This really depends on how big the sheet of plywood you’re working with is. If it’s quite large, then using a belt sander is your best bet for reducing the thickness of plywood. However, you will need to be extremely careful so that you don’t accidentally end up ripping through the layers.
It’s about finding the perfect balance, here. A 150-grit sandpaper would be ideal, but if you go for anything higher than this it will be too rough on the veneers.
You can also use an orbital sander to reduce the thickness of plywood but, again, it’s about finding the perfect balance. Use a 60-grit sandpaper and make sure you’re applying a very small amount of pressure to the plywood.
If you’re working with smaller sheets of plywood, you can reduce its thickness using a handheld block plane. This will allow you to shave some of the thickness off without exposing the layers of glue that hold it together.
This will take a bit longer, but it’s the right tool to use if you want perfect results. You’ll need a steady hand and some patience, but the pay-off will be worth it in the end.