Blisters in E-Coat Over Zinc-Plated Steel

Question:

We are a manufacturer of window hardware that is formed from mild steel. Some of our components are zinc-plated by outside suppliers. The challenge we have is that sometimes we E-coat the zinc plated parts fine, and other times they blister. Why? F.M.

Answer:

We had a similar question earlier this year. As I check with various experienced operators about it there still seems to be some mystery about why some coating systems have more problems with this substrate than others. Also, why some parts are good and some are not, as they are coated on the same system.

You might check to see if the problem parts are showing up in the same area of the rack each time. If that is the case, you may have some hot spots in your oven that are causing the zinc to partially melt or to out-gas.

Another theory I have heard is that the parts may be on a section of the rack that is not exposed to enough drying action of the E-coat film before entering the oven. If this were the case it would place more of a dehydration shock on the electrocoat film and the zinc substrate in those locations as parts enter the oven.