Sanding for the purpose of removing flaking paint and roughing the surface to give a better mechanical adhesion for the new paint is normal. If you think you want to remove ALL existing paint (this would be for other reasons than just refreshing the existing surface) many will opt for a chemical stripper to speed the process. If you're doing this project in a commercial yard (rather than your own backyard) you need to check on any restrictions. Most common are to have a scavenging device attached to your sanders and in some cases enclosing the vessel completely with plastic tarps may be required. You may be able to avoid this if you WET sand.......... but by all means check your contract.
Even if you're not in a commercial place, a good neighbor will contain the nasty dust the best he can. The dust can settle on your neighor's cars, plants and kids playthings... so it's good karma to be considerate.
Normally, ablative paint doesn't flake off the way hardshell paint can if it wasn't properly applied. Sometimes a good power wash will solve that problem. But here's my first piece of advice.... and it's probably too late to do it now...... but you should have the bottom cleaned just before haulout while the existing scum is wet, since it is easily removed in that state. If that's not possible, then get on it as soon as the boat comes out of the water. Using a scraper and the power wash gun you should get most of it. Again, letting it dry will increase removal time dramatically.
Once you've removed any loose paint and are satisfied that no repairs are needed (such as blisters or gel cracks) you can lightly sand the entire surface to begin the painting process that will be described on the paint can's label. My experience is to always refer to the manufacturer's instructions. Temperature and humidity are especially important. So pay attention to their instructions.
Here's my comment on sanding: do not be too agressive!!!! Stay away from disc sanders and grinders... they will chew right through gelcoat and fiberglass in an instant... Your goal is to simply rough up the surface so the next coat of paint sticks. I prefer a random orbital sander, 5 or 6 inch is easy to hold with one hand. Regarding grit .......... unless you're trying to remove something, 120 to 180 is more than enouogh. If you're trying to remove paint from an area to make a repair then you could go with heavier grit. but just be careful... (I say this from experience, trust me)
Using ablative paint is common in seasonal areas. It's also common to apply new coats right over the old ones at the beginning of each season. It's really an easy job if done regularly and shouldn't take more than a day. Here in So Cal where everyone leaves their boats in the water year round, it is much more common to use the hardshell paint so we normally go a few years between re painting.... which requires the cost of a haulout and often additional prep work.... on the other hand... most of us hire dive services to regularly clean the boat's bottom which in turn extends the time between haul outs and keeps us informed on existing conditions.
In summary, my points are 1. know the regs for your area regarding sanding and painting. 2 Try to clean the bottom right before or right at haulout to make it easier to remove scum. 3 Be careful with sanding... no disc sanding or grinding, try orbital sanding with medium grit. Or simply use a sanding block with the heavier grits (unless you're my age, heh, heh.) 4. if you think you need more than just repainting, check here or with the boatyard for advice. 5 AND MOST IMPORTANTLY.... READ THE PAINT MANUFACTURER'S INSTRUCTIONS
Just had another thought, since this sounds like your first time working with bottom paint. There is nothing delecate about bottom paint. It's nasty stuff. 90% of the time you will be simply rolling new paint over the old as part of your boat's periodic maintenance. As I said, it is nasty. Besides copper there are other goodies in it that can be harmful. Many locals are trying to restrict copper content in bottom paints... that's something you might want to research for your area. So......make sure your wear protective gear and for god's sake don't try to clean brushes or rollers. Just go to home dept an pick up a half dozen 3/4 inch chip brushes, a 4 pack of inexpensive roller covers and a sack of shop rags. Oh and a 3 foot extension for the roller handle will reduce fatigue during application. Remember, just throw everything away rather than trying tp clean any tools. If you have your boat on jack stands you might ask the yard manager to move them so you can paint under the pad. Or you can do what I've always done and leave the paint and a wide chip brush nearby,.. a six pack and some cash will make a nice tip when you ask him to paint under the pads before re launch. He will let it hang in the slings till it's dry enough to drop back in the water. He'll probably offer to do it anyway. Okay, can't think of anything else. good luck have fun.
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