Fibreglass boats
Practical Action
Remove any irregularities by grinding or by hand sanding. Repeat this process of hand
finishing until the plug is smooth enough to receive a layer of hard tooling gel coat.
This especially formulated gel coat is the beginning of the coating, which will eventually give
the polished surface from which the mould will be lifted. The plug and mould gel coat should
be black in colour, to contrast with the finished hull. There is no shortcut at this stage. The
more time that is spent hand sanding the plug with wet and dry sandpaper, the better the
plug will be. After applying the gel coat, further filling with resin putty and sanding may be
necessary for a good finish. In some cases the gel coat layer on the plug is omitted and only
cataloy paste is used to fill imperfections.
Credit Practical Action South Asia
Finally, polish with rubbing paste and then with 5–7 coats of non-silicone wax to give a
brilliant sheen, before applying a coat of PVA release agent, which is the last step before the
mould itself is begun. Five workers would take at least a week for the final preparation of a
plug for a 10 m hull.
Making the mould
If the shape of the hull is such that the mould has acute release angles, a split mould, rather
than a one-piece mould, is necessary. The two or more pieces of the mould will be bolted
together at a flange. To create the flange, a pattern of it has to be made on the plug on each
side of the joint line. It is possible to use the first mould flange as the pattern for the second
half. This will provide a very accurate joint and will allow boltholes to be through drilled in
situ before release.
To build the mould, first, a uniform layer of gel coat
should be applied to the plug, using paint brushes with
unpainted wooden handles or a high-pressure spray gun.
The resin is prepared by adding 1% catalyst to pre-
accelerated resin. In tropical countries this mixture will
set hard in 20-40 minutes, so only the amount required
for the immediate job at hand should be prepared.
Credit Practical Action South Asia
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For best results, panels of surfacing mat should then be
laid down to create a smooth skin. Once the panels are
ready the resin should be prepared as above. If a brush is
being used to impregnate the fibreglass mat, short
jabbing motions of the brush should be used. Once the
resin has been applied, it can be spread using a washer-
type or treaded roller, which helps to spread the resin out