Been there in when I first got mine. A Chassi is the only place for Chassi Saver and it wants some of that rust to hold onto. Don't get to smooth and fancy with your prep. Better concentrate on getting rid of dirt and grease. After your wire brush and or sanding, brush or roll it on and don't be skimpy go for the build.

DON'T use it anywhere your going to have real paint down the road. It won't feather for... Shoot what's that word?
It peels off in sheets from smooth new metal also... it's kind of a one trick pony to my mind. An expensive trick. Especially if you get it in your lungs.
A screw in the lid helps for multiple use and it's better to buy quarts than a gal.
But epoxy primer is my preference.

Give me ambiguity
or give me something else