Painting Aluminum Panels

Q.We paint embossed aluminum skinned panels filled with foam insulation. These panels are bolted together to build walk-in environmentally controlled rooms. Some rooms, which are run at 37ºC and 95% relative humidity, have paint peeling problems. The paint comes off to bare metal like the strippable coating for spray booths.

Since the panels are too large to go through our power washer, we solvent-wipe them two times with a lacquer-thinner-based washing solvent. Water-based degreasers, we have found in the past, do not dry fast enough to finish properly. Water, which is trapped by a rubber gasket at the top and bottom of each panel, comes out during the spray painting operation and causes water spotting. We spray the cleaned panels with a thin coat of wash primer and finish with two coats of catalyzed polyurethane.

Can you give me any insight as to what we are doing wrong or what other products we could use to get better bonding of our finish coat? Any advice you offer would surely be appreciated. G. W.

 

A.Your paint adhesion problem on embossed aluminum building panels is caused by improper surface preparation, which is common with this substrate. The adhesion failure may be caused by the presence of oily soils, oxidation products or both. Solvent wiping will remove oily soils if properly applied. Since lacquer thinner-based solvents dry so fast, I don’t recommend them for solvent wiping. Furthermore, they will not remove oxidation, which must be removed to ensure good paint adhesion. Water-based degreasers are also ineffective in removing oxidation. Aluminum is generally pretreated for painting by cleaning, etching to deoxidize and conversion coating. A wash primer can also be used, but only after the cleaning and deoxidizing steps.
Since your panels are too large to be handled by your pretreatment equipment, you should consider using precoated aluminum panels. This alternative puts the finishing burden on the coil coated.