Rough Plaster - Acid wash or sand?

Hello All,
I opend my pool last week and noticed that the plaster in the spa evidently became rough over the winter. Strange thing is the pool it self is fine..go figure. Anyway, shouldn't be a big job to either acid wash or sand myself. My question is which is better. I can certainly take the proper precautions for doing an acid wash(rubber gloves, proper clothing, breathing mask, etc.) if that will work to smooth out. But what would be the proper acid/water ratio to use? Or if sanding what grit sandpaper should I use.

Thanks in Advance!

Mark