Old epoxy degrading?

[JeffM]

About to paint the (Michalak) Piccup Pram that lived outdoors after I finished building it last spring.  Found my sandpaper--all grits from 150-80--rapidly clogging with dark brown or sometimes amber crud.  The crud can mostly be knocked out with sharp taps.  It doesn't go away no matter how much I sand, so it isn't just surface.  It has to be from the epoxy, since there's never been anything else on it.  What's happening here?  Did the epoxy degrade in the sun while I (stupidly) left it out?  Can I do anything about it, or should I just get the paint on?

[Craig]

The epoxy will degrade pretty fast without UV protection Jeff.  Try a scraper to get it all off.  If the boat will not live in the water then paint will be all you need.  Use latex and she will dry right out between uses.  Just store her where she will  not stay wet or collect fresh water in the hull.  Both are sudden death for fresh water boats.

[Greg Luckett]

Did you wash off the epoxy prior to sanding?  It could be the effects of amine blush.

[wkisting]

Just another thought to toss out: Did you store the boat under trees? Here in GA, the pines drop a sap that will clog sandpaper the way you describe, and there's also a yellow pollen that coats everything in Spring (although the pollen is visible on surfaces even without sanding). Not sure if there's a good way to remove them, but it may be something along those lines instead of epoxy. However, as others have said, epoxy certainly does degrade too, and typically yellows (amber) a bit as it does so.

[Greg Luckett]

I just reread Jeff's post and am wondering if the boat was covered while sitting outside.  If the cured epoxy was not protected from the sun's UV then he could have some serious epoxy degradation.  I have had no personal eperience with that nor do I know what it really looks like.  My guess, however, is that the boat would need redone on all its joints with grinding out the old epoxy and then new epoxy applied.  I hope this is not the case.  Perhaps Jeff, you should contact the epoxy manufacturer directly and discuss this?

Greg.

[JeffM]

It's definitely the epoxy, rather than sap, pollen or blush: I didn't get to the bottom of it after initial sanding.  It's probably whatever epoxy turns into in UV, since it is present on the (horizontal) bottom, but not the (vertical) sides.  I'm going to paint, and hope the joints are okay.  The joints are pretty thickly epoxied over glass and biax tape, I expect it won't fail without warning, and this isn't a deep water boat.  Besides kicking myself for not covering it religiously, this is beginning to put an idea in my mind for a little testing: when next I mix a good batch, I'm going to use some on a test fillet, cut it into identical segments, and expose some to varying periods of full sun.  I want to see what happens when I stress them.

[Frank Hagan]

I'm not sure on this one; the epoxy I've seen degrade from UV has turned kind of white and cracks, then chips off around the edges.  Not sure where you are Jeff, but maybe you caught it in an intermediate stage before it got to the point of cracking and turning white.  It doesn't sound like its soft and gooey because you said you can tap it out of the sandpaper.  Painting is probably the best recourse, since it wouldn't make sense to sand down through the glass and re-glass it.

[JeffM]

Frank, those symtoms sound familiar: I've had that on the gunwhales and other unpainted wood of the big boat where the tarp doesn't cover.  I've also seen places where the the epoxy seems literally to have evaporated, leaving bare patches of wood with no evidence of chipping.

I did much of the work on the little boat in an unheated garage in the winter, using cold-temp epoxy (sets down to 40 degrees F).  But that was long ago, and the boat has been through a summer and a half.  I suppose I could have mismeasured on the last coat, using a little too much hardener, but the surface isn't soft.

[Frank Hagan]

One and a half years would do it.  I recall reading that epoxy will degrade in three months in the tropics, and take a year or so in the northern climes.  Can't remember where I read that; might have been in a West System or System Three article.

[PAR]

Depending on the hardener formulation used in the brand of epoxy, most will chalk up at the surface (white powder stuff), then turn reddish brown to reddish black after prolonged UV exposure. It doesn't take very long here in sunny Florida, but in Cape Cod land, it would take longer. On surfaces with a direct exposure, such as an up turned hull's bottom planks, this could be just 6 months, with other areas of the hull (which receive a glancing UV blow) less impacted.

The epoxy is literally burning from this UV exposure and showing the signs. The depolymersation of the epoxy molecule (yes, singular tense), breaks the cured resin down into separate elements. The intertwined structure of the cured matrix, sort of holds things together, but most of the strength is gone from the lose of molecular bonds.

The only cure is to grind it off and start again, unfortunately.