Tony, I have successfully straightened several NOS playfields over the years. It all depends on which way the warping runs. If your playfields are warped length wise, its relatively easy to get it straight with the playfield timbers. The older games most rails were simply nailed/pinned to the playfield from the rear, sometimes with the odd screw. Instead of using nails or pins, use screws and you can really get those rails tight against the playfield. This will get rid of 99% of the warping.
If the warp runs across the width of the playfield then it can be much more difficult to remove. As most games don't have timbers running across the playfield, other than the rear piece. Leaving the playfield turned up the other way with decent weights, heavy books etc applied on a hard flat surface will help. But it all depends on the severity of the warping. If possible screwing decent timbers across the playfield, using playfield holes such as lamp sockets etc for the screw. In other words sandwiching the playfield every 100mm or so between two timbers can really help, especially when left for several weeks.
If you go into it with the mind of improving, not making it perfect, you will have good results. As severely warped playfields usually have been that way for many years, excessive pressure can damage the playfield when being forced in the other direction. Such as splintering or cracking the playfield surface.
I've heard of people using steam, but I'd be really wary of that method, especially with the older playfields.
Good luck, let us know how you go.
