The rear arches on my classic look sound and there is no rust under the arch or on the bumper fixing. However looking in through the boot between the inner an outer arch there appears to be some surface rust just starting on the seam below the fuel filler

Its hard to get in there but i have managed to dry/clean it and coat it in rust killer liquid.
My plan is now to allow it to dry and then paint it with Rust Beater paint before coating in a top coat then clear waxoyl, is this the way to go? Any recommendations of what to use?

Fuel filler cap is nice and watertight so i don't know where the moisture comes from to cause the rust?

 

Condensation?

Or it's coming through..........

 

Put you hand right up in the arch and feel for holes get over looked this bit due to shockers being in the way a little, its a rust trap up there

 

underside is fine, i completely rubbed it back last summer and undercoated then undersealed the entire arches etc

Must be condensation as there is no damage to the seal/seam.
Is it worth waxoyling inside the rear arch or should i just use rust beater then topcoat with normal paint?