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UhOh - here I go again.  There have been a lot of great ideas, theories and
explanations about this controversial topic the last few weeks that started,
I think, with discussion about sanding and solvents.  I can't resist
sticking my neck out once more.

I've been using a method that works well for me and results in a very strong
bond when I use the process well.

I tried using the rollers with grooves in them at first and felt that I
couldn't move the resin around in the layup as well as I wanted.  So I tried
a piece of 2" ABS pipe and found I could squeeze the juice where I needed it
and get a more even percentage of resin to cloth with that method.  I liked
that a bunch but then I started thinking about how I could squeeze more
juice out until the layers were just barely wet to get a lighter result.
That was OK, but then I learned that it was very difficult to get a good
bond - there were lots of bubbles and the material wouldn't stick to the
prepreg, so I ended up doing a few tapes more than once.

Once again, a concept in dentistry came to mind that explains the problem
and suggests a solution.  We use highly filled resins for fillings in
dentistry.  They are loaded to various degrees with particles - quartz
usually - and they become extremely viscous - you can roll the stuff up into
a ball of paste that won't even stick to your finger.  There is no way to
get this material to overcome the surface tension and wet out the interface
between that material and the tooth structure you'd like to bond with.
Therefore, the tooth is prepared with etching and cleaning with various
solvents and primers.  Then a layer of unfilled resin - a viscous fluid more
runny than Jefco is painted on the tooth surface.  You then blow air on it
to remove excess and reduce it to a few molecules of thickness.  The idea is
to present an interface that will wet the bonding surface, flow into the
crannies you've made and mate it to the stronger, more abrasion resistant
filling material that goes on next.

I tried the concept out with the materials we use and it works well.  I sand
and clean the surface, wet out the layers and roll them out  with lots of
pressure to remove any excess resin.  Then I use a chip brush to paint a
thin layer of resin on the bonding areas.  I find that this takes a lot of
dragging of the brush to thin out the layer. Then the glass or graphite
layup is laid in place and painted out with the chip brush and/or squeegeed
down to work out bubbles.

Try it out and let me know what you think.

John Barrett

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