I had my 1985 Sisu 26 water blasted with volcanic ash two years ago for $1000. The operator put a mesh tarp on the ground which allowed the water to drain through but trapped the ash and 25 years worth of bottom paint. He then folded up the tarp and hauled away the mess. The bottom came out nicely abraded but still quite smooth. I followed in the spring with by rolling on 2 gallons of System 3 epoxy resin, 2 gallons in Interprotect and a gallon of ablative bottom paint.