shouldnt make a huge difference I dont think. The idea is the smoothest flow of water possible with the least turbulence, 60 grit finish is so much less an obstacle then the giant, sharp, casting marks that are 90 degree angles and such, also causing each vein to vary in size. I wouldnt worry about smoothing it or roughing it, just dremel the castings and leave it. The turbulence cause by the valleys created by 60 grit paper would be negligible. The only reason I could see for that is to make lateral sandings the way the water flows to help 'tracking' but thats unnecessary.