Nine Common Issues with Wood Finishes: Causes and Solutions Summarised by Experts

I

Pinholes:

1. Phenomenon:

The appearance of pinpoint holes on the paint film surface. These needle-like pores resemble puncture marks, with a diameter of approximately 100 micrometres.

2. Causes:

(1) Inadequate surface preparation of the substrate, with excessive wood fibres and burrs that are difficult to fill;

(2) Application of a second coat before the underlying layer has fully dried;

(3) Prepared coating left standing for too long, resulting in high viscosity and trapped air bubbles.

(4) Excessively thick application in a single coat.

(5) Incorrect matching of hardener and thinner;

(6) Excessive hardener addition.

(7) High ambient temperature and humidity;

(8) Elevated moisture content in the wood substrate.

3. Countermeasures:

(1) Sand the white base material of the board smooth and level, then seal it with primer;

(2) When applying multiple coats, ensure sufficient time between applications; the second coat should only be applied once the underlying layer is completely dry;

(3) Allow the prepared coating to stand for a period to fully eliminate air bubbles before application;

(4) Maintain appropriate coating viscosity; avoid excessive thickness;

(5) Avoid applying excessively thick coats in a single application; instead, apply multiple thin coats, with each layer generally not exceeding 20μm in thickness.

(6) Use the specified hardener and thinner, applying them in the prescribed proportions.

(7) Do not use under high temperature and humidity conditions.

(8) Prior to application, ensure the timber is dried to a specific moisture content, typically 10-12%.

II

Bubbling:

1. Phenomenon:

After drying, the paint film exhibits raised circular bubbles of varying sizes, also termed blisters. Bubbling occurs between the painted surface and the paint film, or between two layers of paint film.

2. Causes

(1) Substrate preparation is inadequate, such as high moisture content in timber, or failure to remove natural resins and aromatic oils from the wood, which may cause bubbling during natural volatilisation.

(2) Topcoat application before oil-based or water-based fillers have fully cured, or before the primer has dried.

(3) Gaps and voids remain in timber joints and holes due to inadequate filling.

(4) Excessively high paint viscosity.

(5) Inappropriate paint mixing ratio.

(6) Bubbles formed during brushing by dragging the brush back and forth were not eliminated.

(7) Presence of oil stains, dust, or water droplets on the substrate surface, with water pooling around these contaminants.

(8) Water ingress in the compressor or air hoses, or water splashes on the application surface.

(9) Most causes are identical to those for pinholes.

3. Countermeasures:

(1) Timber must be dried to appropriate moisture content, with aromatic oils or turpentine removed.

(2) Surface coatings should be brushed after putty and primer have fully dried.

(3) Cracks and holes in timber joints must be filled with putty before brushing and painting.

(4) Clean debris from substrate surfaces using a clean cloth, avoiding direct hand contact. Paintable surfaces may be coated after cleaning.

(5) Maintain appropriate structural viscosity in the coating.

(6) When brushing, avoid dragging back and forth. First pull straight horizontally, then vertically, finally along the wood grain.

(7) Regularly drain water from compressors and install oil-water separators.

(8) Refer to the pinhole countermeasures.

(9) For minor bubbles, once the paint film has dried, sand them smooth with wet sandpaper before applying a finishing coat. For severe bubbles, first puncture the bubble, carefully smooth the area with sandpaper and clean thoroughly, then repair layer by layer according to the coating process.

III

Whitening

1. Phenomenon:

The coating film contains water or other liquids, resulting in a lighter colour than the original and a white haze on the film.

2. Causes:

(1) Excessive moisture content in the substrate, where trapped moisture evaporates within the paint film over time, causing whitening;

(2) Excessively high ambient humidity;

(3) Moisture contamination in the application surface, containers, or paint;

(4) Excessively rapid solvent evaporation;

(5) Residual moisture in the underlying paint film not adequately removed;

(6) Use of low-transparency coatings on dark-coloured boards such as black walnut, resulting in whitening due to transparency issues;

(7) Excessively thick paint structure;

(8) Incorrectly matched hardener causing incompatibility with the coating and resulting in whitening;

3. Countermeasures:

(1) Ensure boards are dry prior to application, with moisture content controlled below 12%;

(2) For dark-coloured boards, employ paint structures with superior transparency;

(3) Avoid applying excessively thick coats in a single application;

(4) Refrain from application during high humidity; incorporate suitable drying retardants when necessary;

(5) Ensure the application surface is clean and free from moisture;

(6) Prevent water contamination of paint and containers;

(7) Employ specified thinners and hardeners;

(8) Ensure the primer film is thoroughly dry, particularly when water-sanding.

four

Bottom bite:

1. Phenomenon:

Bottom biting refers to the softening and expansion of solvents in the upper layer of paint, resulting in poor adhesion of the lower layer of paint film and causing peeling and flaking phenomena.

