the moisture on the ground is not the problem its when it evaporates and your bare steel is exposed to 80% humidity or above it will begin to corrode instantly. above %80 humidity and your sand blasting media will begins to stick together. say your using black sand blasting sand (the cheap stuff) when your sand gets wet is will smear your steel black and will look like wet paint running as your continue to spray that spot. the black pigment is sand and the runny look is the water pushing the sand around.
best to do it on a hot dry afternoon and as soon as you have an area clear of rust. Prime it. if you stop for an hour... prime it. only takes a couple of minets to dry and it protects it from rusting.

right, thanks nala. hears hoping that tomarow will be dryer

this weekend will be perfect for sand blasting. hot and dry