There is only one way to get rid of rust and ensure it won't come back and that is to completely cut it out and replace with good metal. Apply a good quality primer to all surfaces.

Unfortunately early Holden's were poorly primed on internal surfaces and area's such as sills and rear quarters tend to rust from the inside out. Sills in particular can appear solid until you get them back to bare metal and discover they are thin and pitted, they are notorious for rusting out from the inside.

Vans, particularly windowless with large metal surfaces can accumulate a lot of condensation which runs down to the lapped seam on the inside and makes its way into the joint. This also occurs on the roof seam as well. The best defence is prime and paint everywhere you can get to and then fish oil all seams and internal area's.