One thing that I find I have to do for about every game I pick up is restore the coin doors cosmetically. Either someone has hacked into it with a screwdriver, or it has scratches from coins or general wear and tear. While you may consider this a pain in the butt, it's actually a fairly simple procedure and can really improve the looks of your game. Here's what I do when restoring the average coin door.
Remove the coin door and disassemble it. Clean the pieces which are visible from the outside (in other words, don't bother cleaning the coin reject mechanism if it works already). Soapy water and some 409 should be sufficient to clean the coin door and its pieces.
It's typical to see bent coin door edges and scraping where someone has attempted to pry the coin door open with a screw driver or crowbar. Try and bend these back into shape if possible.
Once the coin door is cleaned, touch up any bare metal spots with a flat black enamel paint (hobby store will have something). At this point, you may be finished if you're lucky enough to get a nice coin door. I always repaint them anyway. Beyond cleaning them, I will lighty sand any blemishes so they don't stick out after they are painted over.
To paint the coin door, I use Krylon semi-flat black paint. It dries reasonably fast, and matches pretty well. Some people use primer when painting metal. I have NEVER used primer and haven't had any problems using Krylon paint, so skipping that saves you some money right there!
Set up the coin door somewhere, then spray left to right from about 10 inches away, starting from the top and working your way to the bottom, going just fast enough so the paint has a wet look (do not dry spray it; that's where it looks instantly dry when the paint hits the metal. The coat will be uneven), but not TOO wet or it the paint will drip! This may take some practice, but you'll get it eventually. Let the first coat dry. You may see evidence of left to right spraying (horizontal lines) in the paint after the first coat is dry; don't worry, the second coat will blend everything together.
I also paint the coin return doors and bezels, and coin eject button bezels separately, using similar procedures, but usually only one coat is necessary for the small pieces. For some reason, the coin return doors tend to be rusty or scratched, so I lightly sand them. The bolts that hold the coin door in typically get rusty or banged up, so I like to put a metal paint stripper attachment to the end of my cordless drill and strip the bolt heads completely (takes a few seconds) and repaint those, too.
Let everything dry and reassemble. Now your game has a beautiful like new coin door!
Krylon Semi-flat black spray paint
metal paint stripper
fine sand paper (optional)
flat black enamel hobby paint (optional)
For coin door parts, just about any amusement distributor has them. Wico and Happs both have popular coin door parts available.
Need to add a "free play" button to a game which doesn't have free play, but don't want to drill a hole in the control panel or coin door? Try rigging up a switch behind a coin eject button. Leaf switches work best for this. Also, an operator may have installed an ugly bar hasp across the coin door, and drilled two large holes in the cabinet on either side of the coin door. Just paint some large carriage bolts black and put them in the holes. Use oversized washers if the holes are larger than the carriage bolt heads.
Sean L suggests putting the free play switch behind a hasp bar hole. While it will be hidden to others, a pen or pencil can easily trigger a free play.
Wayne W suggest to use Novus on the plastic inserts and replace the lightbulbs.
Russ P suggests hiding a small credit button inside the coin return box behind the coin return doors.
Scott W suggest magetic reed switches used as credit buttons. A small magnet in the right place will trigger the switch. This is an old trick used by ops to reset credits on poker and cherry machines.