The patient:
This is a 4 or so step process:
Rinse your tank as best you can, remove any heavy rust by swirling around a handful of sheetrock screws or whatever else you feel confident you can fish back out of there with a magnet. FLUSH FLUSH FLUSH it to get all the debris out of it.
Remove anything and everything, the petcock and screen, the gas cap and latch and cover, and get a rubber stopper from the hardware store for the petcock outlet. Mask the petcock threads. If you're doing this and not repainting, mask everything else.
Clean it with a strong solution of their Marine Clean. It really is a magically delicious cleaner, just as Dave says. Then rinse it really well.
Etch it following this with the Metal Prep (phosphoric acid solution I think). That should eat up most or all of the rust and etch the metal so the coating will stick. Roll it all around in various positions and leave it in each position for 20 min or more, so each surface has a good while to be wet with the solution. Rinse it really well again. A 5-gallon bucket for a gonna-be-repainted tank worked pretty well to do this.
DRY IT. Dry it with a heatgun or hair dryer in the neck and the petcock stopper removed to flow maximum air. The tank will get HOT. Pick it up now and then and roll it around so any residual water gets onto the hot metal and evaporates and dries.
Coat it as soon as you are certain it's dry. But first STIR IT REALLY WELL. Like, use a metal ruler and dig up all that good gooey stuff that is stubbornly stuck to the bottom and then STIR STIR STIR till its a uniform color. Plug it back up with your stopper, pour the can of liner in and SLOWLY roll the tank in all orientations to be certain that the coating has had a chance to hit every surface. After 20 min of obsessively rolling and rotating the tank, tilt it way over so all the excess coating will pool in the front left of the tank, remove the stopper, then upright it with the petcock well low and let the excess coating drain out (roughly 1/4 of the can). They say you do NOT want a pool of it left in there to dry. Get the excess out, and leave it propped in that position so it continues to drain out the petcock bung, you will be surprised how long it continues to run out after the first big amount.
They include a teeny foam brush, I used that to coat the filler neck and the lip of the inlet too.
Give it three or so days to dry, then send it to Blake Conway to paint it
. I'll post an 'after' picture later. The coating looks great, silvery and smooth and feels as hard as a stone. So far impressed with it, but it's a multi-hour process you don't really want to stop once you've started, beware. FIN