With the frame completed, we'll attach the four trim blocks next. However, before we do anything else, this is the right time to sand all of the pieces, both front and back. Use progressively finer sandpaper and sand all surfaces, including the coves that you routed in previous steps. This will be the final sanding for this project so take your time and your efforts will be rewarded.
To remove the sawdust from the open pores of the oak, use your air compressor with a blower nozzle to blow the sawdust out of these pores. Wipe the stock with a clean, dry rag while blowing with the air nozzle. You should see a lot of fine sawdust being cleaned out of the pores of the oak.
Once you've blown all of the sawdust out of the open pores, wipe each piece down with a clean rag and some denatured alcohol. The denatured alcohol will clean any remaining sawdust but will not affect the final finish.
Place the frame face up on your woodworking table. We're going to be clamping the four trim blocks in the four corners with the chamfered edge facing up and toward the inside of the frame. Place a moderate amount of glue onto the back of the trim block and spread it evenly across the block. Place the block into position with the corner of the frame and clamp into place using spring clamps (as shown in the picture above). Wipe off any glue that escapes from the joint immediately with a clean, damp cloth.
Repeat this procedure with the remaining three trim blocks, and set the unit aside for the glue to dry. There are no mechanical fasteners necessary at this point.