Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2004

Pin Holes in Powder Coatings

Q. What is the reason for the appearance of pin holing, only in the machined surface after powder coating a grey iron?

A. Typically pin holes are caused either by some type of contaminant on the surface or by solvent popping. Because you are using a powder coating we can eliminate the second cause. My first response would be is the machined area clean? What type of cleaning/pretreatment process do you perform on the parts prior to the application of the powder coating? Do the parts go through a cleaning/phosphate process prior to coating, or just a solvent wipe.

Check to be certain that the parts are very clean prior to the application of the powder coating. Take a clean white cloth and wipe the machined surface with the cloth, and take note to any discoloration of the cloth. Perform the same wipe test on the entire part, and look to see if you find any discoloration.

If after performing these quick tests and you still are unable to determine the cleanliness of the parts, you can perform what is called a water break test. The water break test is just like washing your dishes at home, after cleaning the parts well, coat them with water and take note. The water should appear as a continuous film, no break in the film. If you detect a break in the film, the part shows a contamination on the surface, and is not clean.