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Fibreglass (GRP) Repairs and Construction

Glass Reinforced Plastic - GRP or fibreglass for short - is a popular and relatively cheap material often used for car repairs and, in some cases, construction.

Polyester resin is supplied as a thick liquid, and has a shelf life of approximately six months - it becomes thicker and gel-like as it ages and eventually becomes unusable. Available in a multitude of pack sizes such as 0.25 litre up to industrial size 45 gallon drums and larger.
It must be mixed with a catalyst (activator), after which it has working time of 5 - 30 minutes before it sets completely solid.
Note, it produces heat as it cures. This may not be noticeable in thin layers or small amounts, but if for example a quantity of unused activated resin is left in a container, it will become very hot and may even self combust. Take care with mixing tins etc.!

Cellulose thinners or Acetone is needed for cleaning brushes or tools. Once the resin has been activated and allowed to cure it can not be washed out!

The glass fibre component is exactly that - hairlike strands of glass are woven into various grades of ‘matting’ to produce a fabric sheet which can be cut to shape for easier handling or to make complex shapes.
Coarser (heavier) grades of fabric are used for the substantial layers of a repair or moulding, and can be topped off with a finer surfacing grade if needed.

The fibreglass fabric is embedded into the polyester resin liquid, adding substantial strength and flexibility to the finished article which would not be achieved using polyester alone.

DIY Glassfibre kits usually contain a balanced quantity of resin, catalyst and matting sufficient for smaller repairs or mouldings. The resin supplied in these kits is what would usually be called ‘laying up’ resin - it is a general purpose resin which sets quickly when mixed and is dry to touch when cured.

Car body repairs using GRP are ideal candidates for these kits.
Rusted or holed panels (not structural) can be cleaned back to shiny metal, tapped down a little, and any holes bridged using resin and grp matting. A stiff brush is all that is needed to stipple the resin thoroughly into the matting. Temporary support can be achieved using a piece of thick polythene - the resin will not stick to it when cured and it can be easily peeled away.
Of course any repairs of this nature must be kept below the panel surface to allow for final finishing with body filler etc. Glass fibres must not be visible in the finished surface.

For anyone considering making larger articles from a mould of any sort, it would be wise to consult your supplier, who may recommend a ‘gel coat’ resin for the first coat in the mould. Different grades of resin and activator are also available which may be better suited to different uses or working conditions.
Your supplier will also be able to advise on mould release agents which prevent your resins etc. sticking to the inside of the mould.

A Gel Coat of course will be the surface of the finished product, and must not have matting of any sort embedded in it. It can also be mixed with pigments for a self-coloured surface.
If a gel coat is used, the structural resin/matting is layed in the mould after the gel coat has cured. Two, three or more layers of grp may be added depending on the strength required from the finished product.
Angles or corners of a moulded product which may be subject to flexing or stress may benefit from extra reinforcement, either with extra layers of grp matting or occasionally grp fabric tape.