Soda Blasting is significantly less aggressive then Sand Blasting and is much less likely to cause damage to your gelcoat. Personally, I would not have my boat bottom Sand Blasted.
When I was considering what to do with Cara's bottom to remove all the layers of old anti-fouling, I contacted Olivier Beaute. Olivier had done our pre-purchase survey and as part of the survey report he had recommended that we do something to remove the large accumulation of old antifouling at our next haul out. When I asked him about Soda Blasting, he wrote:
"This is a good way to remove the old anti-fouling coats, as long as the operator is careful and does not insist too much on a specific area. In fact, as soon as the white gel-coat can be seen, the operator must move its tool. With 22 years of practice, these people surely know what they're doing. What would be bad is to remove the gel-coat and get to the GRP, which would mean the glass scales coat is affected. This sometimes happens with dry sand-blasting, more rarely with wet-sand blasting."
I also asked about adding a barrier coat after Soda Blasting, and he replied:
"AMEL spreads polyester resin with glass scales, in the mold, after the gel coat is sprayed and before the first glass cloth. No epoxy as polyester and epoxy are not compatible when wet. This is supposed to fight water penetration and it works rather good. The epoxy coat as a « barrier » is not necessary because you already have one. Moreover, before you could apply epoxy, you would need to wait for the hull to be very dry. All you need before the anti fouling is a coat of the primer recommended by the anti fouling manufacturer."
We decided to do two coats of Interprotect 2000e before applying out bottom coat anyway since we had her down to gelcoat for what we hope is the only time in our ownership.
SM #440 Cara
Deale, MD USA