How to remove white marks from PLA
A quick blast from a heat gun will very slightly reflow the surface texture and eliminate white marks. 

However, it's important to avoid over-heating the perimeter layers or you'll see them soften and sag into the infill. So wait for the heat gun to get fully hot and then use a short duration of high heat. Let the part cool between attempts if you don't get it all the first time, or need to clean up a large area. 

Incidentally, the heat gun will also help clean up strings from travel moves.