Polyester (poly) urethanes are considered the ‘best’. Compared to acrylic (poly) urethanes the polyesters are more abrasion resistant, stain resistant, and more chemical resistant. You’ll find polyester urethanes on jet airplanes, and on the floors of the hangers these airplanes live in. Boat owners should note that operating a boat in water, especially seawater, is very much a chemical environment. Two very well known 2-part marine paints, (we will not say their names) are polyester urethanes.or acrylic/polyester urethane blends.

Acrylic urethane coatings are a bit cheaper and generally one notch down from the polyester urethane paints in terms of toughness and chemical resistance, but still above ‘regular’ paints. Acrylic urethanes are found in clear-coats used in the automobile industry and ‘city water towers’ that grace many small towns in America. Boat owners should note that Awlcraft 2000 (tm) is an acrylic urethane, as probably are most of the 2 part urethanes sold in marine catalogs that don’t specify if they are polyester urethanes or acrylic urethanes.

APPLICATION PROPERTIES: Both kinds of urethanes contain large amounts of solvents and thus have a strong solvent smell. That said, additional solvents are often added during application. In the urethane world solvents are called reducers. There are ‘fast reducers’ for spray application. These speed up the time it takes for the urethane to ‘gel’ on the surface. “Slow reducers” slow down the gel time for more working time when applying by brush.

Brushing on a two-part urethane is not like brushing on a varnish or oil based enamel. The thin, almost watery urethane (you’ll probably need two coats or more to cover) starts to ‘gel’ on the surface quickly. Unlike varnish, you’ll get 2 or 3 brush strokes and then, like it or not, it’s time to move on. An application method called ‘tip and roll’ gets almost sprayer like results by applying the urethane with a roller and then gently removing the roller marks with the tip of a brush. If is likely you will not be happy with the results of a brushed on application on such things as boat hulls, epoxy table tops, etc...

In my experience, both the acrylic and polyester urethane coatings go on about the same, but the general view is that the acrylics are slightly easier and friendlier to apply and, perhaps, repair.

Urethane’s weakest link is their adhesion. Because of that they are often applied over an epoxy primer. Besides priming the surface the epoxies tend to ‘level the surface’ too, important because the high gloss urethanes will show every flaw in the subsurface.

Recoat window for two-part urethanes is about 6 to 16 hours. Beyond that, sand lightly. Apply only in good, dry weather, as urethanes are moisture sensitive during application and curing. The coating will become dry overnight, hard in about 3 days, with maximum hardness in 7-10 days.

EVALUATING YOUR TWO-PART URETHANE: Obviously, the first question is, is it an unmodified polyester polyurethane or an acrylic urethane? Next, what is the price (two part urethanes sold in marine catalogs are priced sinfully high)? Finally, look at percent solids.

Percent Solids: these coats, like most other coatings, consist of some amount of solvents which evaporate away (called VOCs - volatile organic compounds) and what is left behind is the ‘solids’ of the coating. A coating with 40% VOC has 60% solids. Apply a 10 mil (1/1000 times 10 in inch units) coating of this product and when dry you will have 6 mils on the surface. Some manufacturers describe it in terms of coverage for 1 dry mil of their product. A coating with 0% VOC (most epoxies) will have a dry 1 mil coverage amount of 1604 square feet. A coating with 50% VOC will have a 1 mil dry thickness coverage rate of 802 square feet ( you applied 2 mils over the 800 square feet and 50% of it evaporated away).

The higher the solids (the lower the VOC) the more paint you are actually getting on your surface. One of the leading vendors of ‘boat hull’ 2-part urethanes does a wonderful job of providing their technical product information on their web site. Their web site reports 1 mil dry film thickness of 570 square feet for their original polyester urethane and 846 square feet for their version 2 polyester polyurethane. Their acrylic urethane product (introduced around 2000) reports 512 square feet at 1 mil dry coverage.

LPU paints offered by Progressive Epoxy

(cannot be sold in California):

Find the Polyester or Acrylic Polyurethane UV in our Marine Catalog - click here -- LPU paints