[Destroyer]

Spanishmak, I agree with everything that Step up here said.. Please allow me to add a few things.

First, if you're going to trailer the boat there is no reason to paint the bottom. Bottom paint is only for when you leave your boat in the water in a slip. It prevents marine growth due to it's leaching action. An unpainted hull is very smooth. The smoother the hull, the less resistance in the water, the faster and more economical your engine runs. (Plus, personally, I think that unpainted hulls just look better...but that's just me)

Second, NEVER sand blast your hull.. the sand will eat through your gellcoat and fiberglass like a fat woman with a chocolate cake.. It will be gone before you blink.

Soda blasting works differently than sand blasting.. when the Bicarbonate of Soda hits the surface of whatever it's being propelled at the grains of soda actually shatter from the force of the contact.... the force of that shattering propels the remains of the grain sideways, and in the process they scrape away the paint off the hull. Soda is an extremely friable material that micro fragments on impact. It literally explodes away the surface material without damage to what's underneath. Multiply that effect by thousands of grains per second and you see how soda blasting works. You need a big nozzle, a really large compressor, etc etc. It's not a job for the home handyman.. call in a professional. They normally charge about $9 a linear foot .. so a 20 foot boat will be about $180 plus travel, set up and clean up.. Under $300 total.

Also, be aware of the fact that you may have blisters in the fiberglass hidden under your paint, depending on the age of the boat, maintenance, etc. If that's the case you'll have to recoat the bottom, but if you're going to trailer the boat you don't want to recoat with a soft anti-fouling paint.
Instead go for one of the plastic based hard shell paints. It will last longer and will not mark up your trailer the way that a soft paint would.

Hope this helps a little