When I had my home done this summer, I took the siding down to the framework, had a professional come in and do the stucco. They did two layers of black felt paper, chicken wire then did the scratch coat. It sat for two weeks to set up then they came back and did the brown coat. That sat for another 3 weeks. The last coat was color and texture all in one. I have no idea what would happen if you sandblast, how far you could go with that without having a problem. After watching these guys work, I was glad I didn't try it. Maybe someone else on the board knows something about sandblasting existing stucco.