Technical

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FIBREGLASS REPAIRS
Reproduced from the Marcos Club Magazine August 1979 - original article by Marshall Price

INTRODUCTION

It has been known since about 1500BC that molten glass could be drawn into monofilaments. The Egyptians, and later the Romans, used glass strands in jewellery and ornaments, and for over three thousand years these were the only uses to which they were put. At the beginning of the Eighteenth Century, it was realised that glass strands could be woven into fabrics, either on their own or in a combination of textiles. Since glass was hand drawn, however, these fabrics were both coarse and expensive and were really only much use as thermal insulators.
 

The first true fibreglass appeared in the mid-1930s, with the introduction of the continuous filament process. In this technique, which is still in use, glass of a suitable composition is melted in a platinum crucible and falls under gravity through a series of plates known as Bushings, which contain progressively finer perforations.
 

After passing through the final plate, the by now semi-solid glass threads are drawn out (attenuated) at high speed, cooling and solidifying as they stretch, until the resultant fibres are of the desired thickness; for laminating purposes this is between 0.025mm and 0.01mm.
 

To put it very simply, the finer a fibre becomes, the higher is its strength/thickness ratio. A fibre of the type described will have a tensile strength well in excess of 250KgF/CM2 and a ‘bending’ strength of at least 4,000KgF/CM2. A mass of such fibres, when bonded in a suitable resin matrix, provides one of the strongest engineering materials in common use, pound for pound far stronger than steel.
 

The results of such early attempts to produce Glass Reinforced Plastics (GRPs) were not very satisfactory, owing to the properties of the resins then available. These required very high moulding pressures, which resulted in crushing of the glass fibres and consequent loss of strength. In the early 1940s, however, a new group of resins was introduced. Known as contact resins, these could be used without pressure, and could be made to cure at room temperatures. Of this class, those having a polyester formulation are the cheapest and easiest to use and are therefore the best known and most widely used.
 

The starting point for the manufacture of these resins is the reaction of an Organic (Carbon-containing) acid on an Alcohol, yielding a compound known as an Ester. If the acid is Polybasic (having two or more reactive hydrogen atoms within its structure) and the Alcohol is Polyhydric (having two or more reactive Hydroxyl (OH) groups within its structure) then the product may take the form of a chain of Ester molecules strung together, this macromolecule being known, not surprisingly, as a Polyester.
 

If the Acid or Alcohol contains within its structure an Unsaturated (multiple) Carbon-Carbon bond, then the Polyester so formed may react further with other Unsaturated molecules (Monomers, commonly Styrene or Diallyl Phthalate). The result of this reaction is to interconnect the long but mobile Polyester macromolecules to form a rigid cross-linked three-dimensional molecular framework, the liquid resin thus becoming solid.
 

The commercially available Polyester Laminating Resins consist of solutions of the Polyester dissolved in the cross-linking Monomer. In order to initiate the reaction between these, it is necessary to add another component, the Catalyst, which usually consists of an Organic Peroxide and comes dispersed in an inert liquid or solid carrier in order to counteract its otherwise embarrassing tendency to explode spontaneously! It is normal to include yet another component, called an accelerator, which allows the reaction to proceed at room temperature and in a reasonable time. These accelerators are usually based on a class of Nitrogen-containing compounds called Tertiary Amines, and often come already dissolved in the resin, when it is referred to as a pre-accelerated resin.
 

It is worth mentioning here that if ever a resin is encountered which requires the addition of a separate accelerator, this should be well mixed in before the catalyst is added, and on no account should the Catalyst and Accelerator be mixed together (because if you do, they’ll explode!).
 

THE PRODUCTION OF GRP MOULDINGS
 

The first stage in the production of a GRP moulding is the creation of a full-sized replica of the item to be reproduced, known variously as the Plug, the slab or the Dummy. Any imperfections in this dummy, no matter how small, will be faithfully reproduced in the final moulding and a good deal of time is therefore spent on the preparation of its finish. The final inspection is carried out by giving the whole thing a fine coat of black gloss paint and then viewing the reflection of a light source from the surface, thus revealing any minute imperfections which may remain and allowing them to be removed.
 

