Class A Finish

Question:

We have to make composite parts requiring a Class A finish. Could you help me understand what that is? What standard or method is used to define a Class A finish? Actually we make composite parts with glass fiber and vinyl ester. First we want to evaluate the surface quality of the original parts and then apply a gel coat. Since the parts will be used in a marine application, they need to be water-resistant. Then we have to evaluate them again to see what kind of surface quality we have after water immersion compared to the original one. Normally, how do you qualify the surface quality? W.L.

Answer:

You have two issues here, a Class A finish and a water-resistant finish. Class A finishes generally have the fewest surface defects and most consistent color and gloss. A good example would be a refrigerator door or an automobile hood. However, a Class A finish, like beauty, is in the eye of the beholder. To provide a Class A finish for a potential customer, you must know his finish specifications as to color, gloss, allowable surface defects, etc. These properties vary from manufacturer to manufacturer. He could use company specifications, industry association specifications or standard specifications such as ASTM or others.

I gather from your question that if your composite part does not have a Class A finish as manufactured, it must be gel coated. Furthermore, if after immersion to check for water-resistance, the composite absorbs water and consequently does not maintain a Class A finish, it must be gel coated. This is because the presence of glass fibers on the composite surface will not only enhance water absorption but will also not provide a Class A finish. Not only will they look bad, the surface fibers will act as wicks allowing the water to enervate the composite.