Also known as: Pinholes, Pores.
What it looks like
Very small, deep holes in the coating film, often in clusters. On dark coatings they can be almost invisible; on thicker films they may look like tiny craters. A holiday detector will usually find them before the eye does.
Common causes
- Air or solvent escaping from a porous substrate, such as zinc metal spray, castings or galvanising, while the coating is wet or curing
- Solvent trapped by a thick coat or by applying the next coat too soon
- Spraying too far from the surface, so partly dry droplets land
- Outgassing from galvanised steel during powder coating cure
- Contamination such as oil or moisture on the surface or in the air supply
Prevention
- Seal porous substrates with a mist coat before the full coat
- Keep to the data sheet’s film thickness and overcoating times
- Pre-heat or degas castings and galvanised parts before powder coating where appropriate
- Check compressed air is clean and dry
Repair
Light pinholing can often be sealed with a further coat once the cause is fixed. Where the holes reach the substrate, the area may need abrading or re-blasting and recoating to the original specification.
Inspector’s note
For immersion service and linings, pinholes are critical and holiday testing is usually specified. For atmospheric service, the specification should say how much is acceptable.