Join Date: Jul 2007
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Any polyurethane will be a temporary finish, repaint in two or three years.
If the gelcoat finish can be restored, search posts from "mainsail" for methods and products, then this would be a better approach.
If the gelcoat is beyond restoring and you can live with a good enough finish that will require repeating in a few years then prep really well, light sand, tape off or remove deck hardware, and use recommended primer.
I would not thin the polyurethane but if you do, only use the recommended thinner formulated for this application.
Hand paint with brush; paint a rectangle within a forearm sweep, 18" square, then cross hatch paint strokes within, move on to next area and repeat. Don't over brush each area. Sweep the paint brush from your shoulder or elbow not wrist for a more uniform brush stroke, and don't stop until the entire area is complete.
This is a least worst finish option.