Russ, welcome to the forum.

Need to ask some questions to help you with your problem.

What kind of plaster do you have, Pebbletec, Diamondbrite, Marcite etc.?

Did you carefully follow the plaster vendor instructions?

Are you getting any of the plaster in your waste line when you backwash?

Did the plasterer add any sand in the mix when applying it?

These questions will help us give you better answer to the roughness you mention.

Hope this helps.

Pat