name of article
Highly polished, the new heads sole board/shower tray looks very clean and tidyLayers of gelcoat provide a smooth finish
Heat speeds up the curing process Glass tape was applied to the edges... ... until the whole thing was nicely sealed
Brush on a layer of resin – not too thick Lay on the mat and brush in more resin A resin roller consolidates things nicely
View from the boatyard
bond well to the plywood.
Another reason for using just enough
resin is cleanliness. Some folk seem to get
GRP everywhere and end up with a huge
mess, and much of this comes down to
the uncontrolled sloshing of resin which,
when you come to consolidation, will
spray the stuff around in every direction.
Using polyester you have a great deal of
control on cure times compared to using
epoxy resin systems.
Adding a little more or less catalyst is a
bit of an art, and I find it hard to explain
how much to use at specific times as I
seem to manage to add just enough.
You have no such flexibility with epoxy
systems, but one sure-fire way to speed
things up with polyester is to use a heat
lamp which can allow you to get a
workable cure in an hour or so.
Where epoxy wins is in bonding strength
to a substrate such as plywood. However,
to mitigate this using polyester resin,
creating a dust-free and well-scored
surface helps immensely.
When sheathing the top surface we
followed up the layer of CSM with a layer
of 300gm biaxial woven cloth. Again, a
light coating of resin and a good roll will
pull up any surplus resin from the lower
layer. Use slightly more resin on the edges
to help the cloth round the corners and
stay stuck down during the cure.
After trimming and a quick sand using
100-grit paper round the edges, it's time to
add 100mm-wide glass tape round the
edges. By applying just the right amount
of tension you can usually get the tape to
lay nice and flat without too many creases.
The tape is thin, flexible and needs very
little resin to wet out. In this case I was
adding the edge tape on a Friday
afternoon, and I added a little white
pigment so that on my return the following
week I would remember exactly which
point I'd reached.
The whole base was then rough-sanded
with 100-grit paper, washed and dried.
The sheathing was now essentially
complete, so it was time to add the
gelcoat. Surface coating using gelcoat
has one big issue, and that is air bubbles:
you add air during the stirring process and
then again during application with a
brush, roller or spray.Thin layers
Gelcoat is thick, and it's very tempting and
perfectly possible to add 2-3mm of
gelcoat at one time. After an initial cure it’ll
look pretty good... until, that is, you start to
sand and then cut and polish the surface,
after which you will see lots of tiny holes in
the surface. Sanding or cutting further
might remove these initial holes, but only
for others to emerge underneath.The way round this is to apply multiple
thin layers. I like to use four layers and
then a top layer with added wax in styrene
(also known as topcoat or flowcoat).
The wax comes to the surface, seals off
the gelcoat from the surrounding air and
allows it to cure, otherwise it will always
remain tacky.
Once cured, the area can be sanded flat
using 100-grit paper, which removes any
major bumps and hollows and provides a
key for the finish layers of gel.
The radiused edges needed to be
squared off, so we put together a wood
frame. A resin-proof tape was applied to
the surfaces of the wood so nothing would
stick to it, then further layers of gel were
applied and the edges filled. Once cured,
the frame was simply removed, leaving a
solid, square edge.
The final process is marking, sanding,
marking and more sanding until you’re
happy with the surface fairness, followed by
cutting with a suitable cutting compound.
Most fast-cut compounds will remove
scratches made by 1,000-grit wet-and-dry
paper, and you’ll get a pretty good surface
finish. I like to go a couple of stages
further with 1,500 and 2,000-grit paper
before cutting with a fine-cut compound
as the finish in my eyes is far superior and
seems to stay cleaner for longer.