2. Reason

(1) Apply topcoat before the primer is completely dry. The solvent in the topcoat easily dissolves and softens the primer, causing the bottom to bite.

(2) When brushing the topcoat, the operation is not fast, and repeated brushing too many times may cause the phenomenon of biting the bottom.

(3) For oily paint films and some synthetic resin paint films modified with dry oil, the primer paint will be corroded once it encounters strong solvents before highly oxidizing and polymerizing to form the paint film. If phenolic paint is used as the primer and nitro paint is used as the topcoat, the solvent in the nitro paint will bite onto the oily phenolic paint and separate it from the original bonding base material.

(4) Insufficient amount of pre coating curing agent and insufficient cross-linking;

(5) The front and rear jackets do not match.

3. Countermeasures

(1) The topcoat should be applied after the primer has completely dried.

(2) When brushing solvent based coatings, the operation should be proficient, accurate, and fast to prevent repeated brushing.

(3) Primer and topcoat should be used together.

(4) For severe undercutting, it is necessary to completely remove the coating and wait for the substrate to dry before selecting the same type of coating for brushing.

five

grain:

1. Phenomenon:

Dust, splashing flakes, foreign objects, etc. adhere to the surface of the coating.

2. Reason

(1) Dust, sand, and other debris mixed into the paint.

(2) When preparing coatings, the bubbles generated can be applied without dispersing in the coating liquid, especially in cold weather where the bubbles cannot disperse, causing the surface of the paint film to become rough after drying.

(3) The construction environment is not clean, with dust, sand particles falling into the paint, or debris sticking to brushing tools such as oil brushes.

(4) The grassroots treatment is not qualified, the grinding is not smooth, and the dust and sand are not cleaned thoroughly.

(5) Improper use of diluents can result in poor solubility and incomplete dissolution of paint, leading to particle formation.

(6) Improper use of curing agent, incompatible with paint, resulting in particles.

3. Countermeasures

(1) Before brushing, the prepared paint must be filtered with a filter cloth to remove debris.

(2) After the coating preparation is completed, it should be left to stand for 10-20 minutes until the bubbles dissipate before use.

(3) Construction should not be carried out in windy weather or dusty areas, and new paint should be dust-proof and free from dirt.

(4) Apply putty to fill uneven areas on the base, then sand them with sandpaper, remove dust, and apply paint.

(5) Use matching diluents and curing agents.

(6) The surface of the coating has become rough and can be smoothed with sandpaper before applying the topcoat again. For high-end decoration, sandpaper or sand wax can be used for smoothing, followed by waxing, polishing, and finishing.

six

Oil free:

1. Phenomenon:

Lithium oil is a localized shrinkage on the surface of a coating, similar to water splashing on wax paper, where spots exhibit potential flower like phenomena, also known as fisheye, smile, shrinkage pores, etc.

2. Reason

(1) The coated object contains moisture, oil, or oily wax, etc;

(2) The air compressor and pipeline are contaminated with water and oil;

(3) The working environment is polluted, and the spraying facilities and tools are not clean.

(4) Unclean objects such as water and oil. Accidentally mixed into the paint;

（5） Unauthorized addition of defoamers and other chemicals;

(6) High solvent vapor content and poor ventilation in the environment.

(7) Poor quality or contamination of the underlying coating;

3. Countermeasures

(1) The coated object should avoid contamination and require thorough polishing;

(2) Use an oil-water separator and regularly drain water every two hours;

(3) Avoid contamination from oil, wax, and other substances in the workplace and containers. Contaminated clothing and wiping cloths should be cleaned before coming into contact with the work object. During the construction process, please be careful not to let debris fall into the paint bucket. Keep facilities, paint mixing tanks, and tools clean;

(4) The old coating should be wiped clean with solvent and thoroughly polished before painting.

