Should plywood be sanded before putting in under a mattress?

Yes, plywood should be sanded prior to putting it under a mattress. When you are either building a bed frame, or otherwise using a plywood board under the mattress, you need to sand it, in order to make it smooth. If this is not done, the mattress could catch on the rough parts of the plywood, and cause it to rip or tear, thus ruining the mattress. If the plywood is sanded and made smooth, this will not happen and the mattress will not be ruined. Another reason to sand the plywood is in case you want to paint it. If it is not sanded, then the paint will go on all bumpy, and not spread smoothly on the wood.