We talk a lot about gelcoat on this blog. We have stated on multiple occasions that gelcoat has to be applied with a certain thickness, generally 15-20 mils thick. How do you know exactly how thick your gelcoat coverage is? The answer is that you use a mil gauge, also known as a wet laminate gauge.
A mil gauge is a simple piece that you place on to your sprayed surface when the gelcoat is wet. When you pull it up, you look to see which teeth have gelcoat on them. You look at the highest number that has coating on it and it will give you a mil thickness; simple, right?