Epoxy - what is blush?

I read about this several times but I’m still not sure what it is.

Would recommend…
you raise the same question at:


Though I am sure there are those on this site who can easily answer this question…you will definitely get a full response on that site, as well as finding it in a search of their archives.

My understanding of amine blush, is the discoloration that occurs…a whitish look…when using epoxy. I believe it occurs when curing the epoxy… if the temperature in the environment is rising…you get blush in the epoxy. So…builders suggest to use epoxy when the temp. is starting to drop.Believe it has something to do with humidity…though those in ‘the know’ will hopefully chime in.


You may have missed this note…
From the One Ocean site:

“The greasy films 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 non-blushing epoxies?

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.”

Blush is
a byproduct of the curing process for epoxy. It’s recognizable as the shinny wax like coating on freshly cured epoxy. The problem with the blush is that it will interfere with the mechanical bonding of any subsequent coats of epoxy or other finishing materials. As a result blush is an impediment that must be removed from the surface prior to completion of the job.

While the blush is water soluable it has the habit of rebonding to the surface if the washed area it is not diligently rinsed, (it’s also preferrable to mechanically abrade any cured surface before addiding additional coats in order to increase the effective surface area for subsequent mechanical bonding). West Systems promotes an alternative method to avoid the necessity of removing blush between epoxy coatings - they suggest exploiting the chemical rather than mechanical bonding properties of epoxy by applying subsequent coats of epoxy to the tacky, but set surface area. I believe the theory is that the various layers will cure as if they were one surface and hence yield only one episode requiring blush removal. This alternate method does present two potential problems - the thicker surface will evidence more catalytic heat and may promote more gasification and trapped bubble issues.

As stated on the other thread, some manufacturer’s products have more pronounced blush. In addition, I find the slower cure hardeners to generally produce less blush than the faster cure varieties. Deciding on the appropriate epoxy mix and the strategy for application should help to make the job somewhat easier.

One final word of caution, both the fumes and skin contact with curing epoxy can cause nasty reactions. Reactions to skin contact sometimes evidence themselves as poison ivy like rashes so be careful and cover skin areas. Open air environs and control of sanding dust can also help reduce unhappy contact results.

Not much to add except 2 points
First at some point you’ll need to let the resin dry completely because you’ll need to sand to remove the valleys created by the glass texture. At that point if you have blush you have to get rid of it. Best plan is you have enough choices with epoxy these days so pick one with no blush problems (both MAS and Raka performed well for me in this respect).

Second epoxy exposure is important. My experience is people take care to use gloves during application of the epoxy (it’s messy and they don’t like the sticky mess on the fingers). That’s good practice, but your greater exposure is during sanding. Even if you sand outside you will inhale huge amounts of fine epoxy particulate so get yourself a decent cartridge respirator ($30) and stay away from those paper dealies.

Sorry, third thing as an afterthought. A trick I picked up is after I wet out the epoxy and it dries tacky, rather than use a roller/brush for the next fill coat I use an autobody squeege. This minimizes the buildup of highspots (glass fiber) while leaving epoxy in the valley (glass weave). You’ll minimize the sanding you have to do, but prepared to have epoxy on the floor cause this is messier.

Have fun.


Sand after each coat.
Before I built my Cape Charles I read about amine blush and worried about it. If you use a slow cure epoxy, let the epoxy fully cure and sand the surface after each coat, the blush will be removed. In all situations, you sand after the epoxy cures. However, it is important to sand after each coat of epoxy. The final coat being either a polyurethane paint or varnish. Remember for great finishes sanding is the key.

If I remember System Three (or maybe Pygmy)says NOT to sand after the saturation coat as you will damage the cloth. You will get a chemical bond if recoated within(I think)48 hours or maybe it’s 24.

So plan your coats so that you don’t have to sand until you have a few fill coats on the cloth. GH

Great info. Thanks. Boat building is
a whole new aspect of paddling for me.

Paddling a boat you have built…
…is very special indeed and you will get praised by someone every time out…

I’m selling one of mine and it’s like selling one of my children…

sanding does not remove amine blush
If you don’t wash the amine blush off you can smear it around while sanding. Wash blush off before sanding. It’s not something to worry about but for many folks first building according to The New Kayaks Shop Book there is some information that is skewed to marketing or CLCs specific designs so that when CLC started selling the kits with no-blush slow cure MAS a lot of information would hype no-blush while denigrating blushing epoxies. A lot of constructions would take 5days to cure if you used slow cure MAS at 55-60degrees so if you used a MAS Cold Cure (advertised as no blush) it would blush in higher humidity conditions. Cold Cure has since been discontinued. I’ve heard the same can occur with System3Silvertip epoxy. The point being that blush is an aspect of most epoxies and some “no-blush” epoxies might develop some blush in high humidity/temp conditions.

The Snoz
I blame my lack of olfactory snozzle sense on epoxy or acetone use. My nose ain’t what it used to be except in size. Keep the room ventilated and wear the respirator while resining and sanding. The respirator, if a cartridge type, should cut out the acetone smell while cleaning tools as well as the mild epoxy odors.

I also wash down twice with ammonia water (not sure where I read that, probably West) and then an alcohol wipe before next coat.

I rarely use a roller now, mainly an automotive squegee for nearly all surfaces. When it runs fast, sometimes a disposable brush and squeegee. I use the squeegee on initial application of the resin on the cloth to spread it, 2nd and 3rd coats. Goes quick, no waste except for some drips.

Guess I was WAY off…however…
if you are building…definitely check out www.kayakforum.com to find out about the benefits of curing the epoxy when the room temp. is dropping vs. rising. I believe you may be able to avoid the developement of tiny bubbles in the finished product…one of those ‘senior moments’…