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5.1 Mixing Resin and Gelcoat

Resin is stored in drums in a cool, shaded part of the yard preferably a separate resin room. The drum in use will be laid horizontally off the ground on a steel cradle (Figure 9) with a transferrable brass tap (Figure 8) screwed into the drum lid.

There should be a drip tray beneath the tap. Between the tap and the tray will be enough room to place a 5–10 1 plastic bucket which can be filled with resin for use by a laminator. Outside the resin room should be a separate store cupboard for catalyst and a second cupboard, well distanced from the first, for accelerator if the resin is not already pre-accelerated. Accelerator can be added during the preparatory phase and well mixed in with a clean, disposable wooden stick. Catalyst is only added AT THE VERY LAST MOMENT before laminating commence and when the laminating crew have agreed that EVERYTHING is ready.

Both these additives are supplied in 20 l plastic drums (not in steel containers because resin reacts with rust) and are used to refill one litre plastic bottles fitted with a graduated pourer for final mixing. This preparation is relevant to gelcoat and laminating resin, but in the case of the former a colour pigment will have been added to “unpigmented” (uncoloured) gelcoat according to supplier's mixing ratios. Standard white pre-pigmented resin is widely available as it is the most popular hull and superstructure colour.

5.2 Application

After the mould has been cleaned, waxed and polished, the first step in the moulding process is the application of gelcoat by the laminating team of 2 persons with mohair paint rollers or by spraying. Some rollers have handles extended to 2 m to gain access to distant areas. A single heavy coating of 0.6 mm thickness or 2 medium coatings of 0.5 mm each is required on all surfaces which may curve from vertical topsides to the horizontal bottom area. Paint brushes with unpainted wooden handles can be used to cover restricted corners. A pot life of 20–40 minutes will be planned which will require a 1 % catalyst content in the tropics.

It is preferable to have a follow-up bucket of resin ready for use immediately the first is emptied so that a good molecular bond between the two separately mixed batches is achieved. The second laminator can take care of this or a labourer may be employed if the mould is large and requires many buckets of resin. At the end of the gelcoating the tools will need cleaning. Buckets should be left to drain out and any resin which remains will harden and can be cracked off some hours later. Rollers and brushes need more care and will need washing in acetone. This liquid is also HIGHLY FLAMMABLE and evaporates quickly. Personnel should rinse any exposed skin in clean acetone followed by the tools in the same liquid before washing skin and tools again in soapy water then fresh water. Brushes and rollers must be dry before re-use.

Figure 9

Figure 9 Drum cradle

5.3 Preparing reinforcement

Reinforcements also deserve to be stored in a separate room or at least a section of the moulding shop which is kept dry, dust free and clean. It is quite usual to allot the task of storing and tailoring the reinforcement to one labourer who then becomes skilled at this task. It is initially stored in its delivery cartons thus kept free from most contamination. For preparation, a cutting table is needed with a vertical rack for the rolls of different types of reinforcement from which appropriate lengths can be drawn and cut and then re-rolled and labelled with a felt tip pen to indicate to the laminators precisely what it is for. The cutting tailor will prepare the reinforcement for a whole vessel according to a cutting list which has been previously written for each type of vessel in production.

Figure 10

Figure 10 Reinforcement dispenser

Lengths up to 10 m can be prepared on a shorter cutting table by folding the reinforcement. The table edge should have graduations at 10 cm intervals to ensure precise measurement. A sharp knife is needed for cutting and as glass reinforcement blunts steel blades very quickly, they should be sharpened often. A straight edge is needed to cut the glass to length, and aluminium strip is best as it is light to handle and does not contaminate the cloth.

For cutting curves, a felt tip pen or chalked line can be drawn for guidance and the cut made freehand whilst allowing a 10 cm margin. Straight cross cuts in mat and cloth should be made using the straight edge but for WR a transverse strand can be pulled out which will indicate the line to be cut even if the material has deformed in the unrolling process. Industrial quality scissors may also be of use.