（5） Do not add other chemicals without authorization;

(6) If you notice a "smile" while brushing your teeth, stop brushing immediately and clean the "smile" area with a solvent. After the surface is dry, brush and repaint.

seven

None or slow drying:

1. Phenomenon:

After a period of time, the coating still does not dry or harden

2. Reason:

(1) The coating surface contains moisture;

(2) Too little or forgotten to add curing agent;

(3) Use diluents with high moisture and alcohol content;

(4) Temperature too low, humidity too high, not reaching dry conditions;

(5) One layer of coating is too thick, or the interlayer spacing is short.

3. Countermeasures:

(1) Spray after the water is completely dry;

(2) Add curing agent to the coating in proportion;

(3) Use the matching diluent provided by the manufacturer;

(4) Spray at room temperature;

(5) Use two or more construction processes to extend the interlayer construction time. If the coating surface cannot dry, the coating should be removed or cleaned with a cloth dipped in acetone.

eighty percent

Lost Light:

1. Phenomenon:

The coating fogged up, unable to achieve the expected glossiness

2. Reason:

(1) The surface of the coated object is rough and porous, with a high oil absorption capacity;

(2) Excessive amount of diluent added and insufficient amount of paint sprayed;

(3) Poor exhaust, spray paint mist falling on the surface of the spray film;

(4) Using diluents with low boiling points can cause rapid evaporation and drying;

(5) The polishing coating has not been completely dried, ground, or polished, or the polishing wax is too coarse;

(6) The construction environment is too high, the humidity is too high, the solvent evaporates too quickly, and the paint film turns white;

(7) Rough grinding, or grinding with coarse sandpaper, with sand holes;

(8) Dust adheres to the surface of the coated object without removing the paint powder;

(9) The coating is left for too long after adding the curing agent.

3. Countermeasures:

(1) Choose a good board, or apply 1-2 times the base Debao (sealing paint) first, close the wooden eyes, apply putty to fill the gaps, and polish smooth; (2) Control the appropriate viscosity of the coating and spray the appropriate amount of coating liquid in the correct way;

(3) Maintain good exhaust;

(4) Choose the matching diluent provided by the manufacturer, or add a solvent that evaporates slowly;

(5) After the paint film is completely dry, it can be polished using the fineness of the wax;

(6) The painting operation must have good temperature and humidity, and appropriate slow drying solvents such as anti whitening water should be added;

(7) Carefully polish with fine sandpaper;

(8) Clean the surface to be coated;

(9) After adjusting the paint, the construction should be completed within 4 hours as soon as possible.

nine

drooping

1. Phenomenon:

Paint flows in the grooves on the coating surface or corners of the production line. The thickness of the paint film is uneven, with severe ones resembling sagging paint curtains and shallow ones resembling teardrop marks.

2. Reason

(1) Excessive diluent results in viscosity lower than normal construction requirements, and the coating cannot adhere to the surface of the object and move.

(2) The temperature at the construction site is too low, the drying speed of the coating is too slow, and the film-forming fluidity is high.

(3) The selected paint brush is too large, the hair is too long, too soft, or there is too much oil and paint when brushing, causing the paint surface thickness to be different and the thicker parts to flow.

(4) The unevenness of the brushing surface or the edges, corners, and grooves of objects can easily cause uneven brushing and varying thickness, with thicker parts flowing.

(5) The surface to be brushed is not clean, with dirt such as oil and water, and cannot adhere and flow well after brushing.

3. Countermeasures:

(1) Choose good paint materials and appropriate diluents.

(2) The temperature and humidity of the construction environment are suitable.

(3) The selected paint brush and bristles should have elasticity, thickness at the base, coarse bristles, and even mouth. Oil brushes should be carefully and frequently soaked in oil.

(4) During construction, the base layer should be as smooth as possible, and edges and corners should be removed. When brushing your teeth, apply even force, brush vertically first, then horizontally, and do not apply horizontally. Gently pick up the corners and edges of the lines with an oil brush, and smoothly immerse the excess paint to avoid the film from becoming too thick and flowing;

(5) Choose a diluent that matches the coating;

(6) Thoroughly clean the coating surface of debris, oil stains, water, and other impurities;

(7) When the paint film is not completely dry and the paint is sagging on one side or surface, excess paint can be removed with a scraper and then painted again. If the paint film is completely dry, sandpaper can be used to smooth the loose flow. For large areas of slump, it can be smoothed with sandpaper.