View Full Version : UV damaged epoxy - what happens?

09-05-2007, 07:54 PM
I've got some friends with lapstrake ply boats, glassed inside and out. Some of them have not been really good about keeping them varnished, or may have used epoxy and no varnish.

What happens when epoxy, particularly over fiberglass cloth with no paint, gets UV damaged? Does it usually turn white, or just flake like old varnish, or peel off in sheets, or ?

Tom Hunter
09-05-2007, 07:59 PM
In a totally non-scientific experiment I spilled epoxy in the inside of my wherry while doing a repair.

I left it there unpainted, (it was over paint) figuring that nature and sunlight would eventually do thier work.

3 or 4 years later it pretty much all came off when I put a finger nail under it. It was nolonger stuck to the paint, and was brittle and easily broken or crumbled, not at all like the brand new stuff that I have been laying down on the sloop.

The boat is in New England, and sits outside in the weather from May to October or so.

Hope that helps

09-05-2007, 08:24 PM
My boat spent a few years without maintenance along the way (shame). The varnish on the transom and gunwales was badly scratched and scraped in spots. The epoxy where exposed was whitish, and just flaked off, leaving bare and stained wood. I had to scrape and sand back quite a lot to get to a stable base, removing all the varnish in large areas. I had to paint the transom opaque after repairing with epoxy. It is now a few years later and all is well and I have learned my lesson.
Also, a friend epoxied a window frame in his shed, and did not follow up with paint. Within 8 weeks, about 80% of the epoxy had flaked off. Plenty of UV here in summer!
I haven't experienced the epoxy/glass issue, but would guess that going white would be the first stage. I think it indicates that there is air under the epoxy. A bit of gentle probing with a sharp implement might give more clues.
As to treatment strategies, I'm keen to hear from more experienced people. It is a future we epoxy users might one day face.

Todd Bradshaw
09-06-2007, 12:06 AM
Some types, like WEST epoxy will turn yellowish and kind of rubbery, losing their ability to protect the surface and eventually crumble away with very little effort if they're not protected. It makes a pretty good case for using a varnish with a good UV filter and renewing it regularly and an even better case for painting over epoxy on anything that's going to be left out in the sun for very long. Let the paint catch the UV and protect the epoxy under it. Save your bright finishes for boats which live under covers or in garages. In any case, good, solid epoxy looks like good solid epoxy. If it seems to have discolored, softened to the point where you can dig your fingernail into it, delaminated or otherwise no longer looks quite right, it probably isn't any more.

09-06-2007, 12:36 AM
How long is it "safe" to leave exopy exposed to UV? 1 day? 1 week? 1 month?

Is degradation linear with time or exponential?

I ask because I can work on Amie about once a week. I plan to epoxy and glass over a cabintop repair this weekend, but I'm not sure I'll get primer over it the next day-it could be a week or two.



P.I. Stazzer-Newt
09-06-2007, 03:25 AM
I had a kayak in Epoxy Kevlar with the then standard(ish) clear resin hull and metalflake deck- one high-sunlight trip (Khartoum to Aswan) just about did for it, the Epoxy lost the ability to stick to the Kevlar in the hull - It turned a sort of browny-yellow colour.

You can sometimes take a 4 metre long kayak as luggage, by cutting it in half and stuffing the remains of the bow inside the remains of the stern.

09-06-2007, 07:07 AM
My boat´s on the hard and the deck was totally redone about 18 months ago in the hangar. It has not sat out in direct sunlight, though.

Had planned to varnish immediately but.........will diffused sunlight damage a glass and epoxy deck ?

09-06-2007, 10:43 AM
I think you'll need to define "diffused" -- UV bounces off some things a lot more than others.

I've seen varnished transoms damaged by light reflected off water in the California Delta, with the boat fully sheltered by a metal roof.

09-06-2007, 11:06 AM
Here is a photo of my mailbox attached to a railing on my house.

It is in New England, facing south. Was built about 7 years ago (as an experiment with new materials) with wood scraps, 3.1 oz fiberglass cloth, epoxy(RAKA or MAS, don't remember) and helmsman semi-gloss varnish.

1st re-coat of varnish was about 3 years after it was made, and again at 5 years.

Not exactly a "boat":D, but you can see the crazing in the surface due to poor maintenance. Varnish them boats frequently to avoid this!


P.S. I have other projects, stored outdoors, made at the same time, using the same materials, but are covered by an opaque cover when not in use. They don't show this type of degradation.

Todd Bradshaw
09-06-2007, 11:44 AM
The Gougeon Brothers say that 200 hours of direct exposure to sunlight can be enough to cause epoxy to start breaking down - so put a tarp over that fresh repair if it's going to sit out naked in the sun for a while. Unless you're getting some serious reflected light, I doubt that diffused light inside a hanger is going to be much of a problem.

I'm not sure about the UV absorbers in varnish, but many of the common ones work by converting the UV to heat, at which point it dissipates without harming the finish or object that's being covered. In the process, the blocker bits slowly get "used-up" and lose their ability to make the conversion, which is why periodic recoating is important. We used to mix in a small amount of a yellow powder called "UV-9" into resin to increase the UV resistance of the resin itself, but it still needed the varnish topcoat which could then be recoated from time to time. I don't know whether the UV-9 made much difference in the lifespan, buy since one of the canoe companies had given me a can of it, the price was right and we figured it wouldn't hurt to add it to the mix.

09-07-2007, 05:26 PM
Sometime ago on WBF, a forumite who knew more about UV- inhibitors that anyone one of us has chalked up hot dinners, shared his experience and knowledge with us.

I cannot recall the chemical nomenclature for the UV inhibitors but they turn an otherwise clear varnish - or epoxy - into an amber colour. So a perfectly clear (colourless) varnish, has zero UV protection.

I was informed recently of a mono-component polyurethane varnish that uses ferrous oxide (reddish in colour) for the UV-inhibitor, but its UV-blocking performance is anyone´s guess.

Has anyone come across epoxy-based glue where the hardener is colourless ? Not me, anyway, as all epoxy-glues I´ve used to date have a darkish-brown UV-inhibitor in the hardener.

09-07-2007, 06:56 PM
CPES seems very clear, but it is not epoxy per se, a thinned epoxy sealer. Definitely must varnish or paint over it.

09-17-2007, 12:30 PM
Any comments about longevity with 207 hardener from West that has UV inhibitors in it?

How does carbon dust applied in the epoxy protect or not the epoxy from breaking down? Should this also be sealed with varnish?

Thanks in advance for your thoughts.

Todd Bradshaw
09-17-2007, 01:43 PM
207 has U.V. absorbers added because they know that it will be used for clear finishes, which are the most vulnerable to UV problems spoiling the look, followed by structural deterioration. The U.V. absorber is added as an increased layer of protection, but is not there to take the place of a properly U.V. filtering topcoat. Do not skimp on your topcoat or it's maintenance just because the resin itself has some U.V. absorber added to it.

Graphite powder mixed into epoxy coatings at about 10%-15% by volume is a good U.V. blocker. It is a physical barrier, not some sort of U.V. absorber. It works by limiting the U.V. deterioration of the resin to only the exposed surface, so deterioration can only happen at the top few percent of the thickness of the coating layer. The same is true with metal powders and opaque pigments where the sunlight can't pass through the opaque particles and get down deeply into the resin. The surface may chalk in time, but the deterioration isn't likely to get much deeper.