I've used the POR-15, and found it to work very well. The basic kit(do a google search, I think they're in NJ-Vech sells it as well), includes everything you'll need, and costs about $35 or so. The stripper included is excellent, but you may need more(MEK will work OK). You may not like the finish though, a silver, rather than the original red.
Another option is to use RedKote stripper, it is the original red color(very close, anyhow) for the R51. The problem is that it is only sold by the gallon-commercially-and you won't even need a quart. I called a few radiator shops, and one fellow explained all this-then he suggested I go to a hardware store and buy a new one quart paint can, and he sold me a quart for $20. Good guy.
Etching can be done with full strength muriatic acid-but drain it after about two minutes(when it starts to smoke!), and rinse it for about twenty minutes with the garden hose. Dry it immediately with a hair blow dryer. Works well.
I just reread my post, and realize you must have found the POR-15 site. Also you're looking for tips on the application of the POR-15.
It is very important to run a couple pieces of old cables or wire all the way through the crossover tubes, leaving a short length exposed on the outside of the tank. After it cures(about 4 days), you can pull the cables/wires out with pliers. The POR-15 hardens like steel, and it took some effort to get them out. Also, use an unwanted gas cap to seal it during application-the sealer will seal up the venting in the cap. Obviously, do it before painting-all this stuff will ruin the paint.
I don't have spare caps for both of these tanks, so I intend to close off the filler with some duct tape. Is there any reason why this is not likely to work satisfactorily?
It has been awhile since you posted this, but I wanted to know how your tanks came out. I have used the POR-15 stuff in the past, and it is pretty good, but just doesn't look original...that's my only problem with it. I have also tried a PPG epoxy primer which is red, and looks exactly like the original, but I am still experimenting with it. It looks like this epoxy primer is very dependent upon how thick you coat the tank, which then determines how impervious it is to fuel. I know that doesn't sound legit, but that's what I found so far.
I have just found another urethane type product that is also red, like the original and am going to try it. I'll let you know how it turns out. I wish I knew what the stuff is that they used from the factory. Nobody seems to know, and yet it is really hard, durable stuff. Amazing for almost 50 years ago!
Neither tank has yet been used on a bike, so I don't have any experiece yet with how the POR-15 stands up to today's weird gas blends.
You're right about it not being the correct color for the postwar bikes, which had a red coating. But the tank on my R51/3 was already nearly devoid of the red paint when I got it anyway, and the silver color of the POR-15 isn't that much different than the plain steel that was showing when I started. On my R12, I think that tank was never originally lined; at least, there was no trace of it in my tank.
One is from my 1941 R12. The tank is in good shape and there appears to have never been any kind of sealer on the inner surface before.
The other is for an R51/3. This tank has a red sealer that is flaking off badly. Also, the tank is for some reason missing the two hinges for the tool box lid, so I need to have those welded on.
I have often heard the praises of POR-15 tank sealer. After reading their web site, I'm not completely sure of what I need. My questions are:
1) Do I need to use the POR-Strip in both tanks, or only in the R51/3 tank? Although I can't see any signs of a previous sealer in the R12 tank, I also can't see all of the inner surfaces. Does any one know if these tanks were sealed originally?
2) Must I get the welding done on the R51/3 tank before going through the sealer process? My painter has said he can add the hinges before he paints the tank, so it would be easiest for me to do the sealing first. It seems like the welding should be so small and remote from the inner surface that it won't harm it...
3) (Maybe I should have asked this first!) Is POR-15 really the way to go? Are there any tricks to doing the job?