For coating the inside of installed sheet metal panels use 3M 8851 Applicator Wand Kit.
3M 08852 Cavity Wax Plus, Aerosol 18oz
The 3M 08852, Cavity Wax Plus is a low viscosity, corrosion resistant cavity wax coating that is non hardening and self healing formulated for rust repair and corrosion prevention. Cavity Wax Plus is designed for application to internal body panel cavities where corrosion likes to start, (specifically cab corners, rocker panels, quarter panels, etc...) Cavity Wax can be used on replacement parts or with an existing corrosion repair panels. The convenient aerosol can gives you control for application exactly where you want it.