Using router and box joint jig to cut plywood panels

How do you avoid tearout when routing plywood?

I just got a Box Joint sled and started to make small drawers from Baltic birch plywood. Is there a trick to prevent the tearout from the spiral router blade on the plywood? I was thinking of either rolling glue onto the edge or sandwiching the plywood between two sacrificial pieces of wood. Any suggestions? - Abbott Fleur

Tim Inman: Perfection is always more difficult. If the little tearout is too much, then some sort of "sacrificial" surface is probably your best answer. Sometimes a little tape can do the trick. Adhere masking tape along the edge before you make your cuts. That jig makes it easy to lay another piece over the "real" one. I'd suggest something like 1/8-inch Masonite or hardboard to protect the finish wood, and let the bit cut clean. You may need to sandwich the workpiece between two sacrificial ones so both the front and the back cut clean.

Chris Marshall: Abbott, I second Tim's motion to use a thin "backer" board behind the plywood and in front of it, if you are getting tearout there too. Make sure your spiral bit is razor sharp and clean. Here's another suggestion that once worked for me with Baltic birch. I was using a dado set for cutting box joints and was getting unacceptable blowout behind the workpiece. So, I brushed on a coat of Zinsser SealCoat (it's just dewaxed shellac) first to lock the face veneer fibers. It dries super fast and worked like a charm to help me achieve cleaner cuts. Then, I used more SealCoat as my final topcoat for the drawer boxes.