Epoxy and sunlight exposure

[Noklin]

I see people using epoxy with different additives such as copper or graphite on the hull bottom. With an epoxy hull joint exposed to sunlight (southeastern US) without any form of UV protection, how long before the joint is damaged to the point of concern. Are we talking weeks or years? Can you tell by looking if the joint has been damaged or would you have to do some form of strength testing?

[Frank Hagan]

I don't know about joint strength. I have read that when used as a laminating resin, unprotected epoxy will start to chalk and deteriorate in the sunbelt within one season.

What I don't know is how deep that chalking and deterioration goes, and how additives to the epoxy itself minimize that. I think you have to cover the epoxy with some kind of barrier, like paint, to really get good UV protection. Yearly applied coats of varnish work too, but it is easy to miss that the varnish has worn off and get into the chalky epoxy stage.

[Charlie Jones]

Straight epoxy will show deterioration within about 6 months, less here in our sun. It will yellow and get chalky on the surface. That chalkyness can be sanded off and recoated and you are pretty much good to go still.

After the chalky stage, it starts to crack and curl. If it has reached that point, you must grind it away and recoat.

The Gougeon's sell an aluminum powder that can be added to the epoxy to give you a grey surface. That surface will stand several years of exposure before it begins to seriously fail. It will still deteriorate on the surface, but at a slower rate. You would still need to sand and recoat, or sand and paint.

I've seen people build kayaks and leave them unprotected for a season of sporatic use with no ill affects. Left totally exposed to the sun would be different.

So to sum up - several weeks won't hurt a thing- months will probably damage it some, years and you need to re-do it. And yes, you can easiily see the deterioration.

[Garry]

I understand that mixing graphite with epoxy will add UV protection. Check with mfr about mixing guidelines.

I built two hatches for Rosebud using graphite filled epoxy and 1/4" teak slats. You can just see the hatches in this pic:

www.pbase.com/image/33329788/large.jpg

After more than three years of tropical exposure, the teak had weathered and left the graphite seams standing a little proud. There was no sign of any loss of strength in the exposed seams.

[PeterP]

Check out this link on epoxy exposure http://www.oneoceankayaks.com/Epoxyhtm/epox12m.htm

PeterP

[Tom Lathrop]

I understand that mixing graphite with epoxy will add UV protection. Check with mfr about mixing guidelines.

I built two hatches for Rosebud using graphite filled epoxy and 1/4" teak slats. You can just see the hatches in this pic:

www.pbase.com/image/33329788/large.jpg

After more than three years of tropical exposure' date=' the teak had weathered and left the graphite seams standing a little proud. There was no sign of any loss of strength in the exposed seams.[/quote']

Garry, did you use graphite or lampback (pure carbon)? Both are carbon but lampblack is the black pigment used in paint and graphite is a crystalin form of carbon. Lampblack will protect epoxy from chalking very well. Nothing is perfect and some greying will happen eventually. This is what plastic telephone and power cables are pigmented with and they are exposed to the sun in all parts of the country. I have seen some degradation, but not much.

[Garry]

I can't remember what it was but it was from West Systems and specifically for UV protection as well as color. It did grey a little.

[PAR]

Here in Florida sun, epoxy will darken and show signs of break down in six months and will be coming up in about a year. This is direct exposure on a horizontal surface, vertical surfaces will last a little longer because the light is not a direct blow. Aluminum oxide and graphite will offer some protection, but will eventually have UV break down issues. Aluminum oxides intended use, is as a fire retardant in high heat areas and graphite is for making a hard, slick surface. Graphite is generally used on the bottom of race boats and wet sanded to a baby's butt smooth surface, to get the last ounce of wetted surface resistance out of the boat. If exposed to direct southern sunshine, the color alone may cause weakening problems in highly loaded pieces. A black or nearly black horizontal surface, in midday Florida sun, will easily reach the temperatures that cause epoxy to release it's grip on things.