The last thing I did before diving in the deep end with diagnostics is to use a wire brush mounted in a grinder to clean out the leading edges of the inner arches at the back, these had been welded before and had surface rust. As its a mud trap I cleaned them out thoroughly, wire brushed them back, painted the entire area with Vactan rust converter, then hi-zinc primer, then matt black enamel paint to seal them properly. Its not that pretty, but will be all but invisible once the car is back on 4 wheels and will prevent the rust for years to come
I’ve found Vactan to be brilliant stuff and really easy to apply. it gets painted on like milk, goes purple in the presence of rust as it neutralises the rusted bits, and dries black. You can paint right over it, but I prefer to apply hi zinc primer on top, especially if its a non-visible section like this as you can slop it on without worrying about the finish then aerosol enamel over the top of that to give a really solid, durable finish. I’ve used it on much mor evisible areas too, like sills, arches and doors and you can still get good results but needs more care in the prep and between layers/coats.