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Fiberglass Bond Coat

Okay, so let’s review.

Fiberglass (of more accurately glass-fiber reinforced plastic) is a composite laminate of hardening plastic goo reinforced with Ziggy Stardust-esque silvery spun glass fabric.  We have already applied the “seal coat” and need another several layers before we’re done.  I found the Glen-L instructions for applying fiberglass for you.  This is pretty much what we used.

Here’s the application sequence for the “dry method:”

  1. Seal Coat – seals the wood surface prior to applying the cloth
  2. Bond Coat – used to wet out and bond the cloth to the surface
  3. Fill Coat – fills the weave of the cloth
  4. “Finish” Coat – provides enough resin build-up for final sanding and finishing
  5. Aren’t We Done Yet? Coat – enough epoxy hopefully to hide the texture of the cloth
  6. Seriously, This Is The Last Coat – finally enough to more or less completely hide the texture underneath

And then after that we have to protect it like a new born baby from dangerous UV rays with paint or varnish.  We’ll do varnish.

We agonized over this day for weeks.  We thought it would be stressful and it did not disappoint.  We enlisted the help of Jen to complete our three person team.  This was one of those days we talked a lot about The Plan and worked out roles beforehand.  Kai was the epoxy mixer (and later the touch-up person).  Jen was the epoxy applier/roller/splooger.  And I was the smoother downer/problem fixer/epoxy redistributor. 

All summer, it had been ridiculously hot and sunny.  But the day we chose to do the stressy bond coat, the sky threatened to rain and spit occasional little dribblets on us.  Grrr. 

We stapled the cloth in place to the hull smoothing out the wrinkles best we could.  Though the instructions suggested removing staples before the cure, we found it was better to wait until the epoxy was completely cured before removing the staples lest you pull up the cloth.  Note that putting the staples in, we were careful to make sure they stuck out enough to allow them to be easily pried up with a screwdriver.

The Glen-L instructions suggested that for places where the cloth met it be double-lapped with tapered edges tapered.  That is, lay one layer of cloth, sanding the edge to a taper after it cures.  Then lay the other layer of cloth overlapping, and then sand that other edge after it cures.  That meant that since we couldn’t do the overlap in one day, we needed at least two days to finish the bond coat.  

So we got all clever and did half of the bottom and one side on Day One.  Then the other half and other side on Day Two.

Each of us had a set of tools.  Kai used the usual measured containers and an endless supply of stir sticks.  Jen used a dense foam roller with a long handle.  And I used a wide trowel and a fiberglass roller that I handmade with alternating-sized washers and a paint roller.  My job was to go behind Jen and trowel any pools of excess resin into areas that needed more of it, and then roll down the stubborn cloth that had bubbles or warbles in it with the roller.

The magic of this process is that as soon as the glass cloth is covered with wet resin it turns transparent.  So it is easy to see if you’ve applied enough resin to wet it out.  Here I’ll interject a warning:

Do not skimp on resin during the bond coat.  Make sure every bit of the fiberglass cloth is wetted out and transparent.  The next coat WILL NOT cover any areas that got too little resin.

We had some spots like this.  We learned the hard way.  Take a look at the very edge of the corner on the photo above.  Tiny but still irritating.  These places where there are air bubbles behind the fiberglass are potential places for air and moisture to gather and rot to form.

However, other blemishes, like little bits of resin-saturated glass fiber that stuck out (got snagged by my trowel in many cases) or awkward corners, were easy enough to sand down and resin over in the next layer.

In general, places we had worried about, particularly the seams looked great.  After they cured I would have to sand them down in preparation for the next layer.

After this first bond coat layer cured overnight, we sanded down any wacky blemishes and tapered all the places where the glass cloth would overlap.

Then we laid the cloth over the other half of the bottom and the remaining side and applied epoxy over that.

When this second half of the bond coat had cured, we tapered those edges and the result was quite beautiful.  The texture of the cloth was clearly visible, so the next step is the fill coat.

You’ll notice there are few photos of us actually working this work day.  Uh, no surprise.  We were definitely eyes on the prize in order to get this done in a way that didn’t suck.

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