General Information
The use of wood finishes in carpentry and cabinet making has been recorded as far as from Egyptian times. The first materials that where used consisted in natural oils, waxes and earth pigments, that made it possible technologically to reach our times where the use of fossilized derived solvents, resins and man made structured materials give out modern wood coating systems.
The three main objectives why furniture is finished are:
- To protect and increase the wood’s durability. (External wooden constructions)
- To protect and enhance the beauty of the grain especially exotic timbers. (Furniture in general)
- To protect for hygienic reasons. (Hospital and clinics furniture)
What ever the reason and the job one can always adapt the most efficient finish coat to obtain the required result. Specially formulated finishes together with technological advanced spraying systems nowadays have contributed to the furniture industry to achieve better results with less consumption.
| PROBLEMS WITH PAINT BEFORE USE OR IN PREPARATION FOR USE | |
| CAUSES | CURE |
| SETTLING OR PRECIPITATION (RESIN KICK-OUT) | |
| Wrong thinner used | If precipitation is slight, add proper solvent with constant agitation, or use stronger solvent. |
| Paint too cold | Warm paint to 21-25 C or 70-77F |
| Over reduction | Add fresh paint to bring back |
| Reducer or catalyst added too quickly or without sufficient stirring | Add slowly, and stir constantly |
| Pigment or flattening agent settled | Check bottom of can with spatula or stick before using, and stir thoroughly |
| VISCOSITY OR HEAVY BODIED | |
| Solvent in paint will evaporate at normal temperature and "body up" | Use same solvent that manufacturer used to make paint. Check viscosity with cup |
| Used improper solvent | Use recommended solvent for thinning |
| Catalyzed material beyond paint life | See directions-some products can be recovered by adding additional paint and catalyst |
| SKINNING OR GELLING IN CONTAINER | |
| Air drying material that is exposed to air is partially full | Float thinner on surface of pail and seal it, make sure container is sealed or put in new can |
| Gelled material | Usually cannot be recovered and must be discarded. |
| Stored at high temperature, outside, or beyond product's shelf life | Store inside at room temperature and use before 6 months if polyester, polyurethane, or pre-catalyzed lacquer. |
| DISCOLORATION OR FLOATING PARTICLES | |
| Presence of vapors or fumes in spray area | Investigate fumes like ammonia or amines from adhesives or other sources |
| Water born or acid cure react with steel parts or oxidize aluminum spray equipment | Use stainless steel or solvent resistant plastic parts and liners for spray pots |
| Improper drum or pail lining comes off | Strain paint through fine filter or cheese cloth |
| Old nitrocellulose products | Become amber on age in can or react with steel. Use fresh material |
| Chemical reaction to surface treatments | Check pH of surface since acid cures can react with alkali materials |
| FOAMING ( WATER BASED) | |
| Mixing too rapid | Mix slowly; do not shake with mechanical shaker |
| Foam trapped in container | Use defoamer recommended |
| DEFECTS OBSERVED DURING APPLICATION | |
| CAUSES | CURE |
| DIRT OR SEEDINESS | |
| Unclean conditions such as dust in spray room or drying area, dirt in air or paint line, sanding operation near air in-take of both | Check air supply for oil and dirt by spraying air only from spray gun at white cloth or tissue and look for staining or dirt. |
| Resin kick out from improper thinner | Use correct thinner |
| Paint is very cold and created seeds | Warm material to room temperature |
| Dirt in paint | Strain paint before use |
| Particles appear after spraying but not while spraying the wet film | Make sure with magnifier that it is not air bubbles. Dip or pour paint on glass and let dry in enclosed dust free area. Examine film when held up to light for defects. |
| DRY SPRAY | |
| Wrong solvent used | Choose slower evaporating solvent |
| Spray gun problems | |
| Atomized air pressure to high | Lower air pressure |
| Gun too far from surface / Spray past surface-bounce back of spray | Hold 6-10 inches and at right angles to surface, release trigger when gun passes target; check that spray booth is sucking out overspray. Increase fluid pressure and check by cutting off air to gun and adjust stream of paint to fall approx. 3 ft. from gun if gun at shoulder height. |
| Wrong air cap or fluid tip | Use correct combination based on manufacturer's guide |
| ORANGE PEEL | |
