Two-Part Urethanes & Clear-Coats
By Paul Oman
The following is an introduction into 2-part urethanes and 2
part urethane clear-coats. These 2-part paints are perhaps the
best performing coatings (low yellowing, high gloss, durable and
tough physically and chemically) available for brush, roller or
conventional spray application. We’ll apologize up-front
for any errors that our more urethane knowledgeable readers are
certain to find!
Most, of not all, 2-part urethanes are either acrylic polyurethanes
or polyester polyurethanes. Sometimes the prefix “poly”
is left off. These are also called linear (or aliphatic) urethanes,
or LPUs (linear poly-urethanes). In any case, lots of ‘keywords’
for generally two kinds of 2-part polyurethanes.
Polyester (poly) urethanes are considered the ‘best’.
Compared to acrylic (poly) urethanes the polyesters are more abrasion
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 acyrlic/polyester urethane blends.
Acrylic urethanes are a bit cheaper and generally one notch down
from the polyester urethanes 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.
In my experience, both the acrylic and polyester urethanes 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.
Progressive Epoxy Polymers (www.epoxyproducts.com) sells a white
unmodified (i.e. not a blend, but 100% polyester) polyester polyurethane
with a 1 mil dry coverage rate of 960 square feet ( 40% VOC –
$135 for 1.5 gallon unit). Our white acrylic urethane has the
same coverage and VOC level ($70 per 1 gallon unit) while our
clear acrylic urethane UV Plus (contains max. UV blockers) has
800 square foot 1 mil dry coverage (50% VOC). The difference between
the clear and the white is the addition of the white pigment to
the polyurethane which increases the percent solids.
Urethane Clear-coats are almost a different topic from pigmented
urethanes and much of that is due to UV blocking. UV rays damage
and fade coatings. It yellows and damages epoxies. One of the
best blockers of UV rays is pigmented paint. The pigments block
the UV, limiting their affect to the very surface only. Clear
coatings, of course, have no pigments to block UV, hence, while
they add additional gloss and ‘depth’ to a fine paint
job, they are generally considered to be performance inferior
to the pigment coatings they commonly go over.
There are UV blockers that can be added to clear 2-part urethanes,
but surprisingly a very minimal ‘UV Package” is generally
added to these clear urethanes (I’ve heard that automobile
Clear-coats are an exception). The additives are expensive (about
$8 per gallon) and since most clear urethanes are applied over
pigmented urethanes the manufacturers tend to let the pigments
in the bottom coating perform the UV blocking.
The Acrylic Poly UV Plus offered by Progressive Epoxy Polymers
(mentioned above) is also an exception. This clear-coat acrylic
polyurethane has the maximum amount of UV blockers that can be
added ($78 gallon), while their regular Acrylic Poly - available
in white only - ($70 per gallon) has the tradition minimal amount
of UV blockers found in most other pigmented acrylic urethanes.
Progressive Epoxy also sells both ‘fast’ and ‘slow’
urethane reducers, as well as moisture cured urethanes and, of
course, all kinds of epoxies!
THE HIGH SOLVENT LEVELS IN ALL THESE 2-PART URETHANES PREVENTS
PROGRESSIVE EPOXY POLYMERS FROM SHIPPING THESE BY AIR OUR OUTSIDE