Boat GRP Repair & grp modifications

GRP components are used in a very wide range of applications. some of these may require a long life.

Any component that has a true working life may suffer damage and, in some cases, the original mould may not exist any more, so a repair will become essential. There will also be occasions when an existing component does not quite match the demands placed upon it, but the cost of making a completely new replacement would be prohibitive, in which case the original item must be modified.

Basic marine GRP repair techniques

When a laminated polyester component has been damaged and the laminate completely split, the ideal treatment would be to put the component back into the mould, which would ensure perfect alignment of the damaged portion during the repair process. However, this is seldom possible. So we will assume that the repair is to be carried out without the use of the original mould.

When subjected to an impact of sufficient strength, GRP components will typically display some compression damage, like a dent, where laminate has failed around the compressed area, and there will inevitably be tearing with the glass fibres protruding from the torn parts. To repair this type of damage, the main portions of the affected area must be held as near to the original shape as possible. This will make some form of bracing necessary, and wood or metal will do. Since the work will begin on the reverse face of the laminate, the bracing must put on the outside.

Any dent-like areas should be pushed out to resemble the original shape as closely as possible, while torn parts should be held in place bracing. The fraying of the fibres around tears may make it difficult to the parts together. If this occurs, run a fine hand saw down each tear to remove the frayed fibres. This should make it easier to align the damaged parts. Rapid-set body filler is an excellent means of sticking the bracing in place. Using small blobs of filler and some battens, simply align the aged parts and hold them in position. If any parts have broken off  completely, but you still have them, hold them in place by the same method-When the damaged area has been realigned as closely as possible and held firmly by the bracing, any gaps or portions that are missing completely must be filled in. For small gaps and holes, modelling clay will do the job; for larger missing pieces, use card, aluminium or any other material that can be made to represent the missing piece. This type of infill does not have to duplicate the original shape exactly, but it should be as close to this as possible.

With everything aligned and firmly braced, the next stage is to sand the back of the laminate, working over the damaged portion and on to the undamaged surface around the area to be repaired. This not only provides a clean surface to which the repair can be bonded, but it also levels off the damaged area by removing any protruding pieces.

The next task is to laminate a patch over the damaged area, but there is a decision to be made at this point: should the patch be laminated with normal polyester resin or epoxy?

Polyesters make excellent laminating resins and, if the laminate is polyester, they will effect a reasonable repair. However, polyesters are not the best adhesives, which may call the long term adhesion of the repair into doubt. If the component leads a hard life and is likely to be damaged again or often, or the component is not intended for a long life, using polyester resin for the repair would be perfectly satisfactory. While repairs made with polyester could last a long time, if the component is important, valuable, rare or, for any other reason, needs the best repair possible, the patch should be laminated with epoxy resin, and the reinforcement should be woven glass fabric. Another good reason for using epoxy resin is that it produces a stronger repair.

The lamination on the reverse of the damage is the main strength of the repair, and the number of plies necessary for the patch will depend on the type of fabric used. Although almost any type of woven fabric will do, it is best not to use a very fine weave. Three or four plies should be strong enough for most GRP repairs.

When the lamination on the back of the repair has cured fully, the bracing can be removed from the front of the component. The next stage is to take an electric sander and use the edge of the disc to cut along the torn Darts, through to the newly laminated patch on the back. Take care, however, not to cut through the patch. Then open up the cuts to a wide Vshape, as wide as practical.

Finally, use the face of the sanding disc to sand the entire damaged area, removing any paint and gel coat. If there are holes where pieces are missing, taper their edges at as flat an angle as possible. Again, take care not to cut into the repair patch.

Now the damaged face of the component can be repaired and, as before, you must choose between polyester and epoxy resins. While epoxy is stronger than polyester, the patch on the reverse face of the component provides the main strength of the repair, so the decision is not quite so critical. Again, woven fabric should be used, but not a heavy weave. The aim is to laminate strips of fabric into the wide V-shaped cuts and any areas where pieces of the original laminate are missing.

Put in as many plies as practical, increasing the width of the strips as the cuts fill up, but there is no need to attempt to fill these areas completely. The laminated strips should be left to cure, after which the sander can be lightly run over the surface to remove any protruding bits of laminate. If the repair is being carried out on a very thick component (6mm or more), it will be possible to sand enough of the front face away to laminate a single ply over the entire area of the repair. However, for most normal thicknesses of laminate, there will not be enough material to do this.

The next stage is to level off the repair with commercial body filler. If there are any areas where this is likely to be very thick, it is best to build up the thickness in two or more applications. This will prevent too much heat from occurring during the cure, which can cause shrinkage and cracking when the filler has been sanded down to the original contours of the component.

At this point, the repair may be primed and painted, but it could be taken a stage further before painting. By sanding down the repair area a little more, a gelcoat can be applied, which can be rubbed down and painted over. If the component was painted originally, this means that the new paint will sit on the same type of base and should weather in the same manner as the original. If the component has a self-coloured coat and no paint finish, it is possible to colour the gel coat of the repair which can be rubbed down and polished.

However, this is more difficult than it sounds, particularly if the area is quite large. A thick gel coat will be required, and the best method of achieving this is with two, or even three, coats, allowing each to cure partially before applying the next. When all the gelcoats have been applied, allow the repair to cure fully before starting to rub down the finished surface. If time allows, it is best to leave , gel coat for several days before carrying out any further work: the harder the gel coat, the easier it will be to obtain a good finish.

A major problem is obtaining a good colour match, even if the same pigment is used, since the percentage added affects the density of colour. Another drawback is that tiny air bubbles may become trapped in the gelcoat, although this can be avoided by applying several thin coats     .. Despite the difficulties, if the original component is rare or valuable. this may be the only method open to you.

Repairing stress cracks

Another type of repair that may need to be undertaken is old age. Quite often, older GRP components suffer from fine cobweb-like cracks that radiout from certain areas.

This form of cracking is caused by the release of stress, which builds up within the laminate as it cures when the component is made. Many such components are rectified by sanding down and painting, but this is a very short-term solution because the paint merely bridges the cracks, and it's only a matter of time before they reappear... Although an old component can never be made as good as new, with a little more effort, a longer lasting repair can be achieved.

Most GRP repairs are variations of the methods outlined here. However, for minor damage, such as a dent, begin by sanding down the reverse face to the original contour, even if this means going partially through the laminate. Add a patch to the back, as before, then make good the front by filling. Epoxy laminates can be repaired in the same way. Stress cracks are unlikely to appear on well-made epoxy components, so most epoxy repairs will be necessitated by damage.

Small dents and chipped self-coloured gel coat finishes, etc, are relatively straightforward to repair. The former can be filled with body filler.

Modifying existing components

Modifications to boat GRP components require slightly different techniques to achieve the desired result. If the proposed GRP modifications are an extension of the edge of a component, or the addition of a flange of some kind, begin by tapering the edge with an electric sander. Also, sand back through any gel coat for a distance of approximately twice the width of the proposed addition.

As the new piece will be added to the edge of the component, both sides of the laminate should be sanded back. Then calculate number of glass plies needed to make up the component thickness. and laminate half on each side of the prepared area. Ideally, this should be carried out with epoxy laminating resin, but if this is not available, polyester will make an acceptable modification.