The thin lashes that make up the layers of plywood are prone to cracking, particularly in plywood produced from softwood. Moisture escapes the plywood because it ages as well as the wood shrinks. The tiny cracks that form in the veneer layers are known as checking. Another frequent cause of small cracks is irregular contraction and expansion of the plywood. This problem tends more toward exposed exterior plywood than it does to plywood shielded from the weather.
Clean any dirt or debris out of the cracks together with the toothbrush. Dirt and debris may prevent the putty from making a good bond with the wood.
Scoop up a small amount of wood putty together with the putty knife. Press the putty into the cracks and fill each one along its whole length. A tiny sum on the knife allows you to fully work it in the cracks before it stinks.
Scrape excess putty from the cracks together with the putty knife. Allow the putty to harden and cure in accordance with the manufacturer’s directions. Most take less than one hour.
Apply another coat of putty if necessary. Putty applied to wider cracks may shrink as it stinks. Slightly overfill the cracks onto the second coat when the putty shrinks.
Sand the hardened putty and the surface of the plywood smooth using 120-grit sandpaper, a handheld sanding instrument or an oscillating tool with a sanding accessory. Sand another time together with 220-grit sandpaper to get a fine finish on appearance-grade plywood.