, Rust is a huge issue on fuel tanks. Many ways to remove it. I don't know what way is best or if it matters. Just get the rust out.
Regarding liners/sealers I think you'll find they are pretty much a must to prevent future rust & rust holes.
It seems the original tanks did pretty well at not rusting too bad, but once rust started it changes things. After cleaning rust they rust much more readily for some reason. I've observed this on several tanks. If you just clean, it will rust again & will perforate. It just will.
Ethanol fuel tends to promote rust also. Under the fuel level is not much a problem, but the upper inner surfaces exposed to air is very prone to rust & rust perforation.
After a derusting using Kreem my tank perforated on the upper surface after only 4 years. I did not use a liner or sealer.
I checked tank upper with mirror often & could see rust getting worse as time went by. One day I was getting gas seeping out a pin hole a few inches to rear of gas cap. Again down low was ok. I always parked bike with full tank, but that doesn't help the top.
I had really nice original paint. There was only the tiniest pin hole. Hardly noticeable on paint. I was committed to save paint & wanted to do it myself to save money.
Kreem liner is not durable at all, so don't use it. However Kreem paint mask works very well & you can protect paint from acetone & the sealer you want to use. Use 3 coats of paint mask. Put glad wrap over paint mask while doing the sealer as paint mask is very fragile. Paint mask is water soluble, even 1 drop of sweat will compromise it.
After much research I narrowed my choices to POR15 or Caswell tank sealer. Both can be applied over some rust, or so the instructions say. So some remaining rust in seams seems to be ok.
I decided on Caswell after reading up on both the above. I knew I had leak & could see more rust perforation was soon to follow. I felt Caswell was a really good sealer & it also stops further rusting according to Caswell.
I really do not know which sealer is best, so you research & decide. Whichever, follow instructions very closely.
With confidence I can say if you do not seal tank it will rust & fairly quickly. If your paint is good, you can remove rust, seal & save paint.
The liners can be removed by various companies that do this, plus clean & reseal. Often the paint is lost in that case. These companies can save paint in rusty tanks in many cases similar to how I did. You can remove liners with certain chemicals at home also. Not easy, but possible.
I could not find a good used tank. All the used tanks I saw were damaged fairly badly or rusty. Most had much worse rust than mine. New tanks I don't know, how are they? Looking at the forums some don't fit right, leak, rust. They may or may not come painted. No easy answers. New tank + paint is very costly.
At your stage I feel you cleaning & lining yourself would be the best solution.
If you decide on Caswell get at least the 1 pint can. The little 3 gal. size can is not quite enough to cover easily. Also get a 1/4-19 BSP straight pipe tap to clean any sealant from tap threads. I got an eBay
$4 one from China. It actually came in a few weeks & worked good. Get a large black rubber "stopper" from Ace Hardware to block off fuel filler neck. That worked good. I made plugs for tap bungs. Old fuel taps with screen cut off would be good. I found it impossible to keep all traces of sealer from threads. The China made tap cleaned sealer from threads perfectly, but pull plugs & wipe threads to remove sealer as you can before it cures.
I don't know about POR15, but with Caswell, let it cure at least 1.5 weeks. Do not short cut the cure time. Let it cure much longer than instructions say. That seems to be a key factor it long life.
All this was a lot of work, but paid off. Also I get no more rust flakes in fuel filters. So the sealer actually does keep rust at bay, at least until it deteriorates. As the testimonials say, the good sealants last many years.
Paint some sealer right up to very top of filler neck. This seals the crimp of the filler neck & prevents leaks there also. Not on outside of neck, but inside. Gas pump nozzles have not damaged the Caswell sealer in neck. It's quite tough material. But I'm careful to not try to chip it.
Good luck on your tank, take your time & you will be good.