pine sap?

I found an excellent product. It is sold by NAPA auto parts and is called "NAPA Rust Treatment". Item # 765-1232. I've used it on old rusty metal I've wire brushed, but wanted to retain the "old" look. It works fantastic even when submerged in water. It dries clear and has a dull finish. Hope this helps.
Mike A.

Many thanks Mike - will give it a try.

You could try Rustoleum Red Primer. It has fish oil in it, and actually works quite well in penetrating existing rust and preventing additional rusting.

You'll want to brush off loose rust, apply a good coat of red primer, and then let it dry for a couple of weeks. Longer in cold weather. Once it's hard to the touch, it can be painted with a good oil-based enamel.

Don't remove all the rust - the primer needs rust for the oil to penetrate. Otherwise, there will be a slick of oil on top of the primer, which could make top-coating difficult. However, I've had good luck by wiping down dried primer with a degreaser and had the top coat stick just fine.

The Rustoleum Red Primer contains no lead, although early red primers had red lead oxide in them.