| Paint not thinned properly | Use correct amount of thinner and slower evaporating solvent |
| Spray gun problems | |
| Stroke to rapid or at angle | Slow stroke and at right angle to work |
| Air pressure too low | Increase air pressure |
| Fluid pressure too high | Decrease fluid pressure |
| Spray gun too close or too far from surface | Distance should be 6-10 inches from surface |
| Air temperature too hot | Use retarder solvent |
| Overspray striking wet surface | Avoid creating over spray or spray so that it does not strike previously spray surface |
| DEFECTS OBSERVED DURING OR AFTER CURING | |
| CAUSES | CURE |
| POOR ADHESION (TO SUBSTRATE) | |
| Unclean surface or oily wood or stain | Clean with degreaser solvent and avoid oil base stain |
| Primer or sealer lifts from surface | Make sure recommended sealer and top coat are used |
| Wood sanded too smooth especially close grain hardwoods | Use 120 grit sandpaper on close grain and 120-180 grit on open grain or soft woods. |
| Stain comes off with sealer | Make sure stain is dry properly and wipe it off. Apply thinner coat. |
| Stain applied too heavy or not wiped and did not dry | Be sure stain is dry before applying sealer |
| Higher solids coating used as sealer | High solids need a better profile to anchor. Avoid polishing wood when sanding and use 120 grit. |
| POOR ADHESION ( INTERCOAT) | |
| Primer or sealer contaminated | Clean surface |
| Sealer or primer sanded too smooth | Sand 220 to 280 grit maximum |
| Catalyzed finish dried too long between sanding and recoat | Check recoat window of paint and sand within 8 hours of top coat application |
| Sealer and top coat not recommended | Use only recommended system, avoid using supplier's sealer and another's top coat. |
| BLEEDING | |
| Organic reds, yellows, oranges used in stains have not been sealed properly | Avoid bleeding colors; Use stains that are recommended by the manufacturer to use under the lacquer being used. Check that stains used do not contain bleeding pigments or dyes. |
| Certain woods contain tanins that bleed | Seal with barrier coat. |
| BLUSHING (WHITE HAZE IN FILM ) | |
| Humid weather above 60% humidity | Add up to 16 fl. oz. retarder solvent in place of thinner |
| Paint sprayed cold | Bring to room temperature |
| Moisture in spray equipment | Check air line for moisture; compressor should have air cooler to prevent water condensation. |
| BLOCKING (OR PRINTING) | |
| Insufficient drying time before packing | Allow longer drying time or use heat |
| Too heavy film causing solvent entrapment | Apply lighter coats or multiple passes |
| Poor drying conditions | Change conditions by using heat or forced air |
| Improper catalyzed material | Check measurement of catalyst |
| Catalyzed material beyond pot life | Follow recommended time to use after catalyzed |
| BRITTLENESS | |
| Paint over catalyzed | Check measurement of catalyst. A good indicator may be a shorten pot life |
| Paint baked at high temperature | Check recommendation of manufacturer |
| Paint not formulated for wood | Paint for metal are harder but do not have elasticity of coatings formulated for wood. |
| CRACKING | |
| Excessive dry film thickness especially with acid catalyzed materials | Use wet film gage during application. Do not have a dry film thickness of more than 4-5 mils with acid-catalyzed materials or pre-catalyzed lacquers. |
| Grain or veneer cracking | Cracking following grain shows wood is cracking and not caused by the paint |
| Cold cracking by repeated cycles of freezing and return to room temperature | Paints are normally formulated to resist 10 cycles. Put painted wood in freezer for one hour and return to room temperature for an hour and repeat 10 times to see if fresh paint will crack. |
| CRATERING | |
| Film pulls away from areas of substrate. Cratering caused by contamination from silicone, wax or oil. | Find source of contamination in sanding aids on belts that may contain silicone or lubricants or oils or greases for machines or spray equipment, hand creams, or polishes as possible sources and eliminate their use by finding substitutes. You can use anti-cratering additive but silicone will continue to cause equipment contamination and higher amounts of additive will be needed. |
| DRYING TIME | |
| Humid weather | Use heat or reduce humidity |
| Cold weather | Drying area should be at room temperature |
| Oily or unclean wood | Clean wood by solvent wipe or use barrier coat for oily wood. |
| Oil stain prolongs drying | Use non-oil base stains, or leave them to dry for atleast 24hours. |
