I was very lucky mainly due to spending 21 yrs in a garage and only doing a few hundred miles a year. And as such had minimal very light corrosion on the ends of the outer sills with nothing at all on the inners or any where else of note. Any that needed treating was taken back to bare metal then treated with bilt hamber hydrate 80, followed by several coats of bilt hamber Electrox (zinc rich) followed by either dynax UC or dynax S50 depending on whether internal or external. All told a total cost of just under £200. But a lot of time as removing liners, sills, and front wings plus cleaning and prepping the rest of the underside is very time consuming, ultimately though I feel that I’ve done a more thorough job than paying someone else to do it. As they certainly wouldn’t have been cleaning and repainting fasteners, spare wheel etc. hard work but satisfying