Painting defects
Below we have provided you with a list of the most common defects in painting and coating. For the errors you will find the possible causes and tips on how to avoid them.
Assistance
Flaking, peeling
Cause
- Unsuitable coating system (lean on fat)
- Change of the substrate
Applause, margin marks
Cause
- Substrate reversible or not cured
- Pre-materials revised too early
- Layer thicknesses that are too high
- Drying times too short
- Sandpaper too coarse
Avoidance
- Solvent test
- Filler only on bare sheet metal
- Insulate with a 2Pack-filler; spray the filler in several thin coats and observe flash-off times.
- Ensure proper drying of the starting materials
- Do not grind too roughly
- Observe processing instructions
Wetting disorder
Cause
- Silicone contamination (e.g. hand cream, welding spray, neon tubes, sealants containing silicone, etc.)
- Residues containing oil, wax, grease
- Spray air contaminated by condensation or oil residues
- Dust incidence
- Dew fall and moisture exposure during drying and curing
- Exhaust air time too short during heat curing
Avoidance
- Silicone-free hand creams (Stokolan)
- Extreme cleanliness (when processing hammertype coatings complete separation incl. wash thinner!)
- Careful cleaning of the substrate before starting the painting process
- Regular maintenance of the spraying equipment
Bubble formation
Cause
- Residues of chemical pretreatment of sheet metal
- Substrate not sufficiently dried out and cleaned
- Pores/voids not ground out
- Polyester materials not insulated
- Spray air contaminated
- Condensation due to temperature fluctuations
- Hand sweat
- Salts and minerals in grinding water
- Influence of humidity during filming
- Inclusions from porous substrate
- Too short flash-off time for baking enamels
- Lack of primer
Avoidance
- Allow pre-materials to dry out well, careful cleaning of the substrate
- Polyester products insulate
- Carefully grind out or fill in shrinkage marks
- Avoid condensation
- Ensure clean spray air, regularly check oil and water separators
- Use demineralised water for cleaning
Bluish shimmer on the surface and haze formation
Cause
- Influence by interfering substances from the subsurface
- Excessive humidity during drying/curing
- Insufficient ventilation during drying/ curing
- Condensate precipitation
Sediment
Cause
- Exceeding the storage time
- Not stirred or not stirred sufficiently
Opacity, hiding power, missing(s)
Cause
- No correct, uniform base
- Not sufficiently stirred
- Layer thickness too low
- Viscosity too low
Avoidance
- Also stir vertically
- Process viscosity and film thickness according to specifications
- Use light background/ primer
- Spray neutral substrate
Nozzle clogging, turbidity
Cause
- Paint contamination
- Incompatible addition of paint or thinner
Avoidance
- Work cleanly. Sieve material if necessary
- Mix only compatible coatings and thinners
Thickening, gelling
Cause
- Exceeding the pot life
Hue deviations
Cause
- Sprayed too wet, too dry, too thin, too opaque
- Spray gun (nozzle), spray pressure not according to regulations
- Surface weathered by environmental influences
- Deviation in the series painting (multiple shades)
- Substrate is too dark, e.g. with lead-free top coats (red, yellow, orange)
- Not sufficiently stirred
Avoidance
- Use recommended primer
- Colour comparison (sample sheet)
- Coating “droplet method
- Stir not only horizontally, but also vertically (if necessary, mechanically with compressed air stirrer).
Staining (yellow to brown)
Cause
- Peroxide residues in the wood (from bleaching)
Avoidance
- Allow peroxide to decompose completely by heat or ammonia before painting
- Use suitable hardener
Fullness insufficient
Cause
- Absorbent substrate
- Layer thickness too low
Gloss loss
Cause
- Absorbent substrate
- Exposure of fresh coating to moisture (white tint at higher temperature and humidity)
- Too high pigment content due to inadequate agitation
- Disturbances during drying and curing of the coating
Adhesion problems
Cause
- Substrate not sufficiently pre-treated
- Condensation due to temperature fluctuations / dew point undershoot
- Wrong paint system
- Drying times / flash-off times too short
- Insufficient dissolving of the substrate due to exceeding the recoat time (especially with 2Pack-EP and PU systems)
Avoidance
- Observe manufacturer information
Adhesion problems of the clear coat
Cause
- Too high film thickness of the base coat
- Too short intermediate and final flash-off times of the base coat
- Incorrect mixing ratio of clear coat/hardener
- Poor intermediate adhesion
Avoidance
- Adhere to the layer thicknesses of the basecoat according to the product information.
- Observe the intermediate and final flash-off times specified in the product information.
- Observe mixing ratio of clearcoat to hardener according to product information.
Adhesion problems on plastic
Cause
- Insufficient cleaning, drying (annealing)
- Unsuitable adhesion promoter
Avoidance
- Anneal before cleaning
- Thorough cleaning
- Evaporation of the cleaning agent
Skin formation
Cause
- Improper storage
Pull up
Cause
- Grinding through to the substrate
- Not isolated
- Insulated with unsuitable filler
- Filler not processed according to instructions
- Substrate not thoroughly dried
Avoidance
- Solvent test
- Insulate with a 2-component filler; spray the filler in several thin coats and observe flash-off times.
