Cloudy Look in Clear-coat

Q. We apply a clear coat powder material over a steel substrate. We polish the steel to get the aesthetic look we want and then blow it off with compressed air. We need swirls of the polish step to show clearly for our intended look. We sometimes have problems with yellowing of the coating. The clear is kind of milky and clouds the look from the steel. What can cause this problem? How can we avoid it from happening?

A. A cloudy look in a powder clear-coat can be caused by too much film or an some factor in the cure oven. First, check that the film meets the manufacturer’s specifications. If the film thickness is correct the problem could be related to over-cure or oven fouling. Over-cure will occur if the oven is run too hot or the part is in the oven for too long. Run a Datapaq temperature recorder or similar device and make sure that the time and temperature are within the recommended range. If the film thickness and cure cycle are accurate the final possible cause is inadequate exhaust volumes in the oven. If the exhaust rate is too low the oven will build up with gases that can cause the discoloration of the coating. You can check your exhaust volume by measuring the face velocity across the oven opening. With exhaust fan on and the burner off, determine the average face velocity across the opening. Next, measure the width and height of the opening to determine the area in square feet. Multiply the area times the average face velocity and you have the fan volume in cubic feet per minute (CFM). Be sure to measure both openings if you have more than one. The exhaust volume that the oven should have is based on the amount of powder per hour that you put into the oven. That formula is shown below.

1. Pounds of powder per hour x 9% x 360 x 4 = cubic feet per hour (CFH).

2. CFH divided by 60 minutes = standard ft³ /minute (SCFM)                                                    

3. Temperature correction: 

SCFM x 460 + Maximum Temperature/ 460 + Ambient Temperature = Ft³/minute (CFM)

4. Add for products of combustion:    

Max burner BTUH/95 cubic feet per second x 60 seconds = SCFM

5. Correct for temperature as in step 3

6. Add 3 and 5 to get total CFM of exhaust

Compare your actual exhaust volume to the results of the formula and see if the oven is exhausting enough air.

Apply the right film thickness, not too heavy. Make sure you use the right cure cycle. Be sure you have the right oven exhaust volume. That should take care of the cloudy issue.