Wow a chance, at long last to help the General!! Try automotive,non rust primer. This comes in red (oxide) flat Grey and flat buff. Thr best for vehicles in Flat grey; and the buff is best for figures. You can generally (ha ha) find this in the large automotive specialist centers. Being from Belgium ins a plus (for once!!) Cheque in your local GB Auto cinq (if you can still find one!!) ask/seek out the Marque Auto cinq; You'll be hooked. I've been using this for some 10 years with excellent results. The cost is about 100 FB (yeah I know...)for a large spray can. If you have problems let me know. I'm hopefully going to restock this week, as there were cleaned out on my last visit.
I could always arrange to sand you a can or two.
Jack

But also don't forget about the Floquil brand of train paints made for Trains they also have a primer which made for hobby use has less pigment shoots better and if you lay it down real light won't need the barrier coat applied. this is another option General. In real opinion I would use the Floquil