I have three kinds of 27x40 poster frames:
1. Wood frames with the metal tabs you bend down behind the poster backing board. Had them over 20 years. As they're a bit delicate/flimsy, I just keep some Star Wars repros in them;
2. Plastic frames from Michael's (newest) with the metal swivel tabs behind the poster backing board; I keep new releases in these as they're easier to rotate them out.and
3. Metal frames with spring clips behind the poster backing board. They're a pain to rotate posters.
Before mounting new posters I always place them in one of those plastic poster board bags, flat, and weighted to make them nice and flat. Then I place them in the frames.
After a few days in the metal frames, I notice some waves. This is odd, as the frames have some room for the posters and aren't too small.
Here's what I have done:
1. On the wood frames I just use a nice thick piece of cardboard and there are zero waves. This replaced the heavy wood-like board.
2. The plastic frames, which are new, have only rarely had the smallest inkling of waves. No mods done or needed.
3. The metal frames were originally rescued from a defunct school and the "poster" part was just printed onto the foamboard. When I added a poster to these, there were waves in a few of them. When I took out the foamboard and used 2 sheets of cardboard instead, there were fewer waves and not enough room to use the spring clips.
So, in these metal frames, what can I place behind these posters to prevent/mitigate these waves? The extra cardboard sheets have helped, but no more will fit in there.