If the scratchdown the center of the hull has not punctured anything, then it id probably already strong enough for your needs, so don't bother adding resin (thickened or otherwise) in an attempt to reinforce the area.

If the dings and chips are minimal you can just hand sand, or use an electric sander with a medium or fine sandpaper to feather the edges of any dings or chips. If you have some recesses, or deep dings, then I second the opinion that you fill them with thickened resin and smooth them (as has already been suggested) before putting on your paint.

as a trial, get a block of wood and a piece of sand paper -- about 100 to 150 grit -- and start hand sanding the dings and scratches on your boat. If you can smooth them out easily, then keep on going. If you see some progress, but it is slow, then go for a power tool to speed things along. you may want to use an even finer grade of paper with an electric sander so it doesn't cut too fast. Wear a dust mask or respirator so you don't inhale the fine dust that used to be your paint.

If you have gotten 3 years of use from your boat then plan on re-doing this process 3 years from now. Consider it routine maintenance.

hope this helps.

PGJ