Done a lot of this in the past..rusty Triumphs and MG's..
Thought those days were gone..
First thing you need to do is hammer/ lever the sill edge seam straight.. from the photo it looks quite sound..
get a bolster chisel in there first.. between the bent lip and the sill.. in order to open up the gap.. and then using mole grips on the lip.. ease the lip back down to where it should be.. nice and slow.. a bit at a time.. I wouldn't worry too much about it.. you can't knacker it any more than it already is.. finally get a dolly or any lump of flat metal behind the lip and hammer it flat and straight..
Then hit the rust patch with an angle grinder.. small area in the centre of the rust patch.. just to see how deep the rust is.. don't go all the way through.. just use common sense.. Don't pi*s around with a wire brush use a grinding disc.. If you can't get rid of the pitting.. i.e. it's all the way through the metal.. do as follows..
Text book is.. you cut out the damage going two inches back into good steel.. then fabricate a repair section and weld it in. The welding will bugger up any galvanising close to the repair so you will want to get some rustproofing inside the sill..
Bodge is.. pull the dent out.. (drill a small hole.. screw a self tapper in and pull the dent out with mole grips).. then you grind away as much rust as you dare .. treat it with a wonder rust killer and then paint it with wonder anti rust paint.. and then do it again in two years because the rust will come back.. Also bear in mind that a sill is a major load bearing part of the car body so not a good idea to grind it too thin..
If you're keeping the car.. use the first method.. if you can weld.. it's a dead easy job.. would take me about two hours.. I don't do it anymore.. but find a good welder.. shouldn't cost much..
and for christs sake don't let him just slap a patch on over the rust.. makes it rust even quicker..
Last edited by seadart; 05-04-12 at 22:00.