Proper gel coat application is difficult for professionals and is probably the most common problem area for those new to fiberglass work. Most common gelcoat problems are related to temperature variation, catalyzation and a variety of handling techniques. The following list of gel coat problems and causes should help to resolve some of the difficulties associated with gel coat.
A coating less than five mils thick may wrinkle, especially when brush marks are present. Check the thickness using a gel coat thickness gauge. The preferred thickness is .010" to .020".
A wrinkle can also occur if the gel coat is not cured enough prior to lay-up. Before lay-up check the surface for tack. The surface should be sticky but not transfer to your finger. Several things can affect a slow cure; solvent or water entrapment, under catalyzation, cold temperatures and other factors.
If the gel coat film contains pinholes, check the spray equipment for moisture in the air lines or dirt in the traps. It is also important to review your spray techniques.
Any temperature below 70 degrees F will retard the gel time. An under-catalyzed gel coat will also result in a slow cure but this is not a recommended technique for deliberately lengthening gel time. High moisture and humidity will lengthen the gel time.
The most common reasons for gel coat sags would be when the gel coat is applied in an excessively heavy application in one or more passes. To avoid sagging, spray in multiple light coats at a 15" distance from the mold. The maximum thickness to be sprayed at one application is .016".
Gel coat porosity (air bubbles) is normally caused by trapped air or moisture. This can be prevented by limiting the spray thickness per pass to .005". It is also important to verify that the proper air pressure is being used because an over pressurized spray can cause fine porosity. Air pressure should be set based upon the thickness of the gel coat. It is also important to avoid any spray overlap areas. 40-80 PSI, depending on viscosity.
Most color separations are related to improper spray techniques. Reduce or lower thinning agents and lower the per pass thickness of the gel coat. It is also important to avoid any spray overlap areas.
This problem is caused from porosity related to air entrapment while spraying. This can be overcome by spraying the mold surface with several light passes. It is also important to bleed any moisture from the air lines and to be sure that the mold surface is dry.
These surface blemishes can be caused by improper resin to catalyst ratios and improper air pressure both high and low. Another cause for craters would be oil or moisture on the surface of the mold.
Fisheyes are usually caused by contaminates on the mold like dirt, moisture or oils. Bleeding the air lines and using air filters will lower any air related contaminates. Try to isolate the spray area from any oils, especially silicones.
Blisters can be caused for the following reasons:
This condition is usually related to shrinkage of the gel coat. Shrinkage can be caused by:
This visual pattern is caused by too low a gel coat thickness or the gel coat was not properly cured.
This condition is caused by improper releasing of the mold surface. It is important to use release agents formulated for the reinforced plastics industry. Proper training and common sense will eliminate most of the problems associated with gel coats. Proper handling of gel coats will result in a part with a blemish free surface.