After barewood frames are cut and joined, the next thing I do is fill any imperfections, such as dents or holes in the timber (like in the photo above). Then I give the whole frame a good sand, attention is paid to sharp edges like the back edge and corners. Today I filled/sanded 40+ frames, they were then given a size coat of rabbit skin glue, which is like a primer layer. Wood is absorbent and without size coats of glue, the glue in the gesso would be absorbed into the wood and the bond would be very weak. Tomorrow the second size coat will be brushed on, this is a very thin gesso layer which serves a similar purpose to the first size coat. Some people only do one size coat, but if possible I think it is better to apply two coats, especially on dry or absorbent timbers, it is also just as important to allow enough drying time between coats and layers.