Doing a simple visual check may lead you to some big lows that may require filler. When mixing up your fairing compound with resin, try to use the same resin the boat manufacturer used.
But before we put on our barrier coat, we sand the ready to paint bottom with a 6" DA using 80 grit 'Hook It' disks (we use Hook It, since we will be using a lot of disks and the stick it pads deteriorate with all this disk changing). With this step you will have knocked down the 'big bumps' and this will pay big dividends as you attack the bottom of the boat with a long board over your head.
We recommend adding some body to your barrier coat so that you are in effect filling in the minor low spots as you paint. That way, when you hit the boat with a long board, the high spots will knock down quicker and easier and meet your new, higher lows.
The next step is the simplest but most important step to do - inking the bottom. You absolutely have to apply marking fluid to the boat so that as you sand, you know that every inch of the boat has been sanded and your lows (and highs) have been exposed. We like to use indigo ink, diluted with denatured alcohol, wiped on with a micro fiber rag. Using microfiber lets your hands feel the imperfections in the paint, which you can note by applying heavier ink.
After you have inked the bottom, it is time to bring out the big guns of fairing - the 30" long board. Be sure to use a light one since you will be working overhead. Your choice of anti-fouling coating will dictate the grit of abrasive you will use. BaltoPlate or VC Offshore you will want to use 150 -180 grit. If you use SR-21 or VC-17, you will want to use 320 grit.
Once you have selected your paint and hence your grit and the bottom is inked, it is time to attack the beast. Since the task of long boarding is fairly mechanical, and there is really no hard starting line, we recommend starting in the middle of the boat, since this is where the most volume of sanding will take place. We recommend starting in the middle, if, for no other reason than for sanity. As you head toward the ends of the boat, the acreage actually declines and you actually feel like your productivity is accelerating, which will mean a lot to your aching shoulders.
The key to long boarding is to keep the curves fair by straightening them out. By straighten out, I simply mean that you must sand in a big "X" pattern. For example, while you are looking up at the boat, start a stroke beginning at upper left and ending at lower right, followed by a stroke that starts at upper right and finishes at lower left. You must do this all over the boat, overlapping as you go until all the ink is gone. The longer the strokes the fairer the bottom will be. This is key, because the last thing you want to do is go back over the bottom after you are done.
You will want to find a piece of hose with the approximate radius of the curve at the keel hull joint. You will have sanded up to the keel with the long board, but will have had to stop at the intersection with the hull. Long strokes will be tough here. This is also a good place to use Stick-It sand paper. Stick it to the hose and sand in the radius of the keel hull seam in the "X" stroke as well as you can all along the seam - forward to aft.
The other exception to long strokes is up at the water line, where, if you are not painting the topsides, you will want to stay off the paint. You will have to start and stop your sanding strokes more or less right at the line. Good masking tape is key here. You may have to re-mask the boat after sanding just to be sure you do not end up painting up to a jagged or thick edge. This little cosmetic oversight would definitely diminish all the time and effort spent on the rest of the bottom.