Inside is the most important. I don't use the stuff on the outside, I scrape down the chassis and paint with red oxide and enamel.
If you think there will be a lot of mud and crap inside your chassis, try and wash it out first with a pressure washer or a hose. Then let it dry as best you can, a drive around on a hot day will help. Then blast in loads of Dinitrol through all the holes, you can drill a few extra holes in the top or sides of the chassis if needed, and you can get little plastic plugs to seal the holes after![]()
not the best time of year to do this - summer is better.