Sealing and Touching Up the Factory (OEM) Undercoat on the Undercarriage

The truck with undercoating is to throughly warm the vehicle like in a paint booth, this takes care of trapped moisture and helps with adhesion of the undercoat. On spot repairs I have used hot air guns and even high intensity lights to warm and dry the area. Good luck.

Clean the lose rust , hit it with rust reformer , Work it into the metal while wet with a wire brush , Prime and paint or just paint it . Wipe clean with thinner and light buff with a brass brush between coats . That's about the best I've come up with .

If you're under warranty, have the dealer fix this. Otherwise, This is the underside of the car, not a "show" finish. If you have to repair it, peel the coating back as to its loose limit and trim off. Apply primer and don't worry about the depth. Then apply any rubberized seal. I've even seen "Flex Seal" spray used effectively for this. Just apply until the surface is smooth.