Moto Grand Prix
Main Motorcycle: 1973 Triumph TR7RV
Join Date: Oct 2013
Location: Pleasant Hill, CA USA
Other Motorcycle: 1964 Triumph Mountain Cub
Hi Tiger Dale, I have no answer, but a few thoughts. I'll attempt to explain why I feel about what I found in my tribulations.
Not certain the deposits are all from Sea Foam. I sometimes use gas to wash parts. I had a 1 gallon can of California E10 91 octane fuel in a gas can for about 2.5 years. Chevron brand. I needed it to wash some parts. It had rust in it from my tank rust so couldn't be used in motor. Was in metal gas can, lid on tight. Much to my surprise the gas had really gone very bad. Smelled like solvent, was turning dark amber & was thickening. Was beginning to put a coating of some sort inside can. Can was perfectly clean prior. However none had evaporated out. I expected deterioration, but not this bad! That's why I wouldn't just blame Sea Foam. I've never personally used Sea Foam, but it's sold in my area.
I had my Tiger 750 tank stored with gas in it 24 years. Dumb!! It was only supposed to be a few months. But that turned into much longer & bike was for years behind other stored items. Getting bike out of storage the tank had about 3/16" of black gunk inside it where fuel touched metal. I'd stored with full tank of course! Still had a good 3/4 tank of old fuel. I tried many things, but no normal stuff would touch it. I didn't try acetone though. My hunch is it would have cleaned it. Carb spray could slightly dissolve it, but barely. New gas didn't seem to dissolve it at all, but wasn't E10. Just normal unleaded. I didn't try lacquer thinner as it knew it was death to paint.
The local bike shop sold Kreem tank cleaner. An acid material to dissolve gunk & rust. Then you rinsed with water & finally acetone.
The acid indeed cleaned gunk well, however also removed the protective tin/zinc layer on metal. This allowed flash rust in tank & continuing rust for ever more. E10 fuel keeps metal squeaky clean below fuel line, but promotes rust above fuel line. I had to install real fuel filters & change them often. Rust would easily pass through tap & banjo screens & collect around float needle & carb bowl. Finally top inside of tank rusted so if tank 3/4 full it would seep gas from pin hole. I ended up cleaning tank again & using Caswell sealant. So far has worked perfectly. I got 1 drop of acetone on paint & it ate paint off instantly on the first cleaning. When I did 2nd cleaning I used Kreem Tank Mask paint protector. It 100% protected original paint from further damage. Oddly the acid wash didn't effect paint at all. I wanted to keep my original paint & Tank Masked allowed me to do that.
Back to your tank. I would start by draining & filling with E10 or higher E fuel & let it soak for a week or so & see if it softens gunk.
Next I would try acetone. I would use Kreem Tank Mask paint protector first though. You paint it on with a brush. Reminds me of white glue. I did 3 coats then wrapped with Glad Wrap to keep it from rubbing off. It peels easily & dissolves with water. Even 1 drop of sweat will compromise protector. Modern Harley paint is probably much more durable than old Triumph paint, but I'd still protect it. Kreem protects against acetone for certain, I don't know about lacquer thinner or any other solvents. Test it first.
I'm pretty certain if you used any type acid cleaner it will compromise metal surface & promote rusting. They say the metal was tin or zinc plated very thin to act as a lubricant during pressing of the metal. A side effect is some resistance to rust. I will for certain say every tank I've seen acid washed rusted bad afterwards. Not just mine. I would try all means before acid washing. I basically ruined my tank. I would have been better off leaving the gunk in it.
I've learned a lot since then. My hunch is acetone alone or acetone & 2 boxes of sheet rock screws 1-1/2" in tank would have cleaned my tank & saved a lot of heartache.
So I've given no real answer, but food for thought. Kreem Tank Mask really works good, is readily available online. Nobody sold it locally. 1 bottle was just enough for Triumph tank. Harley tank would need 2 bottles.
If it comes to acid wash & tank sealant, Kreem is the very worst sealant. Don't use it. I felt Caswell worked very well. Some use POR15 to good results. Again try to clean it any other way. Many sealers take at least a week to cure well. I gave it 2 weeks just to be sure.
There are a few companies that specialize in tank cleaning, lining & saving paint. Cost is $300-500+ out here to clean & save paint. Not cheap! But much cheaper than paint job. Tank will need cleaning in any case so add that to paint job.