| No air movement | Use proper ventilation |
| Trying to fill open grain wood with heavy coat of sealer which traps solvent in pore | Use filler or high solids sealer like polyester or polyurethane to fill pore. |
| GLOSS CHANGES OF FLAT SPOTS | |
| Absorbent putty or filler that has not been sealed causing top coat to "strike in". | Seal wood filler or putty. |
| Not enough sealer used or sanded through allowing top coat to "strike through". | Carefully sand especially around the edges and corners. |
| Material from same container changes gloss or after a few hours of spraying changes | Stir material at start to be sure it is uniform. Stir after several hours if highly pigmented or high amount of flattening agent was used which is causing settling in can to occur quickly. |
| Gloss increases with successive coats. | The smoother the finish after sanding, the glossier it looks with more coats. |
| HAZING | |
| Incorrect thinner | Use thinner recommended |
| Over catalyzed with acid | Resand and recoat but test inter-coat adhesion. |
| Blushing | See remedies for blushing |
| Water mixing with paint because of air | Clean air line separator and bleed the line once per shift. |
| HIDING | |
| Over reduction | Add more paint |
| Pigment has settled to bottom of can | Stir before using and check for settling. Low film build. Apply more paint with more passes |
| Edges show through | Edges are too sharp so round edges by sanding |
| Color does not match standard-base color is showing through top coat | Apply more paint to achieve full hiding; Color base coat same as top coat for extra hiding; organic pigments have poor hiding and require thicker film. |
| MARRING | |
| Film not completely dry | Allow more days for drying before put in use |
| Emulsion base does not coalesce below 55F and will not cure | Apply at proper temperature |
| Acid and polyester coatings will not cure at low temperatures. | Must cross link at manufacturer's recommended temperature range. They usually take 7 days to reach final hardness. |
| Pot life has expired | Must use within pot life or may be able to reactivate with fresh material and catalyst. |
| Soft film | Mar resistant additive may help but use proper system for intended use. Don't use nitrocellulose lacquer if you want a hard scratch resistant film. |
| PIN HOLING (BUBBLING) | |
| Water in air atomized | Clean air separator and drain water from line |
| Air trapped in open pore of wood | Use a thin wash coat to penetrate pore |
| Fine bubbles after force drying | Use adequate flash off time before using heat |
| Polyurethane bubbles carbon dioxide causing pin pricks to appear after drying | Urethane reacting with moisture: use retarder and successive thinner coats. |
| Viscosity too heavy trapping air | Thin paint |
| Film too heavy | Use thinner coats |
| SHRINKAGE (WRINKLING) | |
| Surface dries quicker while underlying paint remains wet causing wrinkling | Avoid excessive heavy coats. Force drying may cause wrinkling. Polyester primers shrink if sanded to soon even if dry. Allow overnight cure before sanding |
| Incompatible coating systems-solvent in top coat attacks and softens primer. | Use compatible systems and follow manufacturers recommendations |
| YELLOWING | |
| Exposure to UV in sunlight | Use UV inhibitor in coating or correct paint |
| Yellow on aging | All coatings yellow, some more than others, aliphatic polyurethane's and acrylics yellow least; nitrocellulose lacquers, aromatic urethanes, oil drying alkyds conversion varnishes yellow more. |
| Exposure to strong amines | Some coatings yellow more when exposed to ammonia or other amine cleaning agents. |
| Yellow blotches or brown spots. | Yellow spots can be caused by bleeding dyes or such as hansa yellow coming to surface or oils causing staining. |
Type of Material
There are several materials that are use in furniture coatings, these vary from long alkyd base resins that include Synthetic varnishes in general and paint to be applied by means of brush, and the short alkyd base resins that include Nitrocellulose, Acid Catalysts, Polyurethanes, Polyesters and Water based finishes.
Type of spraying equipment
Spraying equipment has changed drastically over the years and has developed to what today are the state of the art robotic arms and reciprocators. Some conventional spraying equipment and systems include normal siphon and gravity spray guns, now mostly available in HVLP, airless, air mix, air assisted air mix, pressure pumps and electrostatic systems. Mechanized systems include roller coating and curtain coating; both may be used with UV lacquer.