Sandblasting versus sanding or scraping
Not a fun job, aye? You can blast your bottom with soda, black diamond, sand, etc., but your follow up work might be more, but that could be a good thing too. Not sure how old your boat is or if you have blisters, but if you blast your hull, this creates an opportunity to apply an epoxy barrier coat before applying antifouling. Blasting is quicker, but leaves behind more damage to the gelcoat that has to be faired and repaired, etc. For fairing use a VC product (now owned by Interlux) called Watertight, an epoxy fairing compound (2-part). It is easy to mix, apply and sand. But, as I wrote, you might as well apply an epoxy bottom and I recommend Interlux's Interprotect system which includes 1000/1001 epoxy, then the Watertight fairing compound, then their 2000/2001 epoxy barrier coat and then your antifouling paint. Stick with the recommended number of coats, their procedure of application and pay attention to the temperature that you are applying the stuff in so that you can apply as much material as possible in as short a time as you can, but still give the product time to cure between coats, etc. They lay it all out for you. If you are going to sand or scrape, then you can still apply the epoxy barrier coat, but eliminate everything except the 2000/2001 epoxy before you apply your antifouling paint. If you only sand to the point where you smooth out your antifouling, but don't remove it all, you can then apply the antifouling, but don't put epoxy over a bottom that is not totally clean of antifouling.
