Wet sand in rear sills

I've started repairing some rust around my rear arches on my car and just inside the door shuts. I took my side steps off (snapped a bolt ughh!!) I pulled the sill trims off and noticed everything was pretty rusty. All of the clip holes. The jacking pad hole. I soon realised why. I popped a few of the sill plugs out to find both sills were absolutely chocked full of wet silty sand. Slowly rusting my sills from the inside out.I've always thought my cars been seriously waded at some stage as the silty marks are high in the engine bay and also under the boot floor!. Strange how so much was able to get in the sills and not get out though. I'm going to give everything a liberal dose of vactan. Gravitex the exterior of the sills and waxoyl inside. Should sort it out. I've gravitex'd the rear arch lip which gets stone chips from the tyres. Looks so much more durable now.

The evil rust.