5.4 Laminating (also termed laying-up or moulding)

Once the resin, reinforcement, mould, personnel, and tools are ready, the catalyst can be added to the resin and the laminating begun. This assumes that the gelcoat has been applied to the mould already, has cured, has been checked for flaws and is considered acceptable. If gelcoat has been applied at the end of the working day it will be ready the following morning, if it has been applied in the early morning, it may have achieved sufficient cure to be overlaid at the end of the working day. If an attempt is made to laminate over gelcoat which is not cured sufficiently, the exotherm from the lay-up resin may deform the gelcoat and leave problems which will only be visible when the hull (or deck) is released from the mould. If the gelcoat is left for too long (a weekend) it may harden to a point which will not provide good bonding with the subsequent laminate. Reference to the manufacturer's instructions and experience will soon further define these broad outlines.

Inspection of the gelcoat may indicate contamination by rain or sand and this should be remedied before any new resin is applied. Any rough areas may need to be lightly sanded and any dust removed. Any overcured areas can be lightly washed with acetone or styrene to regain some tackiness. The designer and manager will have decided if reinforcement is to be laid transversely or fore and aft, the latter is recommended as it more easily accommodates changes of skin thickness required between topsides and bottom and is the faster method.

A sequence should be worked out so that the binder is dissolving in CSM reinforcement laid in one part of the mould while previously laid CSM with now dissolved binder is being consolidated in another part. While a layer is curing in one side of the mould, the other side can be worked on. This allows application of resin and reinforcement, consolidating and curing to take place in rotation.


It is usual to lay up CSM alone for 1 or 2 layers next to the gelcoat. Subsequent layers should be applied as soon as the resin hardens. These subsequent layers may be augmented by the inclusion of a cloth or WR layer which can be laid in the mould at the same time as CSM and which are consolidated together. This saves time and achieves a superior bond as both layers of reinforcement are using the same batch of resin. WR is applied after its associated mat layer has been lightly rolled into the resin. If there are large dry patches, further resin can be applied before the WR is laid on as better impregnation is achieved if the resin is drawn up and not forced down through the reinforcement. Being of a woven nature, WR is less prone to disintegration than wet CSM during rolling out, it can be consolidated using rollers, but a “squeegee” is probably the best, quickest and simplest tool to use. The sequence for subsequent layers has been described, but it should be clearly understood that the opposite side of the hull should be moulded in sequence with the first side.

On returning to the first side, the laminate may have become too hard and parts of it may need sanding to remove any protruding pieces of resin and glass which would prevent the following layer from achieving a void-free bond. These cured surfaces can be walked on to achieve access to restricted areas of the mould, but this risks contaminating the next bond. Walking on the FRP in barefeet is one solution, but even slight perspiration from skin in contact with the FRP can lead to a weakening of the bond in that area. Lightweight shoes should be provided to the laminators to eliminate this risk. Further protective clothing such as an overall is also a good idea as the inevitable contact between clothing and wet FRP cn rapidly render a laminator's own clothes unwearable. Filter face masks should also be worn to counter long-term respiratory problems but in practice these are usually declined by experienced laminators as the discomfort while wearing one is greater than the short-term unpleasantness of breathing styrene fumes.

For any job which requires working in a confined space such as a fish or fuel tank, the laminator should be ordered to wear a face mask. The work should be in short periods and the space should be force ventilated or exhausted. An electric fan may be used to BLOW fresh air via a long pipe but it should NOT BE USED TO EVACUATE a space nor be placed directly in the space as fire may result from the flammable styrene in contact with the electrical contacts of the fan.

