One of the most neglected procedures concerning gelcoat, as well as all coatings, is proper agitation. Proper agitation is as important as maintaining the correct film thickness and catalyst level. Gelcoat is made up of ingredients that have different densities. Shortly after packaging, these ingredients begin to separate. After a drum of gelcoat has been packaged for thirty days or more, some of the pigments and thixotropes can settle in the drum. The lighter materials such as solvents (styrene) will float to the top, leaving the resin in the middle. More separation occurs the longer the material is stored.
To insure the separated materials are redistributed evenly, proper agitation is imperative. Rolling the drum over the floor or bubbling air through the drum or stirring with a plank will not adequately agitate the material and may also have safety implications. For 55 gallon drums, the recommended agitator should be the type that has pitched blades approximately 14-inches in diameter. Common suppliers of these agitators are MVP Inc. and Binks. Pails also must be agitated.
Mixing must be done prior to taking any sample from the material. Listed below are a few of the most common problems that can occur without proper agitation:
- Pigment float
- Resin tearing
- Poor hide
- Poor color match in cosmetic repairs
- Extended film cure
- Gelcoat film on mold after part has been demolded
- Mix daily 10 to 15 minutes. A higher speed might be needed for the first 3 minutes to get material moving, then a lower speed so that there is just enough rotation and visual pumping that the material turns over but not enough to entrain air.
- Use caution in mixing partial drums and not to over mix.
- Pails can be agitated with a small propeller mixer or even shaken for 2 minutes on a three-dimensional shaker. Care must be taken not to entrain air in the gelcoat.
Allow the material to recover for 15 to 20 minutes prior to use.