Ask Sail: Wax-on Poly Paint

I have built a 33ft Kelsall-designed catamaran with vacuum-bagged cored composite panels. I finished the exterior with International’s two-part Perfection polyurethane paint (in white)...
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Julien Cloet from Montreal, Canada, asks:

I have built a 33ft Kelsall-designed catamaran with vacuum-bagged cored composite panels. I finished the exterior with International’s two-part Perfection polyurethane paint (in white). Last year, to improve the boat’s looks and to make it easier to clean, I applied 3M Scotchguard liquid wax to the hull topsides. One month later, after a few rainy days, the hulls were covered from stem to stern with long vertical black streaks that I can’t wash off. Any suggestions?

Don Casey Replies:

Manufacturers almost always discourage waxing linear polyurethane (LPU) paint. The paint does not need a protective coating, and its surface is slicker and harder than wax. The waxed surface is more likely to attract dirt, which, when moisture is added, creates the black streaks you’re seeing. Softer liquid waxes tend to be most susceptible.

DonCasey

You need to strip off the wax with a strong detergent and perhaps a solvent. The bare Perfection paint will better resist streaking; you can help it by washing the boat frequently. If the paint is beginning to lose its gloss, applying a polymer sealer such as Awlgrip Awlcare should give better results than wax. Some boat owners buff an aging polyurethane finish with an ultra-fine polish like 3M Finesse-It II Finishing Material. This cleans rather than coats, but it also cuts the hard surface of the polyurethane paint, and therefore is only suitable late in the life of the finish. 

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