Blush (amine blush) is a waxy layer that forms as most epoxies cure.
Amine Blush - Bad Things with some Marine Epoxy Resins
Before you purchase marine epoxy for your next boatbuilding project learn about how amine blush.
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Blush (amine blush) is a waxy layer that forms as most epoxies cure. When very bad, it is a visible white layer, otherwise it might not be visible, but still there. It is due to moisture in the air during cure. Specifically the amines in the curing agent can react with carbon dioxide and moisture to from ammonium carbamate (blush). Generally it is the cheaper epoxies that blush (those using a very basic amine curing agent).and the top of the line epoxies (sort of amine hybrids) that do not blush - or are harder to make blush. If our epoxy is blushes often, it is probably not a very high end epoxy. Blush needs to be washed off the surface of the epoxy before it can be recoated or painted.
Vendors with blushing epoxies downplay the problem so it can be difficult to determine how bad this issue is.
The best ways to reduce blushing, besides using a no blush epoxy, is to work in warm temperatures (the epoxy sets up faster, reducing the blush window) and with humidity at low levels. Certainly avoid conditions where moisture is coming out of the air as temperatures fall.
How serious is blushing? Here is an actual post on a boating newsgroup:
Some time ago I decided to build the 6 hour canoe to get some practice for a bigger sail design hopefully to come in the future. I used fir ply and fir trim. The construction went well, barring the gun'l split. Next I coated the entire canoe, in and out, with (a leading competitor's 'regular' - blushing - epoxy) followed by 2 primer coats and 2 finish coats of a marine paint from Home Depot, I don't remember the brand. The paint was sprayed on.
Well, I goofed. I forgot about the blush before painting so naturally the paint didn't stick, and it looked so nice for a while. So once I finished the appropriate amount of cursing a sanded of all of the primer and finish coat, wiped the hull down with acetone followed by alcohol and repainted. I thought I was all set but the peeling has come back. So now the 6 hour canoe has become the 6 year canoe..... and I'm about ready to give up.
The best explaination I found about amine blush I found at:
(a great place for wooden ocean kayaks)
Here is what they have to say:
When some epoxies harden, a byproduct of the curing reaction rises to the surface
appearing as a greasy, waxy layer. On contact with high humidity or water, this coating
turns into an opaque white smudge (during or even after cure) which turns into dry chalky
powder with time. This 'blush' will not wipe off easily with thinners or solvents. It can only
be dry or wet sanded after the epoxy completely cures.
This property has a huge implication on the time and amount of labor you need to
expend to recoat or for bonding. Blush free epoxies allow additional filler coats and
bonding without prior sanding! This benefit also cannot be overemphasized if you ever
experienced allergic reaction to fiberglass and uncured epoxy dust.
The way this works is that if you re-coat before the prior coat is completely cured, the
new epoxy will dissolve the top layer of the 'old' coat and fuse 'chemically'. If let cure for
longer, the absence of the blush (surface contamination) permits a clean mechanical
bond. Despite the clean surface, it is a good idea to wet-sand cured epoxy if subsequent
bonding isn't done within 24 hours or so. This assures a good secondary (mechanical)
Epoxies that suffer from blushing are difficult to trust (without a lot of intermittent
sanding). Filler coats can be applied without sanding but it must be done as soon as the
epoxy starts setting or shortly after. Furthermore, unvarnished surfaces (such as the
insides of the cockpit) may turn smudgy white with time which is quite ugly,
The greasy layers that appears on some epoxies are largely the salts of amine carbonate.
Depending on the type and formulation of the epoxy, amine compounds on the surface
combine to various degrees with CO2 (carbon dioxide) and water in humid air forming
hydrates of amine carbonate. This stuff is supposedly water soluble but it will not leave
without some abrasive scrubbing.
Why do some epoxies blush and some don't and why wouldn't everyone want to make
It just happens that the amount of blushing is very related to the shape of the amines .
These molecules in turn determine the physical properties of the cured epoxy. Low blush
epoxy formulations often contain 'cyclic amines'. The geometry of these molecules as
well as their 'monofunctionality' (one reactive side only) improve the surface, slow down
the cure rate and reduce the strength of the cured mass.
What does it all mean? It seems that very slow, glossy, low blush epoxies harden to
solids with lower moduli of elasticity (softer and more elastic) which allows for
deformation and better impact absorption without cracking. Faster setting, blushing
epoxies are generally far stiffer, harder as well as more brittle. At the extreme of this
spectrum lie high moduli epoxies which are never used for clear coating but are
unsurpassed in their strength. These epoxies are used in high tech, high strength
composites that are post-cured with heat.
Once again, thanks to Vaclav Stejskal at One Ocean Kayaks for the above information. Pleas visit their site
and support their business!.
MORE ABOUT COMPARING MARINE EPOXIES - CLICK HERE
Our standard epoxies are non blushing - BASIC NO BLUSH ™ - and PREMIUM NO BLUSH™ -
. What is a Puddle Duck Racer? ( visit their website - PDRacer.com) Our BASIC NO BLUSH ™ marine epoxy is
the endorsed epoxy within the Puddle Duck community.
The PDRacer is a developmental one design racing sailboat that is basically a plywood box with a curved bottom, and is the easiest boat in the world to build. The rules are aimed at keeping the lower 10" of all hulls the same, but the rest is up to the builder. A simple hull can be made from 3 sheets of plywood. If you work hard for 2 weekends, you can go sailing on the 3rd weekend.
What is a Puddle Duck Racer? ( visit their website - PDRacer.com)
Our BASIC NO BLUSH ™ marine epoxy is the endorsed epoxy within the Puddle Duck community.