How to fix a bad KF flange?

I have fixed flanges like this by sanding. If your flange is aluminum, this won't be too hard; if it's stainless, it might be fairly tedious and painful!

Start with a coarse-grit sandpaper, around P120-P200, and sand out the visible dents and scratches. Then work your way through a couple of intermediate grits up to at least P600. Sand in a circular motion around the flange to avoid making scratches that cross the o-ring. Make sure to moisten the sandpaper (I usually use isopropanol) to keep the dust down and to plug the exhaust tube and clean its outlet regularly while you work, as you don't want any of the grit to migrate into your turbo.

As a side note, I've found Apiezon L to be a very good low-outgassing grease for ultra-high vacuum, but it's always better to fix leaks if you can rather than greasing and hoping for the best.

I can think of three possible fixes:

1) The easiest "work-around" involves using a teflon gasket. Teflon is known to cold flow and conform to the shape that it is clamped between. With sufficient compression, such a gasket should fill in the gaps that obviously exist in your present flange face. Assuming that this works, you will probably want to keep quite a few teflon gaskets on hand, as each time you compress the gasket, the teflon would get squeezed to thinner and thinner dimensions.

2) Have a good machinist resurface the current flange face.

3) Cut off the existing flange and weld a new one on. This is probably not an option, or you would have done it already.