Circumstances (the magic triumvirate of good weather, time off, and motivation) aligned themselves properly last weekend and I finally sealed up Ludwig's gas tank.
I been hypnotized! from stirring the POR-15. If you do this, stir it, don't shake it. POR-15 is the anti-007. Then you pour it in the (taped up except for one hole) tank, tape up the last hole, and slosh it around so it coats the inside all even-like.
Then you dump all the excess into an empty cat litter container (or equivalent).
After it drained for half an hour I looked inside and didn't like what I saw. My sloshing technique had been wanting, leaving some thin spots. The directions say not to do two coats, but I figured that it hadn't cured yet (it takes 96 hours), and so a second sloshing wouldn't count as a "coat". I dumped the POR-15 in again and revised my sloshing thus:
This time around it looked good. I think.
While the tank's out you might as well check out the filler neck, especially if your bus smells kind of fuelly after fillings.
I thought this was cool, the evidence of 36 years' worth of gas nozzles.
Apparently VW didn't seal gas tanks at all after a certain year (if they ever did; I don't remember), so if you seal yours it'll be better than new! Go for it. Full instructions here.