Powdery Gelcoat

the gelcoat is powdering

.....that is known as "oxidization", and best way to deal with it is to use 800-1000 grit sanding water-paper to remove dead surface. Then you can wax and polish if you're old school, or you can use a clear-coat finish like poliglow

http://www.poliglow-int.com/

works great on coloured hulls, white hulls do not shine as good but it is the protection that matters, only have to redo your hulls every 2 years

bhull
Check out this thread:

http://thebeachcats.com/forums/viewtopic/topic/13978

Edited by klozhald on May 28, 2013 - 03:15 PM.

I second what Renovator said. The "powder" is heavily oxidized gelcoat, you cannot repair it, that very shallow layer has to be removed.
Hook up your compressor & buy an automotive air sander, or if funds are tight use your 1/4 sheet random orbital sander with automotive paper. you need water, a trickling garden hose works well...but water & your orbital sander are not the best from an electrical standpoint. Remember the sander is double insulated...I wore rubber gloves, & rubber boots. I had the hull off the boat,(an '83 Nacra 5.7) so I could lay them on their side, & sand on a horizontal surface, much easier, the sander just sits on the hull.
Start with 800, do the entire hull, then change to 1000, or 1200. Have the paper already cut into 1/4 sheet squares, change them as soon as they stop cutting.
I followed up with Collinette polishing compound,(it is just a rubbing compound, ie a very fine grit) & Collinette boat wax. Two years later it still reflects light.
FORGET ANY IDEAS THAT INVOLVE ZERO WORK. You need to remove the powdered coat before you use Poliglow, wax, or any other product. Bite the bullet, put a few hours into sanding, & it will look great. There are no shortcuts, PREP WILL BE 90% OF YOUR TIME.