Sanding composites is a time-consuming but necessary task, and very often it has to be done manually. A gentle touch and plenty of patience are necessary for the best finishes. This surface preparation of composites is necessary for several reasons:
- To finish the surface prior to applying a mold-release compound on a plug or mold before the lay-up of a product starts.
- To key the surface for additional layers of composite when the overlay time has been exceeded
- To correctly shape the item which may have become distorted when removed from a mold – e.g. a sailplane wing or skateboard
- To ‘fair’ the surface – that is, remove blemishes or excess material which stands proud of the surface
- To prepare the surface for a finishing layer – which could be, say, epoxy paint or gelcoat.
- To ‘smooth’ the surface prior to waxing (say on skis or a composite surfboard) or prior to applying anti-fouling paint or primer on a boat.
In many cases, all these steps will be employed during the production process. A really good mold and layup process should result in a product that requires minimal sanding, but some sanding will be almost inevitable.
Power tools (electric or compressed air) can be used for larger areas. Disc-sanders are definitely to be avoided as these can very easily damage the surface through gouging. Belt sanders may be used with care (but only on flat surfaces). Generally random orbital sanders provide the best results, but hand finishing with a sanding block is usually required as a final step – ideally it should be the only step.
Be very careful with power tools – they can easily and quickly over-abrade and heat the surface. It is essential to avoid generating excess heat which can damage the composite and even ignite it.
Use a light touch and a grade of abrasive which is a couple of grades finer than you would use by hand. ‘Wet’ sanding is a technique which keeps the surface cool, though it is done usually by hand – don’t attempt it with electric power tools!
‘Detail’ sanding can be done with a tool such as a Dremel or a needle file.
Do it slowly, and do it very carefully. If you don’t and you cut through to the reinforcing (glass, aramid or carbon fibre) then repair work will be required, and the ultimate result is very likely to be unacceptable.
Choice of Grade and Material
The term ‘sanding’ refers back to the days when sand was used for abrasive papers, but today we have a range of materials available – from aluminium oxide to silicon carbide and tungsten carbide. Harder composites will require materials which have a higher hardness to cut the epoxy. However, the binder which binds the abrasive to the carrier substrate has to be tough enough to hold them together under what is considerable local force.
A good quality tungsten carbide sanding block will finish carbon fibre without the characteristic 'fraying' associated with conventional sandpaper or wet-or-dry – particularly at the edges.
It is usual to work with three or more grit sizes, with 400 grit (or even finer) being used to finish off. The starting grit should be chosen to suit the initial roughness. It is better to work with too-fine a grade than one which cuts excessively.
For mirror finishes a rubbing compound will be used as the step prior to waxing (if required). This will be equivalent to ‘flour’ grade abrasive paper (2000 grit).
Do wait until the composite is fully cured before starting to sand, otherwise the surface will be dragged, balled and seriously damaged.
Health-wise, avoid build up of excess dust which clogs the abrasive surface. Use a vacuum extractor and wear a breathing mask and goggles as the dust and any gases produced during sanding are injurious to health. Ear plugs will prevent any dust getting into the ears, and a disposable protective overall and hood are recommended for larger jobs.
Production line sanding and major projects should include a professional air extraction system – excessive airborne dust in an enclosed space can, in the worst case, cause a dust explosion.