I have a old gas tank that has lot of old gas that has turn to varnish. Was looking for the best way to dissolve and remove from the tank.
Thanks
Fitz

Best way is to buy a new one. Why fight it, I highly doubt you'll get it all out anyway, and chances are the tanks got rust too.

I had similar problem with lots of rust. I made end clamps out of plywood then used pea gravel, then a 3 step cleaner, and internal tank paint. Its good as new now.

I have had good luck with a gallon of lacquer thinner. i use the gallon on several tanks. Be careful.
Rich

Saturated steam is the ticket if you can find somebody with a steam cleaner (not a hot-water pressure washer). It reaches into all the places you can't see along with the added benefit that, when properly done, you can weld on it afterward.

I'll second rotating the tank I used drywall screws to break up the crusty rust and lye for varnish and what ever else might be there. To spin the tank was placed in the cement mixer stuffing rags in to keep the tank from rubbing anything. The inside looked like it was sand blasted when done. After rinsing a light bulb was placed inside to help dry things out.

I have used Lacquer thinner on several gas tanks with good results. Leave it in the tank for several days and shake the tank several times a day before emptying. After the tank is dry I then use Phosphoric acid to remove any rust which leaves a coating inside the tank.
I just finished cleaning two 26/27 tanks which are not being reproduced and am very pleased on how they turned out. Will be bringing the extra one to the next Bakersfield swap meet if someone needs a good tank.

Some 70/30 muriatic acid/water and a handful of nuts and bolts, shake it up good, shake out the hardware then rinse out with plenty of water. Slosh some acetone around inside to absorb any leftover moisture. Air dry then coat with your favourite sealer- Hirsch, Pederson's Eastwood- or go down to your local motorbike shop and grab a bottle of Kreem tank sealer.
Coat the inside, let it dry and its better than new.

I'm not big on coating the inside of gas tanks with sealers. The tank in my 27 that I'm replacing was coated years ago with "Who Know What". The stuff is pealing off in chunks....so large that the chunks would not flow out the 1/2" drain hole and stop the flow. It also looks like water got trapped between the coating and tank and rusted holes in the tank.
If I could buy a new gas tank.....I would do it in a heart beat vice coating the inside. The Oil companies keep playing games with additives in gas.....what sealant works to day may not hold up in years to come.

I actually had some trouble with the Red Kote tank sealer I used in some small engine tanks recently. Seems the gas literally dissolved it in 1 tank in particular.

As for the old gas, Methyl Ethyl Ketone is what I would try. it aint cheap,but it will dissolve dead gas out of fine filter screens and what not. it is also what Red Kote says to slosh the tank with before lining.
There is a small fine screen in 2 cycle engine carbs about the diameter of a pencil eraser. The rebuild kits don't come with that screen anymore. So I pour a dab of Methyl Ethyl Ketone into a bottle cap and dunk the screen in it for a while.Comes out spotless.

I've probably coated 30-40 different tanks for all kinds of stuff- small engines, tractors, cars, construction equipment or fuel storage tanks and at least on the machines I still own, or know the whereabouts of, not one has had any further tank troubles, some for +40 years.
BUT I've also cleaned out previously "sealed" tanks that have failed for unknown reasons- most likely they weren't properly cleaned and dried before coating.

Maybe I'm just lucky, but I go by the dictum that just like painting an exposed surface, coating the inside of a tank is 80% preparation, 10% application, and 10% perspiration.

Dale and others, I certainly won't condemn anyone for trying to re-hab a tank, but..like you said, maybe you were lucky, but lots of times even the best of cleaned and sealed tanks fail, usually shutting things down miles from home! I for one don't like having to do things (like removing tanks) over. Once out, throw it out. Get a new one. Just like a 90 y.o. plus radiator...their lives are limited.

Tim,
I agree, if a repro tank is available I'd go for it- new, consistent gauge metal is the way to go if you can.
Re-coating is plan "B" if a new tank is not available.

Unfortunately Plan A doesn't apply to 26-27. There are no new tanks.