It's not worth the time and effort to sandblast an old bike frame .
You'd have to strip it right down and make sure that before you paint it that the abrasive is cleaned out of every nook and cranny ,chances are that it will get into swing arm or head stock bearings and cause you grief later on .
Better IMHO to use paint stripper or sand and feather the rough areas and apply a good hard finish frame paint like PJ1 if it's still available .
Blessed are the cracked, for they let in the light. - Spike Milligan
"When you come to a fork in the road ,take it ! When you come to a spoon in the road ,take that also ."