So we really want to cover everything here and we feel it should help.
Gelcoat is generally available in ‘clear’ or ‘white’. White gelcoat is based on a clear gelcoat which has been correctly loaded with white pigment. The addition of any other pigment will tend to overload the resin with this inert (pigment) compound, and may affect its setting and curing properties. The addition of any colour to a white gelcoat will produce a pastel shade; very different from the colour of the added pigment. It is possible to add a very small amount of a strong colour to white gelcoat to produce an ‘off white’, without unduly affecting the properties of the gelcoat; however, when colour matching, it is wise to use a clear gelcoat as a base resin. Blue Gee can supply a range of gelcoat colour matching kits.
Colour matching is far from easy, we have a superb blog on the topic, click here to read more. It is often easier to obtain a pigment which is very near to the desired colour and to modify that pigment by adding small amounts of other pigments of a similar colour, before adding to the clear gelcoat. The purpose of this approach is to ensure that the original choice does not vary radically in mixing other colours, the principle underlying the design of the Blue Gee Gelcoat Repair Kits. Often, the parent material colour will have faded and the surface will have lost some of its original gloss, in addition to changes in surface texture. This will tend to confuse the eye when comparing the pigment with the colour of the surface to be matched. When the pigment has been mixed and added to the clear gelcoat, the final colour should be the same, providing clear catalyst is used.
When mixing the pigment and catalyst into the gelcoat, it is inevitable that some air will be entrained into the gelcoat. For small repairs, spreading the gelcoat onto a flat surface and leaving for a few minutes, may allow larger bubbles to escape, however, the very nature of this viscous material tends to retain trapped air.
Trapped air may show up as pin holing in the repair when it has been flattened back to the surface. High quality repair finishing is possible using an airbrush, loaded with gelcoat which has been thinned with styrene monomer, preferably with some ‘wax in styrene’ additive to avoid tackiness caused by moisture in the air. This may be used to spray the repaired area when the filled surface has been cut back to the original profile. Spraying gelcoat may be a time consuming operation, requiring a few coats, remembering that the airbrush will require frequent flushing with acetone, to ensure that the gelcoat does not set leaving the airbrush unusable. Spray application for gelcoat for thick coatings or large areas is not recommended. Adequate ventilation, and use of protective clothing, including a mask, is essential in all spraying operations.
Wet polyester gelcoat is liable to be attacked by moisture in the air. A small repair may be protected by covering with ‘Sellotape’ or a similar barrier to moisture. Alternatively, the product ‘wax in styrene’ may be added to the gelcoat, and thoroughly mixed – usually at the rate of 2 to 5% of the gelcoat volume – to provide resistance to airborne moisture. For large gelcoat repairs, or coating, this additive is essential – do not use wax in styrene additive when gelcoating in a mould for overlaying by a laminate (it interferes with further bonding). A common concern in small gelcoat repairs is that the surf
ace is remains ‘tacky’ – leading to the belief that the gelcoat has not set. Providing that the gelcoat has been correctly mixed, usually this tackiness is due to airborne moisture, and only affects the surface of the work. This ‘tack’ may be removed by ‘wet or dry’ paper, or, providing the gelcoat has fully cured, with acetone – be careful – since an uncured gelcoat may be ruined by the premature application of this powerful solvent!
A gelcoat may be thinned by the addition of styrene monomer, although it must be noted that this will extend the setting time, and overcoating should not be attempted before the excess styrene has evaporated. Thickening of gelcoat may be achieved by the addition of colloidal silica, which should be well mixed into the resin – this will further increase the non-sag properties of the gelcoat, although it must be added that this filler is very rarely necessary since a good quality gelcoat will have adequate non-sag properties for a reasonable coating thickness.
The addition of pigment, styrene monomer, wax in styrene, or colloidal silica will not trigger any setting process in the gelcoat. The setting and curing of the gelcoat is initiated by the addition of the catalyst, commonly of medium reactivity, at the rate of 2% by volume for most commercial gelcoats. 2% (two parts catalyst per hundred parts of gelcoat resin) roughly equates to 6 drops of liquid catalyst per 10 millilitres, or c.c., of gelcoat. Thus 100 c.c. of gelcoat mixing would require 60 drops, or 2 c.c. of catalyst. Clearly when mixing large quantities of gelcoat, this ‘drops’ technique would become tedious, but it has been found to be a useful measure for small quantities.
Temperature has a marked effect on gelcoat setting time, see our blog for more information on this. Gelcoat will ‘read’ the temperature of the surface to which it is applied. It is most inadvisable to attempt gelcoat repairs at temperatures below 5 Deg.C., and at any temperature below 10 Deg.C., additional heating is advisable, although it must be ‘dry’ heat = combustion heaters produce large quantities of water vapour which has a very adverse effect on the resin. A shielded light bulb often provides overnight protection. In hot conditions, beware overheating of the gelcoat in pot.
Finally, on the subject of mixing, never mix up more material than may be decanted from the mixing pot within ten minutes. Depending upon the quantity mixed, and the day temperature, pot life may be even shorter – usually 20 minutes! Material working life may be significantly extended by decanting the catalysed gelcoat into a tray. It should be remembered that the setting and curing time will be much longer than the pot life, or working time.