5.5 Stages of Cure

Storage or shelf lifeWhile pourable Up to one year if stored correctly
Pot life or gel time20-40 min Depends on age, catalyst and accelerator
Hardening time130 min
Fully cured1Three weeks from addition of catalyst

5.6 Using Tools on FRP Mouldings

No special tools are needed on FRP after it has set hard, but metalworking rather than woodworking tools should be used. It can be drilled, filed, sawn and polished but not hammered or bent and not easily punched or sheared. The basic shape cannot be altered and the resin component shows a tendency to fracture and chip. It should be remembered that apart from trimming and drilling very little work is usually required because all the shaping and forming is done while the plastic is soft and uncured in the mould. In this respect it is like concrete.

1 Hardness to be tested by using a “Barcol” Impressor model 934 or equivalent method

Table 5

Gel time and temparature

Table 6

Hardening time with hardness

Table 7

Cure times

The resin component cracks and shatters if overstressed. It has little ductility and it does not deform and yield like a metal, or even like wood. Any fastening which depends on deforming or denting the material will not hold satisfactorily, e.g., a wood screw or nail, and bending is as impracticable as bending concrete. Although the basic shape cannot be altered the natural springiness allows a certain degree of minor distortion. The extent of this will depend on the framework and the age of the moulding. It is much more flexible when newly moulded and still in the gel state than when fully cured. A sawn, ground, or sanded surface will have a matt finish and cannot be polished until recoated with resin.


Holes can be drilled easily with ordinary twist drills. Wherever possible drill from the smooth face towards the rough side as this prevents chipping the resin-rich gelcoat. Masking tape or a punched mark will help prevent scratch marks from skidding drills if a hole has to be drilled from the gelcoat side. For large holes use a tank cutter or hole saw or drill perforations around the circumference of a large holes with a drill bit wide enough to accept a jigsaw blade, then saw it out.


A hacksaw, padsaw or jigsaw should be used for sawing and trimming. Always saw on the face opposite the gelcoat to avoid chipping. Heavy duty electric jigsaws with metal cutting blades or diamond wheel compressed air cutters are the most effective.


FRP files easily much like aluminium. An open pattern file is less liable to clog particularly when the resin is not fully hard. On edges, the cutting stroke should be in the direction away from the gelcoat once again to avoid chipping. Extra care is needed on corners.


Light hammering to encourage mould release or alignment of two mouldings should be carried out with a rubber mallet. Hammered fastenings or blows with a metal hammer will shatter the laminate.


Avoid sandpaper (sand or gold coloured) as it will clog badly particularly if the resin is wet. “Wet and dry” (black colour) is the only suitable kind and it must be used with plenty of water. A small amount of liquid soap added to the water will reduce friction. For power sanding, a very open grade of disc will clog least. Resin bonded discs must be used. Care must be taken as disc edges can make deep scars in a moulding before an operator realize what is happening. Face masks should be worn and dust removed before applying the next layer.


Files, drills, saw blades and discs can be cleaned with acetone when clogged. If left overnight the resin will harden and render the tool useless.

Effect of Fillers

The workability of resins can be affected to a large extent by fillers. Talc has a lubricating or anti-clogging effect. Hard fillers such as quartz, silica, or slate render the operation similar to drilling rock. Where hard fillers are encountered a masonry drill should be used for drilling and a diamond wheel for cutting if the opportunity to trim with a knife during the gel stage was missed.


The easiest stage to trim is just after the resin has set, when the moulding is rubbery and can be cut with a sharp knife. A large moulding must be trimmed in stages as each layer sets. The rubbery “gel” stage lasts only about thirty minutes depending on the type of resin used. Do not trim too soon as the laminate will not have set enough and will be disturbed. During fitting out hard untrimmed areas will be encountered and should be treated as described in the note on sawing.

Sealing Edges

All FRP working will leave a rough, shattered edge where water can penetrate and eventually cause delamination. All such edges must be sealed either with resin painted on or bonded over during assembly. Holes for deck and underwater fittings should be treated carefully in the same manner. Working on sandwich mouldings demands special care as there will be the likelihood of causing delamination between the opposite face and the core. If possible work from the gelcoat face and support the inside with a wooden pad. Seal the hole edges thoroughly.

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