Normally a picnic table isn't done to the same level of craftsmanship inside furniture is and it's not sanded. Having said that though there is no reason it can't. The picnic table I built I sanded it and stained and finished it with a spar varnish. A lot would depend on how smooth the treated wood was to begin with. Some of it is very smooth with only the mill marks and some of it has tear out places where is was roughly surfaced. You will just have to make the call based on the condition of your table.Should I sand it??