Drying

Blistering

Raised bubbles and blisters on the paint surface — isolated or extensive. Moisture or contaminants trapped between the coats push the paint system apart from within.

Cause

Paint applied over an acid or reactive primer without checking chemical compatibility
Polyester filler applied directly onto a reactive primer
Wet sanding of polyester filler — polyester materials absorb water and release it for years
Moisture condensed between coats due to a large temperature difference between the body and the booth
Insufficient flash-off of waterborne products before applying the next coat
Contaminated air in the compressor system — water or oil getting into the gun
Products left open or stored incorrectly — moisture absorbed from the surroundings before use
Cross-linking time between coats too short — insufficient chemical reaction of the hardener
Incorrect mixing ratio — too little or too much hardener causes incomplete curing
Product not mixed thoroughly — a non-uniform chemical reaction leads to weak spots in the coat
Painting at high relative humidity without adequate booth ventilation and heating
Cold part not acclimatised before painting — the cold surface condenses moisture from the warm booth air
Dew point conditions ignored — the part's surface temperature too close to the dew point of the air in the booth, causing invisible condensation directly on the surface being painted

Prevention

Substrate preparation

Check the compatibility of all products before application — do not combine reactive systems with epoxy systems
Sand polyester filler dry only — never wet
After wet sanding (products not containing polyester), wash the substrate thoroughly with clean water and dry it immediately with compressed air
Before painting, wait until the panel reaches booth temperature — a minimum of 30 minutes of acclimatisation for large panels
Clean the substrate thoroughly with degreaser before every subsequent coat — avoid touching the cleaned surface with your hands
Apply the base coat only over the recommended primers, sealers and filler primers

Working conditions

Check the dew point before starting work — the surface temperature of the panel must be at least 3°C above the dew point
Do not paint at relative humidity above 80%
In cool and humid conditions, always allow the panel to acclimatise in the booth before starting to paint
Drain water from the compressor and receiver regularly — ideally every day before starting work
Check and replace the filters in the compressed air system regularly
Maintain a constant booth temperature throughout the drying period — avoid fluctuations

Application and storage

Observe the flash-off times between coats given in the product technical data sheet
Allow waterborne products to flash off until completely matt and dry before applying the next coat
Mix products thoroughly and in the recommended ratios — use scales or a measuring cup, do not estimate by eye
Seal cans tightly immediately after use — open products absorb moisture from the air
Store products at the recommended temperature and avoid fluctuations — especially in winter
Keep tools and equipment clean — a dirty spray gun or contaminated hoses can introduce moisture into the system

Repair

Identify which coat the blistering occurred in before starting the repair — check by pressing gently and observing
Sand the paint system back to the layer in which the problem occurred — do not cover the defect with further coats
If blistering occurred at the boundary of the filler or primer — the entire system must be removed down to bare metal
After sanding, dry the substrate thoroughly with compressed air — especially in corners, grooves and around trim
Before rebuilding the system, eliminate the cause — check the dew point, booth humidity and product compatibility
Rebuild the system in accordance with the technical data sheet, observing the flash-off times between each coat
Do not accelerate drying with an IR lamp too aggressively — rapid evaporation may cause the defect to return
After the repair, monitor the surface for several days — blistering may reappear if the cause has not been fully eliminated