Wax oil and undersell, but make sure you give it a good scrape back on the loose bits first. There's a few threads on what's best for underselling on here, but make sure you keep the drainage holes clear! My side door sill rusted to pieces very quickly from inside out because drain holes were blocked
Run it on a tenner, they only breakdown when the diesel tanks full!
Mk6 53 260 100ps In Colorado red.