Coating Defects

Raised Lumps

Raised bumps in the coating surface.

Large Lumps

Lumps in cured film. Lumps are raised areas typically >2mm diameter.

  • Potential Cause

    Clogging of the powder feed hoses, caused by agglomerates building up in spray equipment.  These are periodically freed by delivery air.

  • Solution

    Change hose diameter, shorten hose.  Optimise the air pressure settings.  Check in-line filters and moisture traps

  • Potential Cause

    Electrical earth.  Powder builds up on the gun tip and periodically spurts onto the coating.  This may be particularly noticeable in dry-blended metallic effect coatings. 

  • Solution

    Check contact areas between parts and jigging hooks and ensure there is no build-up of cured powder coating, or any other insulating material.  Jigs should not be too thin and the earth connection to ground should be secure and tested.  Ideally resistance should be less than 0.5 megaohm, but should certainly be less than 1.0 megaohm.

  • Potential Cause

    Powder falls off booth ceiling onto work below

  • Solution

    Adjust/increase frequency of booth cleaning intervals

  • Potential Cause

    Powder falls off workpiece onto work below

  • Solution

    Test electrical earth, check diameter of hoses.

  • Potential Cause

    Powder moist

  • Solution

    Use dry powder, check air filter and moisture traps in compressed air system

  • Potential Cause

    Powder agglomerating in the box.

  • Solution

    Agglomeration can occur under inappropriate storage conditions.  If possible, manually break up the powder coating lumps.

  • Potential Cause

    Spray gun settings.  A defective gun, or certain gun types may cause a rash of lumps caused by build-up in the gun.

  • Solution

    Often, increasing kV to the gun tip can correct this.

  • Potential Cause

    Worn venturi or pump.

  • Solution

    Replace worn parts

  • Potential Cause

    Powder too fine

  • Solution

    Decrease amount of reclaim to hopper.  Optimise virgin powder plus reclaim ratio.  Contact HMG.

Small Inclusions

Small pinpoint raised peaks in the coating surface, often referred to as 'bits' or 'seeds'.  The particles are often so fine that sieving does not remove them.  Under the microscope, these inclusions may be the same colour as the coating, or a different colour.

  • Potential Cause

    Film thickness of the coating is below the recommended DFT.

  • Solution

    Ensure DFT is above the recommended minimum.

  • Potential Cause

    Single- and multi-headed inclusions caused by contamination from the reclaim system:

    • Compromised reclaim system sieve;
    • Dirt of grit being collected in the spray booth from jigs, flight bars and conveyor system;

  • Solution

    Check the recovery and reclaim system, particularly ensuring the sieve is in place and not ripped.

  • Potential Cause

    Multi-headed inclusions which under the microscope are revealed as fibres.

  • Solution

    The source of fibre contamination is often clothing or a poor choice of cleaning cloth.  Operators in the spray area should wear suitable protective clothing and gloves.  A lint-free cloth is preferable.

  • Potential Cause

    Single- and multi-headed inclusions caused by air movement in the plant.

  • Solution

    The spray booth extraction system will drag contaminants from the local environment into the booth.  Restrict the air movement in the spray area, ensuring that outer doors are closed to prevent dirt and grit being drawn onto the work or reclaim system.

  • Potential Cause

    Deposits of dust and powder on cured film, often these can be felt on the surface as slightly raised bumps.

  • Solution

    Ensure cooling zone is dust free and is protected from cross draughts.

  • Potential Cause

    Single- and multi-headed inclusions originating from dirty parts entering booth.

  • Solution

    Check that the quality of the cleaning system is sufficient to remove all dirt from the parts.  Check for weld spatter and other burrs on the substrate surface.

  • Potential Cause

    Contamination in virgin powder.

  • Solution

    Contact us to discuss.

< Back to Knowledge