[freebeard]

Why not a Rainbird with soap injection?

The nice thing about those higher pressure washers is that they can drive a sand blasting attachment. I would think that would keep the dust down. OTOH:

en.wikipedia.org/wiki/Dry-ice_blasting

Dry-ice blasting is a form of carbon dioxide cleaning, where dry ice, the solid form of carbon dioxide, is accelerated in a pressurized air stream and directed at a surface in order to clean it.

Compared to other media blasting methods, dry-ice blasting does not create secondary waste or chemical residues as dry ice sublimates, or converts back to a gaseous state, when it hits the surface that is being cleaned. Dry-ice blasting does not require clean-up of a blasting medium.[2] The waste products, which includes just the dislodged media, can be swept up, vacuumed or washed away depending on the containment.