Thanks, guys. The lens was coated at one point in its life, but there was no option but to sacrifice what little was left - the front element looked like it had gone ten rounds with a sandblaster and lost. Most Summars are uncoated, and I'm not opposed to uncoated lenses at all, so it wasn't a problem to my mind. There is still some roughness around the periphery, but I made the tactical decision to leave it rather than risk screwing with the optical correctness of the lens any more than necessary.
I polished it with cerium oxide. It's the standard material for polishing glass. It comes as a power, which you mix with water. Ideally, you'd use a form that matches the shape of the glass you're polishing, but lacking such a thing, I did it freehand, very gently, over many hours. I would periodically test it on my Fuji X-E1 to see how it was going and to make sure I wasn't developing any sort of distortion. Perfect it's not, but it's a usable lens now, and from f/4 it's quite good (like most Summars, apparently - some consider f/2 and f/2.8 to be "emergency" settings).