What kind of joint compound over plywood
I personally would never recommend joint compound over plywood. You could wood putty all the screws and other defects as mentioned above or you could try one other idea. How much of a lip do you have between the surface of the sheet rock and the surface of the plywood? My suggestion would be to place a piece of 1/4 inch sheet rock over the plywood if the difference between the two surface is deep enough. Then you could just finish the perimeter with tape and joint compound.