Looking at the sills inside the b posts im thinking about how to treat them. Most of the liquid options appear to be various solutions of phosphoric acid, fertans rust treatment gets good reviews but at £25 for a litre it's not cheap.alternatively i can buy a normal phosphoric acid for £16 a litre.

Im looking for something that will leave me a converted surface i can then plaster in cavity wax, liquid is preferable considering how difficult getting into the bottom of the b pillars