Making Repairs Using GRP (glassfibre Reinforced Plastic)

Using polyester resin and glassfibre is simple providing the instructions are followed. Shown below is
the basic technique for laminating with glassfibre. The type of work carried out of course varies and
the use of the material will have to be adapted to the particular job but the preparation and technique
will remain the same.

The first and most important advice is to make careful and proper preparation of any surfaces
and the materials being used.



Step 1

Cut with a sharp knife or scissors the glassfibre into the required shape and size. The mat can also be torn quite easily into shape and is the preferred method since it provides a frayed edge which enables almost undetectable joints to be made when overlapped. However cutting is quite alright provided the edges are frayed out afterwards. lf any gussets or patterns are required to enable the glassfibre to lie more naturally into the shape of the mould then either tear or cut these in at this stage.


Step 2

Mix up the required quantity of resin, always measure out the hardener as recommended. A 2% (by weight or volume) addition of hardener at a working temperature of around 15’C will give a pot life of 25-30 minutes, To avoid wastage do not mix up more resin than can be used well within this time. The normal amount of resin required for this pack of glassfibre is shown here.


Step 3

When using gelcoat in a mould it is sensible to reinforce this with a layer of surface tissue. This is not applied at the time of brushing on the gelcoat but immediately before the main laminate of glassfibre and polyester resin. When repairing fractures or holes,and not using a mould, then apply a coat of resin onto the prepared surface and place the glassfibre on top. Using a stippling action with the brush apply more resin until the glassfibre is “wetted out” or saturated and becomes translucent. Glassfibre is more pliant when wetted out and therefore it is good practice to apply resin to the more difficult and complicated shapes and allow the resin 30 secs, or so to dissolve the binder holding the glassfibre together, it will then more easily take up the required shape. Avoid laminating around or into right angles if at all possible.


Step 4

The next layer can then be applied, wet on wet, but remember to avoid unsightly joints by fraying out the edges of the glassfibre mat and overlapping, the join will almost disappear. When applying several layers it is important that each layer is well consolidated with the previous layer so as to exclude laminate the trapped air appears as a whitish blister. For small areas a thorough stippling with a brush is sufficient to ensure a good laminate but for larger areas a metal roller is advised. Chopped strand mat when well stippled provides a fairly smooth surface but to make this even smoother a layer of surface tissue, applied on to the wet last layer of glassfibre will provide an even better surface.



When applying over very large uncomplicated areas e.g. boat hulls, garden pools, use a lambswool roller which considerably speeds the work.

How to Calculate Quantities

For repairs two weights of glassfibre mat or a cloth can be used as follows:-

Material Weight                                                 Layers                              Thickness

Glassfibre Mat 300 grams (1oz) sq/M                        3                                          -3mm

Glassfibre Mat 450 grams (1.5 oz) sq/M                    3                                          +3mm

11 Thou Cloth                                                         4                                          -1mm

Resin Usage

Generally the ratio is 2.5 :1 – resin to glass by weight. For gelcoat allow 50 grams/sq.ft. (550grms/sq.m). Therefore every square metre of 300 gram glassfibre mat uses 750 grams of resin. 

When repairing downward or vertical surfaces it is advisable to preimpregnate the glassfibre on a flat piece of board, give the metal a coat of catalysed resin and then carefully stipple the patch into position. When applying several patches allow each to become finger tacky or gel before applying the next. 

Once the resin has hardened remove any high spots, should there be any, with a Surform or course wet and dry paper, skim with Bondafiller, flat back and paint. 

Once the technique of using glassfibre and polyester resin has been mastered, most rusted and dented bodywork can be repaired using these materials. Rusted doors are most certainly caused by internal rust and therefore it is important that the inside of the door is treated as well as the outside if a lasting repair is to be made. Take care to clean the area as instructed. Using a former, this can be a piece of stiff cardboard or hardboard covered in polythene foil or release agent to cover the hole, make sure it is firmly fixed in position with masking tape and then apply the resin and mat preferably from the inside of the door. lf a repair has to be made from the outside then in order to make it level with the surrounding metal it is necessary to tap the metal around the area to be repaired sufficiently below the level of the bodywork so as to allow for a little more than the thickness of the glassfibre repair. Then apply the glassfibre patch and skim coat with Bondafiller.