When using plywood, why is it recommended to prime the back and the sides, but not the surface onto which you will be tiling?

The reason the ply should be sealed on the back and edges is to prevent the plywood absorbing moisture . When the ply is subject to humidity variations it expands and contracts (a small amount) This is obviously not ideal when tiles are fixed to it as the expansion can cause problems with the bond between the tile and the ply. The less the variation in moisture in the ply the more dimensionally stable it is and the more suitable for tiling. For this reason plywood is not (contrary to what many will tell you) a good substrate in wet areas unless it is waterproofed with a tanking membrane first. Personally I avoid using ply where possible. Cement based tile backer boards or wedi boards are a far better background to tile on The reason you dont prime the surface of the ply to be tiled is that the adhesive will bond to the ply better than the sealer. Unless of course the ply is very dusty eg lazy plasterers not cleaning up!! then it might need to be primed Hope thats clear

Its because sealing the adhesive surface will create a barrier which can stop the adhesive being so effective. Plywood has good properties for adhesive to stick really well to and form a water tight barrier which is the desired out come from tilling. hope this helps.

Sam has got it bang on there. Good reply Sam!