
This is what I see with the panel removed (hopefully the photos work as I've just started playing with Google Photos)
The black stuff is thick oily gloop that I can scratch off with a fingernail so I am guessing it is waxoyl? The original wheel arch was rusty and replaced by the dealer back in July when I bought the van so I am guessing he sprayed some waxoyl in after replacing that arch?
Looking down inside I see this. The left-hand side of the photo is the outer body of the van and the right-hand is the wheel arch. You can see a weld at the bottom where the new arch was presumably attached.
So what I need to do now is try to make sure no new rust grows. As part of this I have also considered insulating the van to help fight future condensation. Bongolia has spent a lot of time detailing his process here and here. The basic process is to kill existing rust with Fertan, treat these areas with zinc paint, glue in some insulation and the wax what is left. Having finally taken the panel off so I can see what a Bongo looks like under the skin I have a few questions before I start...
1) What is the best thing to do with the existing weld? Should I just leave it as is or remove the waxoyl currently there and apply something else over the weld? I have read about epoxy primers and seam sealant but have no idea if this is the sort of thing I should consider?
2) It isn't really clear from the photos but there seems to be a gap where the inner wheel arch descends at about where the weld is on the outside panel. So the outside of the van comes down, has the weld in it, and goes down a bit further. From the inside of the van the wheel arch is curving over and descending and I expected it to eventually just meet up with the outside of the van. Instead it seems to stop short, at least in places. Is this normal or fallout from the arch repair? I am a bit unsure as to how to treat this area or how I would be able to get any Fetran or wax into it.
3) With the Fertan do I just spray it everywhere or am I looking just for places with visible rust? There are obviously a lot of areas I can't see (e.g. inside pillars etc) so should I be spraying in these? After 3 days of camping the main area shown in the first picture above is quite dry but if I put a hand up the pillar at the rear of the van (i.e. the left side of the first picture above) I can feel quite a lot of condensation in there.
4) When waxing do I need to get everywhere I can? All I am trying to do is put the breaks on likely rust, not necessarily ensure that I can leave the Bongo to any grandchildren that may arrive 30 years down the line

5) When it comes to applying wax, I don't have a compressor but could hire one. Some people also seem to have used garden sprayers with reasonable success. There also seems to be a dinitrol cavity wax in a spray can that you can get a "wand" for though I'm not sure if it will be up to much or just a waste of money. I assume dinitrol would be a suitable alternative to waxoyl? The same people also do a cordless sprayer which sounds like it might be OK for thinner waxes though perhaps no better than a garden sprayer. Does anyone have any thoughts on these options? I don't have a driveway so will need to do all work on the (very quiet) street outside the house.
Any advice on the above would be greatly appreciated!
Thanks so much
Alan