Rust

That pic your showin, it ain't clean son. You need to remove all the corrosion from the metal to clean bare grey colored steel. If you leave anything behind, then covering it up with epoxy primer ain't gonna fix it. Its gonna keep on going.

Naval jelly and plastic to hold it in place. Keep reapplying until you have clean bare steel. Work small areas maybe 20" square at a time. For your rinse water use a mixture of water and trisodium phosphate and a wire brush to scrub to neutralize the acid and prevent flash rust. You buy powdered trisodium phosphate at lowes or home depot in the paint section for about $7 add a couple tablespoons to a half gallon of water. Stir it up into solution.

Primer your clean steel with a good professional epoxy primer. Not rattle can ****.