I like the POR-15 motorcycle tank kit. I've used it a couple of times. You clean/degrease the tank with their metal prep product, and then dry it completely, then toss in the small can of liner, which is actually a paint of some sort. It is NOT a thick goopy latex coating like KREEM or perhaps some other products. It is very runny and coats the inside of the tank easily. It is thin like paint, or even thinner.
I read the instructions on the rust converter product you linked to and it's just the same - clean/degrease, then apply the product. They talk about applying it with a paintbrush or roller - not sure if its thin enough to spread around on the inside of a fuel tank.
Downside of any of these things is if you get anything on your finished paint job, it will never come off. The POR-15 product is permanent. Nothing short of an abrasive process would remove it to my knowledge.
I like POR15 too, I've (successfully) done three tanks with it recently, but all BS aside, I don't know that it's any better than Kreem. In my experience Kreem was a good product that copped a lot of flak by folks who did a half-arsed job then blamed the product, rather than themselves. The real advantage of POR15 (to me at least) is that it is a silvery color, so isn't as noticeable as Kreem when you pop the lid.
I found a local supplier who would sell me a litre of the actual POR15 tank sealer, which saved me a lot of money over buying three kits, and not needing a degreaser (none of the tanks were from a two stroke bike, and anyway, why would an oily tank rust?) I just re-used the supplied "Marine Clean" (phosphoric acid) from the kit that Sean sent me 3 times, with a handfull of nuts and bolts (don't use brass or stainless as you can't pluck them out with a magnet afterwards) to help abrade any rust.
One tank had good paint on it, so I sealed all the orifices with aluminium tape while I cleaned it out, then tipped in some acetone (MEK is better) to dry it out, then once dry, sealed the tank again while I swirled the sealant around. If you seal the tank completely the POR15 will never dry, so once you think you've swished it around sufficiently, make sure you unseal the lid so the stuff will dry. Even though the weather was still in the 70's and 80's when I did it, I left it a fortnight to dry before I added gas, and so far, no dribbles.
Keep up the good work Ron, I'm looking forward to seeing the finished beast! P.S., I finally got a new tire on that wheel with your beautiful wider Borrani rim and HD spokes fitted, I "loosely" laced it but chickened out of the trueing process, and had my friend Jeff true it and fit a new Michelin tire (3.25 x 19 was the widest I could find, is that what you used?) and tube, and this weekend I'm going to mount the discs and install it on my K1, so I can then use the K1 front wheel with the Bridgestone tire on my K0 to cure the "flat spot" blues on that bike. It's like motorcycle musical chairs in my workshop, ha ha! Cheers, Terry.