Get long lasting rust proofing with 3M™ Inner Cavity Wax. 3M™ Inner Cavity Wax – Amber will seep into gaps and along welded joints for excellent rust proofing. Use as an aerosol or with an applicator gun. 3M™ Inner Cavity Wax - Transparent has a heavy consistency and is usually used for engine compartment areas or where you like to protect accessible bare metal parts, i.e. use it for areas where capillary action is not required.