Is it possible that you didn't prep and finish correctly? I usually clean the area beforehand with naptha, then after the touch up dries seal it with clear acrylic. I don't think any touch up is going to last very long without a protective finish.

Lowes, and virtually any hardware store. Most paint supplies specialty stores would likely have it as well.

Check your dad or uncle's garage. Almost guarantee you there's an old can of naptha up high on a dusty shelf or lingering among the paint cans. Stuff never goes bad.

For the playfield , any local touch up without any kind of “ varnish “ will not last for long. If the touch up is where there is lots of ball traffic , you can get away with a piece of Mylar that will keep the touch up there indefinitely. For the drop targets all you have to do to protect any touch up , is to stick a piece of transparent packing tape on top of the area you want to protect. It will protect the underlying graphics for ever.