The '74 360 I just picked up, in spite of being a mess, had a decent tank (no dents) and the rust inside was quite minimal. I removed the petcock (what a mess) and made a blanking plate from gasket material and after previously filling it with hot water and Super Clean to rid it of it's  greasy junk and scale, I filled it to the top with vinegar (gentle stuff) and the results are great, I will neutralize with water/baking soda. I am now thinking about using Ospho to convert any remaining rust/flash rust and coat tank. Ospho says all good, give it 30mins of rotating tank to coat all areas and then rinse good with gas. Yes, I will look in and see gray vs that nice shiny looking new metal but I like the idea of keeping this ancient tank in good health. Any thoughts? Has anyone done this on this forum? Picked up a nifty trick of using small light chain (brass) and shaking tank with this soft material and some vinegar to break up any scale. Works well and do not have to worry about leaving a screw/nut or two behind lodge in tank somewhere and of course easy to remove. 

If the rust wasn't heavy, scaley and left deep pits then I wouldn't line the tank. Lining isn't necessary unless it is endanger of leaking due to heavy deterioration. If you've got it cleaned I'd put some 2 stroke oil in it and slosh it around and dump out the excess. That'll prime it for fuel and keep it from flash rusting. Keep fuel in it and it'll be fine.

heavy rust. Let the acid sit BUT check every 15 mins.
Same method for 10 plus years. Check for those awesome pin holes !!!

Standard TC rated two stroke oil does a pretty good job of protecting from rust. 

Adding a little oil to the fuel every tank and before storage will help prevent future rust from forming in the tank

Also, ethanol-free clean gas is a useful idea if you can find it in your state...

All good comments and ideas - one correction though, Ospho is not a lining (like Caswells, etc) it is a rust converter that chemically converts the rust from iron oxide to iron phosphate. 

I don't think I ever used Ospho but have used a similar phosphoric acid treatment...  it was a thin green liquid, can't remember the name...  on the inside of a tank that wasn't rusted too bad and it worked very well.

Yep, its thin and green and does change the shiny metal inside to a dull gray during the conversion process
 

I found the bottle... kept it because the stuff worked so good.  (And yes, it turned metal grey)

Brand I used was "JASCO"

Sidebar:  To respond to a quote can be tricky...  you need to click in the quote and then move mouse to lower right and then click red "insert paragraph" arrow...  that will allow you to type below the quote.

 


.