Hi desertT1, thanks for this thread. I'm looking forward to seeing your progress.
About the worn ball track along the top apron arch- I had a*lightly* sanded the lower edge of the worn track using #340 paper before spraying the Varathane. My playboard looked pretty bad immediately following the first light coat of Varathane for the white paint splotches, as the photos below show. 24hrs later they almost cleared up and the splotches had completely disappeared by the last spraying.
I applied 4 coats of Varathane, each heavier than the previous spray. The last coat was really, really very thick.
I was initially concerned because some of my acrylic painting had a somewhat rough surface along with the worn arch. I really couldn't sand very much before spraying because the original paint was extremely fragile in many places, thin etc. The thick Varathane filled all the pits and valleys in the paint along with the worn arch making any indention inconsequential to play.
I used about 2/3 bottle of 91% rubbing alcohol and almost 2 Mr. Clean Magic Erasers to get my playboard clean. I was being very observant to see if I was removing any paint as I rubbed, checking the eraser for flakes or stains but the playboard did well without issue. I ended up going over it twice over.
Before using the ME, I had first removed everything that stuck out of the playboard... even rollover wires, outhole bottoms and masked every slit and hole on the underside with masking tape to keep the alcohol and ME off the back side and out of the switches.
The light sockets under the plastic graphics would have been too difficult to remove, and I didn't want ME or Varathane getting in them so I rolled a small piece of paper towel into a ball and stuffed it into the empty sockets.
I had followed up the ME cleaning with Napthia, and it took some elbow grease to get all the dried ME off.
If you are unfamiliar with matching paint color (as unfamiliar as I am) I recommend taking lots of photos to use when you buy paints at the store. Before going to the store, verify the colors in the photo are a close match to your playboard, and retake them under different lighting if they don't match well. If you choose acrylic then get the "Heavy body", I found the lighter types don't cover very well. Also get a cheap color wheel while you're there so you'll know which colors to mix to get a closer match.
I am confident the original paint is permanently locked down under the Varathane. If you choose to spray your playboard before doing any touchup I would be certain the first coat is extremely thin. I had signed and dated my playboard in the covered area under the bottom steel apron using a sharpie pen before spraying the last application. It is easy to tell the lettering is not written onto the playboard, but rather elevated off the board to some degree, enough to cast a shadow on the playboard itself if placed under a strong light. If that was touch up paint in a seen area it would look rather odd.
Your playfield looks to be in better shape than mine was.