The Mould is made from this Dummy by laying-up GRP around it, to produce an ‘inside-out’ copy. After the Mould has cured, but before it is ‘broken away’ from the Dummy, external stiffeners of wood or metal may be bonded onto it in order to ensure absolute rigidity. The Mould is then broken away, and its inner surface is prepared in much the same way as the outer surface of the Dummy, by filling and rubbing down.
 

To produce a moulding, the inside of the Mould is coated with a Mould Release Agent (in rather the same way as a cake-tin is greased), which is then polished flat. The layer of resin which will become the gel coat of the finished moulding is then brushed or sprayed over the release agent in a layer which should be between 15 and 20 thou’ (350-500 microns) thick, though this is obviously difficult to regulate.
 

Modern Laminators appreciate the importance to the longevity of the finished product of totally eliminating air bubbles from this gel-coat, otherwise said finished product could fall to bits very quickly indeed!
 

Ideally a layer of surfacing tissue (a fine web of glass fibres laid in random directions) should be laid over this gel coat, but this is often omitted due to financial considerations. Next, a pre-cut sheet of Glass matting is laid down. This matting consists of bundles of glass fibres ‘glued’ together with a size, which is chosen to be compatible with the Laminating Resin used. These bundles may be either woven or sized together to form the mat, depending on the properties required, but are most often size into ‘chopped strand mat’ which consists of bundles about two inches long laid in random directions.
 

This matting is graded by weight, the most common and the best for general repairs weighing 1½ ounces per square foot. The resin is sprayed or brushed over this matting, and worked well in, eliminating all air, with a spiked roller or stippling brush, and further layers of matting and resin are ‘laid up’ until the desired thickness is attained. It should be understood that the only function of this resin, which has little intrinsic strength or modulus, is to bind and support the fibres, which have high strength and modulus and transfer stresses to and between them.
 

THE REPAIR OF IMPERFECTIONS AND FAULTS ARISING FROM IMPERFECTIONS
 

Ripples and Surface Imperfections
 

Until very recently, assuming it was too late to take the vehicle back to the manufacturer, there was very little that could be done to rectify rippling that was at all extensive, short of rubbing down and very laborious hand-filling.
 

This job has recently been made very considerably easier by the introduction of GRP-compatible spray-on fillers, which are rather softer than normal body fillers and may therefore be applied and rubbed down very easily.
 

It is necessary to remove all conventional (non-resin based) paint from the area to be treated, preferably with a sanding disc (280 grit) in an electric drill, which will provide a good key for the filler. Any ridges may be ground out without worrying about exposing the matting, as far as is possible without unduly weakening the panel, since these fillers are designed to bond with the glass and resin.
 

The filler is mixed according to the individual manufacturer’s instructions, and applied with a spray gun in as many coats as may be necessary, allowing each coat to partially harden before applying the next. Certain of these materials may be brushed on as well as sprayed but it is important to follow the manufacturer’s instructions since some do not begin to harden until they absorb oxygen from the atmosphere while being sprayed.
 

Once hardened, these fillers can be rubbed down in the conventional way, though is it best to leave them to cure for at least 24 hours. Most can be finished with normal paints, but it is always advisable to use one of the new two-pack polyester resin paints which are designed to bond with fibreglass finished and fillers, unless the area is bounded by cellulose, with which these materials are often not compatible.
 

Hairline Cracks and Crazing
 

These are usually the result of too thick a gel coat, or of the surface tissue having been omitted during laying-up. They are confined to the gel-coat and do not penetrate to the laminate, but should still be treated or at least painted over, since otherwise they will allow water to reach the laminate. When this happens the water may be drawn along the glass fibres by capillary action, which can lead to blistering or even to de-bonding of large areas.
 

Cracks should be enlarged into an inverted ‘V’ with the folded edge of a piece of emery paper. The groove should be deep enough to expose the laminate, though should not cut into it, so don’t be afraid to dig in (see Figure 1). Mix sufficient gel-coat (it comes in small tubs) to fill the crack and press it well in with a blunt knife, leaving the surface slightly proud.

Figure 1 - REPAIR OF HAIRLINE FRACTURE

Click on image to enlarge

The gel will not harden in contact with air, so place a sheet of cellophane over the repair and press it down with a flat object (see Figure 2). When the gel has hardened, in about 1½ to 2 hours, remove the cellophane, rub the surface down flush and paint.

