Clear Coat Over Silver Powder

Q. I had a commercial applicator powder coat a luggage rack for a motorcycle, applying one coat of a silver color and then a topcoat of clear powder. A few months later, the clear finish cracked. The motorcycle is garage-kept. I consulted with another powder coating company, which said the powder was probably not polyester-based. Would polyester be best in this application? Do all silvers have to be clear-coated? Do only silvers need clear coat? J.H.

A. Most silver colors contain metal flake to get that bright silver appearance. Metal flake appearance can be affected by exposure to sunlight and handling, therefore, it is usually desirable to apply a clear coat of polyester over the silver metal flake. Polyester resins can protect against harmful UC rays for a longer period of time.

As far as the cracking is concerned, there are several potential causes. Powder coating requires a series of steps to be performed correctly to achieve optimum results. Indeed, the clear coat could have been an epoxy resin, and that would break down in sunlight over time. However, it is more likely that the material was not applied or cured correctly. First, the second resin must be compatible with the silver metal flake product. Second, the amount of cure in the first and second coats will affect how well the two coats work together. If the first coat is too hard, the second coat may not adhere to it very well. If the second coat is over-cured, it will be brittle and prone to cracking. It sounds like the coater used the wrong clear-coat product for your job and then may not have gotten the correct cure cycle (time and temperature).