Ask the Expert Question-and-Answer Archive

by Ron Joseph

May, 2007

Cracking of "Lacquer"

Q: I primed & sprayed black enamel, cured, then sealed with poly, but in one spot, the poly appeared milky, so I immediately retouched the wet poly with more enamel, which resulted in a horrid crackle appearance. The rest of the surfaces are mirror perfect. How do I best repair that 3" diameter area?

A: Unfortunately, you have given me generic information and haven't told me exactly what products you used to repair the guitar finish. What type of black enamel, did you use? What type of polyurethane did you use; a single component clearcoat that you purchased from a local hardware store, or a two-component polyurethane that can only be purchased from an industrial paint store?

Based slowly on the information you provided, it sounds as if you experienced typical incompatibility between the two paints. The polyurethane and the black enamel appear to be incompatible with each other. Usually the solvents in each coating cause the incompatibility. I suggest that you perform an experiment on a separate piece of wood or metal in which you apply the black enamel and allow it to dry thoroughly, perhaps even for a few days before applying the polyurethane clearcoat.

In most cases after all the solvents of the first coat have evaporated it is possible to apply at the second coat without problems.

Some resins, such as air-dry acrylics or air-dry alkyd modified acrylics are so sensitive that a recoat window must be established. For instance, in some cases it is possible to apply an acrylic over itself within two hours and then again after eight hours. Surprisingly, if you apply the acrylic over itself between two and eight hours incompatibility leads to what I assume represents the cracking you experienced.

In summary, I suggest that you perform your own experiments before going back to repair your guitar.

Regards,

Ron Joseph