Figure 2

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Crazed gel coat may be removed by the judicious use of a sharp chisel. The edges of the area should be undercut as before, but if the area is extensive it may be necessary to ‘rough up’ a few glass fibres in order to provide a good key. The gel coat should then be applied as described.
 

If a panel is extensively affected, as is often the case, the entire gel coat may be removed by grit blasting. This should only be undertaken by a boatyard or an operator used to treating GRP, and the panel must afterwards be treated by the application of a resin based paint.
 

Blisters
 

These may arise as the result of a wrongly compounded or incompletely mixed resin, but are more usually the result of a flaw in the gel coat allowing water to reach the laminate.
 

In the cured resin, there always remains a little unreacted Monomer, which is hygroscopic (attracts water). If a crack or ‘pinhole’ in the gel coat allows water to come into contact with a region where there is a concentration of this Monomer then a solution will be formed which will attract more water, both by virtue of solute’s hygroscopicity and by a process known as osmosis, whereby a strong solution will attract more of its solvent. The end result of this will be a pocket of liquid in the panel, usually between the gel coat and the laminate, which it forces apart.
 

The blister should be completely ground out of the resin. There is likely to be a ‘soggy’ area beneath it, and this too should be removed. The area must be allowed to completely dry out, and the last traces of moisture should be driven off by gentle heating (a hair dryer is ideal). The edges of the ground out area should be undercut and the gel coat repaired as described earlier.
 

Spontaneous De-Bonding
 

This may be caused by any of the factors listed under ‘Blisters’ or by the glass mat containing a size which is not compatible with the laminating resin used. The most common cause however, is the glass mat not being perfectly dry during the laying-up.
 

The sizes used in the production of chopped strand matting will absorb small quantities of water from a damp atmosphere such as found in many industrial premises. If this is allowed to occur, and the moisture is not removed by warming the mat slightly before use, then the bond between the fibres and the resin in the finished laminate will be weak, and will eventually break down. In a severe case, the resin may actually break up, and crumble away from the fibres.
 

The treatment for de-bonding is the same as that for bruising, which is described later.
 

Spontaneous De-Lamination
 

This may be caused by any of the factors already mentioned, though the usual causes are moisture present during laying up at extremes of temperature and allowing one layer of resin-impregnated matting to harden too much before laying-up the next or trying to lay-up with resin that has started to harden.
 

The ultimate effect is that the layers (or laminae) in the laminate begin to separate from one another. This maybe confined to a small area, but more usually whole panels are affected.
 

Small areas can be treated as for bruising, but extensive de-lamination used to mean that the panel was scrap. A process has now been developed whereby de-laminated areas can be injected with a special resin, depending on the cause of the problem, through specially drilled fine holes.
 

This process is expensive, and the life of the repair is a few years at most, but at least it is an alternative to scrapping the part. So far as the author of this article is aware, this process is only available at boatyards, and not many of them at that, but most are quite prepared to work on car bodies and some even carry out accident repairs to GRP cars, and can make an excellent job of it.
 

IMPACT AND STRESS DAMAGE
 

Bruising
 

This occurs as a result of either a single heavy impact or large stress or a series of smaller stresses leading to fatigue. The resin is crushed and the fibres are broken.
 

If a very bright light is shone on the unpainted side of the panel, any bruising will show up as a white area in the resin. In addition, there are usually, though by no means always, cracks visible at the surface of the area.
 

The bruised area must be cut out of the panel with a very fine saw, or better still with a round file in an electric drill.
 

If a coarse cutter is used, it will tear the edges of the cut and cause further localised bruising, which will lead to a weak repair. The edges of the hole should be ground back to an angle of 45O or so, (see Figure 3), and the underside of the panel should be well roughened for about 4 inches (100mm) around the border of the hole.
 

Figure 3
 

Click on image to enlarge

A patch should be prepared from three pieces of 1½ oz per square foot chopped strand mat. The mats should be cut to the same shape as the hole, but the first should be 3½ inches (90mm) larger all around, the second 2¾ inches (70mm) and the third 2 inches (50mm). Lay the smallest mat on a piece of cellophane large enough to overlap the hole by 6 inches (150mm). Mix a quantity of resin according to the maker’s instructions and spread a little evenly across the mat with a blunt knife or spatula.
 

