What I've learnt about de-varnishing and de-rusting '81 CB750 gas tanks so far...
1. More or less every step pretty much sucks balls
2. See 1
3. See 2
4. Don't use lacquer thinner to dissolve the varnish, totally didn't work. Softened it but that's it
5. Use MEK - that shit not only melted the varnish it melted my eyeballs and made inroads into my soul
6. With that in mind, handle MEK with a respirator (I did) and in a well ventilated area (I did. In -15 Celsius)
7. MEK will destroy petcock gaskets - no biggy as I'm replacing mine anyway. But know that, otherwise your garage floor will end up wearing some highly flammable shit
8. Be patient - I let the MEK sit in the tank for a few days, moving it around so it got everywhere. But I had a serious varnish situation happening
9. MEK (in Canadian Tire anyway) comes in small, 1 litre cans. I used 4. Don't do what I did, which is to try draining the used MEK back into separate cans. It's a highly combustible ball ache. Get a larger 5 litre can and drain it all in one go.
10. Once drained, chuck in a couple bottles of alcohol to mop everything up. Don't get impatient and try to speed up the drying process by blowing your shop vac in there. Like I did. And blew a bunch of shit/wire/crap into the tank. Big - and massively irritating - fail there.
11. Go the Metal Rescue, got 2 gallons of that mofo in the tank now and - if you're freezing your balls off in The Great White North - point your heater at the tank to keep things as warm as possible. Like all good things (except beer and snowball fights) it's better toasty.
12. Use a good length of chain (with a piece of thin wire coming out of the gas tank cap to make for easy removal) in the tank to knock off any big rust
13. Find something else to do while the whole thing does it thing - takes days. I've degreased the engine and frame, and now plan on starting a brewery.
Oh, and the whole process sucks balls