Powder Coating Recoat Procedure

Application of a second coat of powder over a fully cured first coat is a common practice. Sometimes a second layer is planned because the first layer is used as a primer that is followed with a suitable topcoat. Sometimes a second layer is added due to the fact that the original coating was flawed and a second coating is necessary to correct the defect. It is also possible that a primer layer is used and the second coat is a topcoat layer. This is a description of the normal procedure and cautions regarding the quality of the finish when applying a second layer of powder.

Make sure that the first coat is clean and free of particles or blemishes that will show through the second coat. All dust, fingerprints, oils or other lose particles must be removed. Rinsing the part surface is advised to be certain that no residual soils remain on the surface.

If it is practical, a light sanding of the surface is recommended. This will provide a level surface and create a substantial profile for adhesion of the second coat. Using a fine to medium grit (60 to 120) sand paper or Scotch-Brite pad, lightly scuff the surface in a uniform pattern taking care to minimize deep cuts in the film. Blow off loose dust with clean, dry compressed air or rinse with very clean water after sanding.  Deep cuts from a heavy grit sand paper or grinding disk will likely show through the second coating layer.

1.       Make the following adjustments to the powder spray gun:

• Increase the normal flow rate by 10 to 15% from the original setting used on the first coat.
• Adjust the amperage down to around 10 to 20 micro-amps or use a factory pre-set for recoat.
• Move the spray gun around 25 to 50 mm farther from the part than the normal gun-to-target distance used on the first coat.

2.       Apply a complete coat over the entire surface so that there is no visible shine of the first layer showing through.

3.       Run the part through a normal cure cycle.

Notes:

• Before adding a second coat of powder over a fully cured existing powder film the material should be tested for inter-coat adhesion. Apply a second coat over a test panel or part and conduct an adhesion test according to ISO 2409 or ASTM D3359 before running a production part through recoat. If the inter-coat adhesion fails it may be necessary to sand or scuff the first coat as described above in order to assure that the second coat will adhere.
• A second coat will add thickness to the film and may affect fit to another process or part.
• The second coat is likely to have more orange peel.