Re: 3.5 liter tank pictures
There are products made for de-rusting a motorcycle fuel tank but Crassius gave you a good inexpensive suggestion. I've used small pebbles, sand and BBs. Anyhting that will break up the rust scale and allow it to be flushed out with a good rinsing will work.
Actually vinegar is a good de-scaler and works almost as good as some of the high priced solutions. Let it soak for a while with some type of abrasive, like those mentioned above, shake the tank and then drain and flush with hot water.
Flash rust can be a problem. That is the rust colored coating that appears almost immediately after flushing and while the inside surface is drying. That can be eliminated by adding a few ounces of oil rich fuel mix and 'swishing' it around after the hot water flush.
Now, even though you've cleaned the tank, install an in-line fuel filter between the tank and carburetor for extra assurance.
And show us a photo of this tank on your project. Did you give up on the big one?
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"