Blush

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Erik, Belgium
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Blush

Post by Erik, Belgium »

2 of us builders here in Belgium have had no blush so far with a local epoxy brand, Wilsor. Another fellow builder however is encountering blush in the fill coat. I thought that it was dependant on the epoxy brand or epoxy type, whether blushing appeared or not. Now I 'm questioning myself if humidity might be the reason of the blush. I 'm interested to know what factors can trigger blushing.
FYI: the temp when the fill coat was applied was 15°C, and it was done indoors.

Erik, Belgium.
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Glen Smith
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Post by Glen Smith »

This is from One Ocean Kayaks:

What is Blush? - technically speaking

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.

Erik, I use a local epoxy which is known to blush but if I use it in my workshop during the Winter when the humidity is below 40%, it does not blush. If I use it when the humidity is above 50% it always blushes. Between 40% and 50% it sometimes blushes.

I always allow this epoxy to dry for 24 hours then wash the surface with warm water and a scrubbing pad. Then rinse with clear water and dry immediately with paper towels. This removes any trace of blush. Then the epoxy can be scuff-sanded and recoated.
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Doug
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Location: London, Ontario, Canada

Post by Doug »

MAS Epoxy ( http://www.masepoxies.com/mas8.htm )

9. What is BLUSH, and how do I remove it?

Blush may form over the fast mix MAS Hardener and MAS Resin. The MAS Slow mix generally will not blush. Blush is noticeable as a slippery film formed over the cured surfaces. It can be removed with warm water and a sponge (rinse and wipe).
"Some people hear the song in the quiet mist of a cold morning..... But for other people the song is loudest in the evening when they are sitting in front of a tent, basking in the camp fire's warmth. This is when I hear it loudest ...." BM
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Doug
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Location: London, Ontario, Canada

Post by Doug »

Epoxy Test ( http://www.oneoceankayaks.com/Epoxtest.htm )

Amine Blush ( http://www.epoxyproducts.com/blush4u.html )

If I am not mistaken the amount of blush is related to the "humidity" and the "cure time".
The less humidity the less blush.
The slower the cure time the less blush.
(KISS principle, for me)

All the best,
Doug
"Some people hear the song in the quiet mist of a cold morning..... But for other people the song is loudest in the evening when they are sitting in front of a tent, basking in the camp fire's warmth. This is when I hear it loudest ...." BM
Tom in MN
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Location: Eagan, MN

Blush

Post by Tom in MN »

Eric,

It is a function of both the type of epoxy used, and the environmental conditions. If it is truly a "no blush" epoxy, then the environment is not a factor. If it is an epoxy that does blush, you will get less under dry conditions, and more under humid conditions. Generally, blush is only a problem when putting a fill coat on, or applying varnish. Wash the canoe with soap and warm water between coats and blush should not be problem.

Not a bad idea, if with a no blush epoxy. Just make sure and let it dry good before adding the second coat.
AlanWS
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Location: Shorewood, WI

Post by AlanWS »

In case anyone encounters the idea of cleaning off blush with vinegar, I want to warn against it. This is based on chemistry, not experience doing it.

In case anyone want to know, here's why: Epoxies harden by reaction between basic amine groups in the hardener and epoxide groups in the resin (at least boatbuilding typoes of epoxies do.) If these groups react with acid or carbon dioxide instead, they can't link up with epoxides. Either of those reactions make the hardener dissolve in water more easily to help cleanup, but if used on epoxy will leave its surface in a form that will not chemically bond to the next layer of epoxy used. Cleaning it off and roughing up the surface will help the next layer get a physical grip. Washing it with base (ammonia) would help get rid of both acid and the carbamates from carbon dioxide, but there's a risk the wet surface would just take up more carbon dioxide as you waited for all the water to evaporate, which is probably the reason I've never encountered that as a suggestion for dealing with blush. It would be the thing to do to undo any problems caused by washing with vinegar.
Alan
The Gutt Clan
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Post by The Gutt Clan »

So, here is a question... Say I select an epoxy that blushes. I apply my first coat, wait out the bubbles and drips. So eight hours later the epoxy has hardened enough for a second coat but there is blush. Do I wash off the blush or do I just epoxy over it? If I wash it off, do I dry it by hand, or let it 'air dry'?

This is one of the reasons I am thinking of going with a less expensive slower hardening epoxy that does not blush. Instead of trying to cram three coats into one day, I can just do it over a full weekend.
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Glen Smith
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Post by Glen Smith »

If you use a blushing epoxy you basically have two procedures to choose from: 1) Apply all the coats as soon as the previous coat is no longer tacky. The amine blush will rise to the surface of the last coat. If blush has started to form on the surface and you apply another coat over it, there will be no adhesion between the coats and it will delaminate.
2) Apply one coat and let it harden for 24 hours. Wash the amine blush off with warm water and a rag, rinse with clean, warm water and dry immediately with paper towels to be sure the blush has been removed. Lightly scuff the surface with a scotchbrite pad or 120 grit paper then clean the surface. Now you can apply another coat of epoxy. Continue this procedure until you have applied enough coats of epoxy. Wash the blush off the final coat before sanding.

Another technique is to wet-sand the epoxy as this will remove the amine blush and prepare the surface for the next coat.

There is a lot of information in books and on the internet concerning amine blush. I use techniques that I learned from the head chemist of the company that makes the epoxy I use. Opinions may differ.
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