Very little. The boat was tarped from the toe rail to the ground, and other tarps were on the ground and over the cradle. So the 'blaster' was effectively working in a closed environment where almost nothing escaped.
>I need to do this next winter, how does the Farrow system work?<
Operator has a double hose from a compressor. One hose shoots the 'media', the other fine spray of water. They hit the surface together. I think the prime role of the water is to cool the abrasion, coz much of it turns to steam - there are not streams of water coming off the boat. The media is sand ground from pumice. Apparently it is milled to different sizes, allowing the operator to load the hopper with media with different levels of 'aggresion'. The surface it leaves is very slightly pitted which apparently provides a good key for overpainting.