Things You'll Need
150- to 180-grit sandpaper
TSP or TSP substitute
Acrylic latex paint
Synthetic filament brush
Although this marine finish is most commonly used by landlubbers, Helmsman spar urethane was designed to serve as a tough but flexible weather-resistant finish that can repel water, oil, salt spray, chemicals and anything nature might hurl at it aboard ship. Painting over this somewhat softer finish will be more challenging than repainting the hallway walls, but fortunately you can find coating systems that will cover virtually any challenging surface.
Rough up the entire surface with 150- to 180-grit sandpaper until it loses its shine. It will not be necessary to actually remove the spar urethane. Wear a face mask during the sanding process, because otherwise you run the risk of inhaling urethane dust.
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Wipe the sanded surface down with TSP or a TSP substitute to further open up the pores of the old urethane finish. Apply the TSP using an ordinary sponge, and follow up by wiping any residual TSP off the surface with a clean, damp sponge. For safety's sake, be sure to wear rubber gloves and a long-sleeved shirt, and it might be wise to continue wearing the face mask. Be sure the surface is completely dry and dust-free before moving on to the next step.
Apply a water-based bonding primer with a synthetic fiber brush or roller. While it is possible that an all-purpose primer might be adequate, a water-based bonding primer is more certain to adhere firmly to the spar urethane, and, in turn, the bonding primer will provide a much better footprint for the eventual topcoat. The typical primer should be dry and ready for the next coat within one to two hours.
Select an acrylic latex paint for your topcoat, and be sure to use a good-quality synthetic-filament brush to prevent brush marks from showing up after the paint has dried. If the area to be painted is larger, a good-quality roller will also lead to a smooth finish. An acrylic latex paint is preferred because it is capable of forming a hard but flexible film over the relatively soft Helmsman spar urethane.
The spar urethane undercoat may make your new coat of paint more vulnerable to chipping or premature failure. To extend the life of your newly applied paint, consider using a scrubbable paint or applying two coats of acrylic latex paint.
If the original Helmsman spar urethane finish feels noticeably soft after completing Step 2, you may want to strip it off before continuing to paint.