- Avoid cut-throughs
Clear coat yellowing
Cause
- Incorrect hardener
Avoidance
- Close hardener container after use
- Mixing ratio clearcoat / hardener according to product information
Curling
Cause
- Not fully cured undercoat, e.g. fresh synthetic resin paints
- Unsuitable substrate, e.g. non-solvent-resistant plastics
- Too thick layers
Avoidance
- Observe prescribed drying times
- Remove or isolate solvent-sensitive substrates
Runner or curtain formation
Cause
- Deviating spray viscosity, spray technique, intermediate flash-off times and layer thicknesses
- Spray gun (nozzle), spray pressure not correct
- Paint material, substrate or room temperature too cool
- Incorrect dilution or viscosity too low
- Substrate has loosened
Avoidance
- Processing according to manufacturer’s instructions
- Use proper spraying equipment
- Heat object and material to room temperature of 20 °C
- Select good combination of hardener/thinner
Pinpricks
Cause
- Substrate not sufficiently dried out
- Polyester materials not insulated
- Pores not ground out
Avoidance
- Allow starting materials to dry out well
- Sand or fill pores well
Optics (unclean appearance of the coating)
Cause
- Insufficient preparation of the substrate
- Dust incidence
- Unclean working equipment
Cracking
Cause
- Change of the substrate
- Unsuitable coating system (lean on grease / primer on paint)
- Incorrect pretreatment of the substrate
- Layer thicknesses too high
Wrinkling
Cause
- Too thick layers if the surface dries too quickly
- Swelling of coatings by water or solvents
- Pull up (overpainting too early or too late especially with alkyd systems)
Grinding grooves
Cause
- Sanding with too coarse sandpaper
- Soft, elastic substrates
- Top coat applied too thinly
Avoidance
- Solvent test
- Isolate the soft ground
- Use specified abrasive paper
- Apply normal layer thickness
Striping
Cause
- Spray viscosity, spray pressure, spray technique
- Spray gun, spray nozzle
- Too short flash-off times
- Unsuitable processing temperature
- Unsuitable thinners
Avoidance
- Observe processing instructions according to product information
- Select suitable spray gun and spray nozzle
- Guide the spray gun parallel to the object
- Remain in the manufacturer’s product system
Drying / curing problems
Cause
- Climatic influences
- Wrong hardener addition
- Too thick layers
- Insufficient substrate pre-treatment
- Plasticiser migration
Processability poor
Cause
- Unsuitable viscosity
- Unsuitable solvents
- Unfavorable temperature of the substrate and the coating material
Discoloration
Cause
- Bleed-through of bitumen, organic pigments and dyes
- Yellowing with phenolic coatings
- Effects of light and industrial waste gases on pigments
- Overburning with thermosetting coatings
Yellowing
Cause
- Incorrect hardener
Avoidance
- Observe data on product information
Course disorders / orange peel
Cause
- Deviating spray pressure, spray viscosity, spray technique, processing temperature
- Unsuitable solvent combination
- Scarred underground
- Unsuitable spray gun (nozzle)
- Too fast drying
- Poorly flowing base coat
- Absorbent substrate
- Incorrect processing technique
- No optimal layer thickness
Avoidance
- Observe processing instructions according to product information
- Proper surface preparation
- Suitable spray gun (nozzle)
- Use specified dilution
Matting / Gloss
Cause
- Layer thicknesses/ humidity
- Substrate soluble
- Curing error or hardener had already reacted with air humidity
- Wrong dilution
- Water or oil in the spray air
- Drying interrupted
- Sprayed too dry
Avoidance
- Comply with the specifications on the product information
- Closing hardener cans
- Ensure sufficient fresh air supply
- Do not interrupt forced drying
- Observe drying time
- Use correct dilution
Water bubbles
Cause
- Sanding water residues in corners, edges, folds and under trim strips
- Spray air contaminated
- Poor insulation of polyester products
- Humidity too high
Avoidance
- Always dismantle attachments if possible
- Carefully blow out
- Check oil and water separators regularly
Water stains
Cause
- Freshly painted, not yet cured coating
- Layer thickness too high, drying time too short
- Curing error or hardener no longer usable
- Use of unsuitable dilution
Avoidance
- Observe processing instructions according to product information
White tarnishing (especially in thunderstorm climates)
Cause
- Condensation formation in fast-drying NC systems
Avoidance
- Add retarder
Cloud formation
Cause
- Deviating spray viscosity, spray technique, flash-off times, spray room temperature
- Spray gun, spray nozzle, spray pressure not correct
- Unsuitable thinner
Avoidance
- Set spray viscosity according to dipstick
- Guide the spray gun parallel to the object
- Select suitable spray gun and nozzle
- Use thinner of the manufacturer
- Observe processing instructions according to product information