Originally posted by ap_exclamationpoint
So... How would I go about repairing that filter ring without making it worse? I'm a little worried that under that battery door is an old, leaky battery. Probably should have asked the seller about that one before I bid...
I get some scrap wood, usually 3/4" plywood, trace a part of the filter ring that's in good shape, and cut almost to that line by hand or with a bandsaw. I smooth out the inside curve with a rasp or sandpaper. That's my outer form for the filter ring. Then I find another scrap piece, a stick of maple. It needs to be longer than the diameter of the lens, about a half inch wide, about as thick as the filter threads. I sand one end to an arc similar to the shape of the filter threads. Put that inside the filter threads where the dents are. Then I tap on the other end with a hammer to bend the ring back into shape. I test with a filter to see how close I'm getting. It's one of those tasks where lots of small taps and testing is better than one big whack. There are special tools for the same purpose.
Pentax used corrosion-resistant metal for the battery contacts so there's a chance that they aren't just greenish dust. Sometimes a leaky battery will corrode the single wire leading from the contact, or sort of weld the battery door to the bottom plate. The plate comes off with the three small screws you can see there, so if the door doesn't unscrew easily, just take the plate off and see what's on the other side. Maybe soaking it in some penetrating oil will help.
The auction photos don't show the original hood for the Sigma, but that's not necessary. The original one was meant for film so you want something deeper for APS-C. Mine is about an inch deep but the lens is on loan so I can't check it now. You will probably want to get a hood for it.