You can buy (or make) removable mylar repair disks.... I have some on order to replace the homemade versions I was using on 1 of my games. Basically concentrically larger circles build up the cupped insert. Good solution for a playfield that isn't worth a huge repair.

http://www.oldpinballs.com/store.html

Hi textrivers, I'm the owner of oldpinballs.com and maker of the playfield insert repair discs here in Australia, and thanks Wickerman for the recommendation.

I designed these discs for one of my own machines many years ago and simply sell to help others in the hobby. All the adhesive discs are machine cut on a wide format cutter which I own.

textrivers if you send me a PM we can discuss a custom design and cut for the multiple inserts on you title.

You can just fill them with clearcoat.

The clear reinforces them from further cupping and makes the playfield dead level.

https://pinside.com/pinball/forum/topic/vids-guide-to-ultimate-playfield-restoration/page/17#post-1717646

Are you using the right description term here? Are the inserts really cupped, or just sunk? I understand cupped inserts are common in other makers EM's of this era, but I have rarely see a 70's Williams with cupped inserts. Sunken, yes, all of the time.

If this is the case, just remove the inserts with a heat gun or blow dryer and then reset with superglue gel and press flat. It will not take you too long to do and the results are very acceptable.

Watch this series of videos on youtube. 4 parts. I have had great success using this

I would like to add, I do not recommend filling with Polycrylic, it is my understanding that you have to wait almost 3 months for a true cure, then sand down the inserts flat. A lot of work that you already said your not ready for.

In these video's, this guy sands down the inserts, that's too much work and trouble and defaces your inserts. Just Novus 2 then 1 then inserts to a nice bright shine. If you do not want to go through all of the videos, just watch parts 1 and 4.

Here are the rest of the videos.

Yes, you have the correct understanding. GL

I'm only 6 years late to this post, but here I am! Same problem as the original poster, but I was thinking of just replacing the cupped inserts with new ones. Most of the inserts on Grand Prix are unmarked, so no modifications needed.

Are there any considerations I should keep in mind if I do a full replace on that many inserts?

These are sold at TheEMScoreKeeper.com

Free shipping and multiple sizes available.

Why not install a playfield protector?
https://www.playfield-protectors.com/Home/Details/y9uRLmPj4EOgKkcLF47sXw?catgoryid=22&name=Grand%20Prix%20%7C%201976%20%7C%20Playfield%20Protector