There's a lad with his series III asking the same thing. It would be good if you could start by running a high pressure hose with suitable nozzle up and down the chassis rails, there'll be a lot of muck to get out. If you've got the holes cut out you'll have better access and be able to see what it's like in there. Once plates are welded on you could treat the inside, perhaps with Fertan or similar first and then with your chosen cavity wax. These will also need to be sprayed in via an appropriate pipe/hose and pump. Well worth doing a good job of treating the inside.I have a couple tins of waxoyl. I take it I should paint it first on the outside then waxoyl, but how should I get the backside of the patch full protected?