A "Not So Fun" Job

In an earlier post I talked about the weather being relatively cool and deciding to do some filleting work, which was a disaster.  More about that later.  The fillets are thickened epoxy used to round or soften any corners in the hull so the fiberglass cloth has a smooth curve to bend around.  It will not adhere well at all to sharp corners.  These fillets have to be as smooth as possible, not only for appearances, but because any bumps or voids will leave a "bubble" under the fiberglass causing a weak spot.  Weak spots, especially in boat hulls that are going to be bouncing around on a trailer, can be disastrous.

On some of the fillets I was at least able to do the initial smoothing and today I chiseled away some of the roughest areas on those.  After filling in a few low areas I should be able to sand them smooth enough at this point.

A not too bad fillet with the worst areas chiseled off--ready for sanding

There were one or two particularly bad areas where the epoxy started hardening before I could even start to smooth it out.

Very bad fillet, most of which has been chiseled away

 It is hard to tell from the picture, but in this case I have chipped most of the fillet away.  It will be sanded and a new fillet put in its place.  I'll post the results when I am able to get the job done.

The way the fillets are supposed to be done is the thickened epoxy is laid in, then smoothed and sculpted with a flat, rounded stick to get it as close as possible to the finished shape.  Then any spills and excess material are cleaned up with a putty knife.   After an hour or two, depending on the temperature, the fillet has started to gel, but is still malleable.  At this point the final smoothing and shaping is done with a gloved finger dipped in alcohol.  This can be done very nicely if the temperature is anywhere between 60 and 77 or so degrees F.

According to the epoxy manufacturer's literature, at 77 degrees the pot life of their resin, using slow hardener, is 23 minutes.  That's how long you have to spread the epoxy before it becomes unworkable.  What they refer to as "thin film set," where the epoxy feels fully hardened but is not yet cured is 5 hours at 77 degrees.  Full cure at this temperature takes 5 days.  So, at that temperature it is best to wait about two hours before the final smoothing.

Again according to manufacturer literature, at 86 degrees the pot life is about 17 minutes, thin film set (TFS) occurs at about 3 hours, 45 minutes, and full cure takes 3.75 days.  At 95 degrees pot life is 11 1/2 minutes and TFS is at 2 1/2 hours.

So to me this means I should have had at least an hour to an hour and a half before the epoxy became unworkable.  That tuned out not to be the case.  As I said, most of the fillets from that "pot" only gave me time for initial smoothing and the last fillets didn't even give me time for that.  I would estimate the actual pot life at about 5 or 6 minutes and the TFS time at the end of pot life at about one more minute.  The temperature when I started the project was 85 and I don't think it climbed above 87 before I was finished.

So what went wrong?  One possibility is that I accidentally added too much hardener.  But according to the manufacturer, this does not make the epoxy cure any faster, but can make it weaker.  As I learned today when chipping away at it, it was definitely not any weaker!  So I don't think that was it.

One response to my previous post was that epoxy starts to become unpredictable above 80 or so degrees, meaning I guess I can't trust the literature completely.  So, one more lesson learned the hard way.  If I remember all these lessons my next boat will be perfect!