I did the same as your doing when I put an overlay on my kiss. I sanded down to bare wood and replaced all the inserts. My inserts were total crap and had to go. I pre-sanded all the inserts to get them close to flat and then flattened again after they were installed. Once everything was flat I used some west system epoxy to seal the wood. I spread it over, let it soak and squeegee all of the excess off. Once that had cured I wet sanded to achieve a super smooth base for the vinyl overlay. Installed the overlay with the wet method to get all of the bubbles out and get it straight. Alignment was off on some holes but I found a happy medium. I ended up keeping everything on the lower playfield lined up great and there are some insert holes up top that are slightly off....best I could get it.
Next I did a light coat of two part auto clear (I used Nasons) and then two medium coats with sanding in between. The final coat turned out so well that I decided not to do a final sand and buff in fear of screwing it up. Turned out great for my first time doing an overlay and spraying clear.
Its good enough that I wouldnt buy a new playfield to swap. Much more satisfying then just swapping a PF.
I relied heavily on Vids playfield restoration guide while researching before the project. I had never done any kind of playfield work before. Give me a shout if there is anything I can help with.
Check out the inserts with art still intact after sanding(2nd pic). Yeah I would say they were cupped quite badly. 3rd pic shows new inserts installed.

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....And the famous before pic. This machine had been stored outside for an undetermined amount of time. I built an entire new lower cabinet and all metal parts were replaced. The coin door was one big chunk of rust....what a mess.

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