Certain models of GM and GMC vehicles (Silverado, Sierra, Express, Suburban, Escalades, etc.) Have a factory coating on the frames that is wax based and applied at the factory by dipping the bare metal frame into the coating.
This coating is somewhat oily and under coatings do not adhere very well to the coating. The under coating may run or “slip” off of the factory coating and may not dry as fast if at all.
This GM coating has been known to peel off and have corrosion present under the coating. (TSB: 19-NA-255)
It is recommended that you do not apply any under coating to these GM frames.
It is best practice to let the customer know in advance about the factory coating and that you cannot under coat on top of the factory coating. The rest of the vehicle can be under coated and rust proofed as normal.
It is not recommended to strip any of the factory coating off of a new vehicle.
On used out of factory warranty vehicles that exhibit peeling of the coating it is possible to remove the coating and retreat the frame as necessary. These frames must be cleaned and free of rust before under coating. TSB: 19-NA-255 outlines a repair process.
All rust must be removed or “converted” using VG-420 Rust Convertor (if applicable) or equivalent following product instructions. Once these steps are completed you may then apply ValuGard Under Coating.