Just like the undercoatings they used to force on new vehicles in the 80/90s; a lot of these coatings can cause more problems, than help.It's slag, to which paint will not adhere. Wire brush + Rustoleum should work. POR-15 is another option.
Personally, I would never Line-X a chassis. If it chips and water gets in between that coating and your frame then you're effed.
I live in CT with salted roads and am not worrying about the surface rust on the slag.