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Glass Fibre Structures

The use of plastic media for stripping paint from glass fibre can be very effective, but can also have its problems.

Plastic media blast cleaning relies on small granules of plastic being propelled by air pressure through a nozzle onto a surface where they scratch and chip the paint to remove it from the surface. Whilst the plastic granules are less dense than alternative blasting media they are still fairly aggressive and do have a 'peppering action' when they hit the surface.

On some glass fibre finishes the gel coat is sound and hard, the panels having been manufactured using a press to ensure that bubbles are excluded from the lay‑up. However, some of the earlier cars were manufactured using, by today's standards, fairly primitive methods, in workshops where there was no temperature or humidity control and when the ratios of resin to catalyst were far more critical, with result that lay-­ups were often inconsistent and air entrapment within the lay‑up was a common problem.

The result is that air bubbles are often trapped in the lay‑up between the first coat of' mat and the gel coat, which is applied to the inside of the mould.

When the paint stripping is taking place the gel coat is the outer layer of the glass fibre panel to which we are normally required to strip. The gel coat is normally fairly hard and can be very brittle. Alternatively on some of the older cars the gel coat is of poor quality and has deteriorated over the years, which can make it comparatively soft. The plastic blast media pellets do not differentiate between the two adjoining surfaces and in the case of a hard surface, the gel coat over top of the bubble is usually very thin and tends to fracture, exposing a void during the blasting process. Conversely where the gel coat has deteriorated and become soft, it will breakdown and expose the void in a similar way.  In some severe cases the gel coat is very thin and will break down even on areas without any air entrapment, exposing the strands of fibre matting.

This situation is made even worse when multiple layers of paint have been applied to the car and especially when relatively hard finishes have been used. The concentration of the plastic blast media required to break down these more durable paint finishes results In more abrasion to the gel coat surface, especially in complex areas such as moulding lines, corners and similar features.

The final result after stripping may be a fairly smooth gel coat finish with a number of pinholes, and especially on older cars we would expect to see many tiny voids where the air bubbles have been broken. Some exposure of mat in more complex areas and areas where previous damage has been repaired, is common place and any gel coat cracks caused by impact damage will be visible after blasting.

The customer must be fully aware that the repairs necessary to the glass fibre surface may be extensive and expensive, not because our stripping process (or any other process for that matter) has damaged the body, but because that is often the way with older fibreglass bodies. In many cases the pinholes can be filled with stopper, flatted and sprayed with polyester filler or similar materials, with intermediate flatting between subsequent coats, to achieve a smooth uniform finish. Alternatively, some repairers believe that surfaces in this condition should be re‑matted with tissue mat and given further extensive repairs prior to producing a surface ready for painting.  Before embarking on a fibreglass restoration just be aware of the potential problems and pitfalls of stripping paint from glass fibre by this or any other method,and the alternatives for the repair and restoration of the glass fibre to an acceptable standard.