As with all cement-based systems, GRC is also subject to moisture movement in that it expands during water uptake and contracts when drying. Great care should therefore be taken in the consideration of any applied coating. Impermeable coatings for external use should generally be avoided, as they can lead to an increased risk of interstitial condensation, particularly with sandwich panels. If the panels are not allowed to 'breathe', then moisture migrating to the surface will cause any impermeable paint to blister or flake.
Proprietary textured permeable finishes, such as those used for coating external masonry, e.g. Muro-plast, Sandtex and Glamrock, are often used. Such finishes were used at Heathrow Eurolounge and at Newton Heath Library.
Clear surface coatings of the silicon type, which are particularly useful in protecting the surface of white GRC panels against dirt and fingermarks, can be used without fear of blistering, as they allow the GRC to breathe.
Care must be taken when applying paint coatings, whether in the factory or on site, to ensure that the surface is free from grease or any trace of mould release agent, so that an adequate bond is maintained between the GRC and the paint system base coat.
Some GRC panels have been produced to emulate the aesthetic features of flat metal systems by incorporating a PVC finish.The PVC sheets are either cast in line integrally with the PVC facing or adhesive bonded to the GRC subsequent to moulding. Such panels were used at the Oxford Ice Rink (architects: Nicholas Grimshaw & Partners) (Fig. 4.14) and at the Sheffield Arena (architect: HOK).
Thus coloured panels can be obtained by the use of pigments and coloured sands or by adding coloured Metakaolin. However; these colours are muted and limited in their colour range, and architects may prefer to specify acrylic-based emulsion applied on site or in the factory. High-gloss finishes are possible but expensive. Also, these high-gloss surfaces have the disadvantage that any surface irregularities will be emphasized in bright light.
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