27 Fibre-glass reinforced plastic moulding corners should be avoided, or the resin mat will tend to bulge over, leaving a void in the structure of the finished mould. The working surfaces of the mould must be filled (resin and talc, Polyfilla, Alabastine, etc.), sanded down smooth, lacquered (polyester wax, polyurethane, shellac, etc.), again sanded down with 400 grit wet and dry paper, and burnished with cutting-down paste to a high gloss, after which it is given a heavy coat of wax polish and left to harden overnight.
GRP former mould The former mould from which the finished moulding will be struck is now made up as in Figure 27:2. The wax coat is first buffed off the pattern, which is then coated with a polyester emulsion wax release, followed with an application of wax polish buffed to a high gloss, and then a layer of polyvinyl acetate (PVA) release agent applied with a sponge. A gel coat composed of polyester resin, catalyst (setting agent) and a small percentage of colour paste is then brushed on, followed by a second gel coat immediately the first has cured. The second gel coat should be of a contrasting colour, so that adequate warning is given during any subsequent rubbing down, and, when this coat has cured, a coat of catalysed resin is brushed on, and a layer of chopped strand glass worked into it, adding more resin with a stiff stippling-brush until the mat is 'wet out' or saturated. Two or more additional layers of glass mat and resin are then added, and the whole assembly stippled and rolled with a split roller to consolidate the layers and eliminate all air bubbles. The mould is then put aside to cure for at least four hours at normal room temperature, any roughness smoothed out. washed over with warm soapy water, given a thick coat of wax polish and allowed to age for a further 24 hours.
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