After your panel is cut out, there is an important step here....you must sand the edges on both side...the corner of the edge, if that makes sense. Because paint will not stick and fail at a sharp edge so we must make it smooth...it doesn't take much just knock the sharpness off.  Once you have sanded it I blow the saw dust out of the cracks so the caulking will get in and fill the porous edges of the plywood. This is the area of the most weakness in MDO so I feel that this requires the most attention. Fill the voids in the edges as well I using the tip of the caulking gun in these areas to force fill them.

This is how I seal the edges I use top quality ELASTROMERIC latex caulking...I just happen to use Sherwin Williams’s products. First I will apply a small bead down the edge to start. Then I will with a wet finger (I recommend not licking it to make it wet, a wet finger is easier to wipe clean) smooth the caulking into the edge. While the caulking is still wet wipe the top edges or top and bottom faces of the MDO clean of the excess caulking with a dampened rag. If you tape the face and the underside before you apply the caulk you don't have to wipe the excess off and it leave a clean face this tip was added by Jane Diaz. The caulking will show when you finish coat and sanding caulking is un satisfactory in my opinion...the wiping is the fastest  way.

When I get my MDO in from my supplier I usually get 2sided and I get it un-primed. I do not trust factor primed MDO (this is another story). I spray out primer on the whole piece with my Grayco airless. I do not spray the edges yet because I will seal them before I finish coat. Then I can store for future job and when an order comes in it is primed and ready for cutting to size.