Stipple the resin into the matting with a clean paint brush, adding more resin as necessary, until the resin is thoroughly impregnated with no white areas or air bubbles remaining. Lay the next smallest mat over this, taking care to centre it accurately, and impregnate this with resin as before making sure that no air bubbles are trapped between the layers. Repeat this with the third mat, then paint the roughened area around the hole with resin, working in well with the brush.
 

Offer the patch up to the hole, making sure that it is accurately centred, and press it well down onto the roughened area with a small roller (the type used for rolling the seams in wallpaper is ideal, or failing that a small glass jar) squeezing out as much of the resin from the joint as possible (see Figure 4). The patch should be left to cure for about 24 hours, the cellophane should be removed, and the outer surface of the patch should be roughened with 180 grit emery. Chopped strand filler, or layers of mating and resin, should then be laid into the hole stippling well into the edged of the ‘V’ to within about 2mm of the level required (see Figure 5).
 

Figure 4
 

 Click on image to enlarge

Figure 5
 

 Click on image to enlarge

A single layer of surfacing tissue should then be applied and stippled with resin and the repair allowed to cure overnight before repairing the gel coat as described under ‘crazing’, rubbing down and painting.
 

Fractures
 

These have the same causes as bruising except that the forces are either very localised or very severe, so as to cause the fibres to part.
 

A fracture is always surrounded by a bruised area, the whole of which must be cut out and the hole repaired as described under bruising. In the case of a stress fracture, it is advisable to reinforce the panel by laying on strips of fibreglass 2-3 layers thick, running across the back of the repair at right angles to the direction of the fracture.
 

Starring
 

This is the result of an extremely localised impact, usually on the reverse side of the panel to which the star appears. It is most commonly found among the tops of wings and around the wheel arches where stones flung up by the wheels strike the inside of the panels.
 

Frequently, these stars are confined to the gel coat especially when this is too thick. When this is the case they may be treated as for crazing. More usually, however, the underlying laminate is bruised. When this has occurred, it must be cut out and the hole treated as already described.
 

When stars occur very close together, it is usually easier to cut out the whole of the affected area than to attempt to treat each one individually.
 

REPAIRS TO BLIND PANELS
 

It sometimes happens that a patch must be applied to a blind panel, i.e. one to which there is no access from the rear. In this case, a slightly different technique must be employed.
 

The hole in the panel must be cut to an elongated shape. A sheet of hardboard must be cut to such a shape that it will pass diagonally through the hole, but when laid flat will overlap it all around, preferably by 2½ inches (90mm).
 

Two small holes should be drilled in the board, about 1 inch (25mm) apart and close to the centre. If the patch is to be applied to a curved panel, the line on which the holes lie must be at right angles to the plane in which the curve lies.
 

A patch three mats thick should be laid up onto the rough side of the board with each of the mats cut to the same size which should be slightly less than that of the board. A length of flexible wire should then be threaded through the holes as shown in Figure 6.
 

Figure 6
 

 Click on image to enlarge

The patch is then passed diagonally through the hole, accurately centred, and drawn onto the back of the panel by the wires. It may be held in place by twisting them into a Spanish windlass by means of a stick supported on blocks, as in Figure 7. When the patch has cured, the wire is drawn out and the rest of the repair carried out as already described.

Figure 7

 Click on image to enlarge

PAINTING
 

The best finish for fibreglass is a two pack epoxy resin paint, which won’t crack or flake and has a high resistance to weathering. Unfortunately, before it can be successfully applied, the entire gel coat must be ground or shot blasted away, but the finish really does justify the effort.
 

Next best is a two pack polyester resin paint. It this is to be used, then all traces of conventional paint and primer must be removed and the gel coat should be lightly abraded.
 

Unfortunately, neither of these products is compatible with conventional materials and are therefore only suitable for complete re-sprays. For treating single panels, or for touching up, cellulose paint should be used.
 

TIPS

 Never carry out repairs with gel coat when the temperature is less than 15^OC.
 Always cover gel coat with cellophane while curing otherwise any surfaces exposed to the atmosphere will remain tacky.
 Never carry out repairs with resin or any resin-based material in direct sunlight. Ultraviolet light can adversely affect the curing properties.
 If resin comes into contact with skin, rub it off with washing powder and wash with soap and water.
 To speed curing, especially in cold weather, hang a 100-watt light bulb a few inches under the repair.
 The best and by far the cheapest source of materials is your local boatyard or yacht chandler. Look in the yellow pages.
 

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