Monday, 20 December 2010

Aerospace composite filling and preparation for painting

Composite surfaces present a range of challenges. Pinholes, voids where resin is absent and weave patterns are the most prominent of these. Although weight is a priority many end users prefer to see a flat smooth surface and want these imperfections to be eliminated.

Pinholes and voids larger than ½ mm diameter are best filled with a spatula applied filler of a putty consistency. Suitable products include Aerowave® Filler or Polystop LP. Smaller pinholes can be filled using a lower viscosity product which can be applied with a spreader, rag or even a brush, products for this include Aerodur® Brushing Filler, 28-C-1 and 467-9. It is important to wipe the product in several different directions on the composite surface, so that pinholes which are angled in from the surface are all picked up. The bulk of the filler can be wiped off the surface after a short cure time, leaving filler in just the pinholes.

Spraying fillers can be used to fill weave patterns and other surface unevenness, pinholes cannot be filled properly by spray application, they usually reappear after curing or sanding. Spray filler products include 8-W-5, 464-3-1 and 10-P30-8. If the spray filler is in perfect condition after sanding, it may be possible to apply a finish coat to it; however the specification may require a further application of primer. A chromate free primer should be selected, as chromate pigments are harmful and their corrosion inhibiting properties are not required. In addition chromates are hygroscopic and can encourage water to pass through them to the composite. Suitable primers include Aerodur® Barrier Primer 37045 and Aerowave® 2002, aerospace primers such as these have low weight and layer thickness of 15-25ยต they are not intended to be built up into thick layers for filling purposes and filling should be done using the relevant products.

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