There are a few different methods that can be used to straighten bent or warped baseball cards. The key is to do it slowly and gently so the card isn’t damaged in the process. It takes some time and patience, but it’s very possible to restore the straightness of cards that have become curved or bent with the following techniques:

One of the most common methods is to use heat. You can use a clothes iron on its lowest heat setting, with a protective material like typing paper or tissue paper placed over the card. Slowly move the iron back and forth over the card, being very careful not to let it directly touch the surface of the card, for about 30 seconds to a minute in each area. Check the progress frequently. The heat will gently relax the fibers in the cardboard and allow it to flatten out again over time as it cools.

Another heat option is to put the card between the pages of a heavy book and leave it for several days. The weight of the book will help flatten it and the heat from sunlight or indoor warmth may also aid in the straightening process over an extended period. This takes longer but requires no direct heat application.

For cards that aren’t too badly warped, you can try carefully bending them back into shape. Handle them only by the edges and corners, and slowly and gently flex the card a little at a time until it is lying flat again. Go very slowly and stop if you feel any risk of creasing the surface. It may take several bending sessions over a few hours or days to fully straighten it.

If the crease or curve is more severe, you can try placing weights on the card while it dries. Cut some heavy cardstock just slightly larger than the baseball card and sandwich it in between with the warped areas face down. Then weight it down overnight using heavy books or bricks. In severe cases you may want to leave the weights for 2-3 days to allow maximum flattening.

For very curved layers, you could try lightly humidifying the card before flattening. Use a humidifier, hot shower, or spray bottle to lightly mist the surface and edges – don’t soak it. Then sandwhiched between cardboard inserts and weight it as described above. The added moisture will relax the fibers further to help it regain straightness.

As a longer term straightening method, you could try building a custom straightening rig. Cut and tape layers of sturdy foamcore or cardboard slightly larger than the cards to use as rigid supports on both sides. Layer the cards in between with weight on top and leave undisturbed for several weeks. The constant mild pressure may help cards that are deeply creased regain straightness over an extended period of time.

Always handle cards by the edges only and go very slowly with any bending or application of heat, steam or moisture. Patience is key – it can take days or weeks depending on the severity. Only use gentle heat and weight/pressure, checking the progress regularly. With care, even badly warped cards can usually be restored to lying straight and flat once more. Proper storage is also important afterwards to prevent recurvature. With the methods outlined here, it is very possible to salvage cards that have become less than mint due to bending or warping over time.