Spray Application Overview
This defect is found in spray applications where there is heavy application of paint on the outside of the spray pattern with little paint in the canter of the fan.
The cause can be:
- Too much air pressure
- Uneven lapping of the spray gun
- Having the gun too close to the job
The remedy may be found from:
- Use at correct air pressure
- Ensure that the overlap of each stroke is 50% over the previous coat
- Hold gun at the correct distance from the job - about 15 cm for lacquers and 25 cm for enamels. To correct a coat that has been applied, re-coat with double coat using thinner that has been specifically recommended for the paint using indicated solvent ratio and pressure, and ensuring that the gun is held at the correct distance.
This is the migration of the color from a previous coat into the freshly applied top coat. This defect usually occurs when a light color is applied over a dark color, particularly reds and maroons which are prepared by using organic pigments not resistant to solvents or application over a surface contaminated with bitumen where the solvents in the fresh paint dissolve the bitumen.
The remedy is:
- Use a bleed sealer before application of the light color
- Wash the surface with mineral turps if it is contaminated with bitumen.
This defect is the appearance of irregular blisters on the paint finish.
This defect can be caused by:
- Not correctly cleaning substrate
- Contamination of the brush, air gun, line etc.
- Using wrong thinner or incorrect amount of thinners
- Old paint surface
- Excess film thickness
- In timber finishes, not allowing the solvent, particularly paint removers, to evaporate before repainting
This defect may be overcome by:
- Cleaning all surfaces free of grease and allowing the solvent to evaporate.
- Using recommended thinner at correct ratio.
- In spray applications, inspect so that water does not build up in the traps, especially in humid weather.
- Check that the new paint is compatible with the old surface.
- Do not apply paint films too quickly and allow solvents to evaporate before re-coating.
This defect gives a bloom or white deposit, like the bloom on a grape or plum, after the paint has dried. The cause is the rising of soluble fractions of the pigment rising to the surface on the paint's drying. The remedy for spray paints is to rub the surface down.
This is a white deposit appearing on the surface of lacquer films only. The defect is caused by painting with lacquers in high humidity conditions where the water contained in the air condenses on the paint film The remedy is not to paint in humid conditions or to add a strong, active solvent that may stop the blushing.
This is a defect that was often observed on cars painted red or blue where after a period of time a characteristic red tone developed on the paint surface. The cause was older types of pigments like phthalocyanine or Prussian blues. The defect is not common with the pigments available today.
Chalking is the powdery deposit on the surface of the paint which dulls the gloss and appears after exposure. This defect is usually associated with long exposures to sunlight and is a natural degradation of the paint film. Some combinations and types of pigments and resins show more pronounced chalking than others.
Checking is the appearance of wide splits with round edges that occur in the top coat. The cause is usually due to the surface not being clean (could be old paint) or too high a film build or the materials not being mixed properly. The remedy is to remove the old paint, cleaning the surface and mixing the paint ingredients properly.
Cracking or Crazing
This defect is a series of irregular cracks in the surface of the paint.
The cause of this defect can be:
- Application of the top coat before the previous coat is dried
- Too thick of a top coat
- Impurities on the surface or the effect of impurities on the applied coat.
The remedy is the let the intermediate coats dry before the top coat is applied, clean the surface well, remove the previous coat or ensure that the top coat is not applied too thickly.
This is the deposition of dirt and dust on the paint film. For certain types of paint, the dirt may become entrained into the surface. The paints that resist dirt retention are high-gloss enamels while the low gloss latexes are the most susceptible to this defect.
Fading is the decrease in the intensity of the color after exposure. It should be tested for after removal of any chalking that may have occurred as this will tend to mask the actual fade of the pigment. In general organic pigments, especially those of low cost, will fade more than pigments that are inorganic. More expensive coatings especially prepared for exterior exposure will resist fading more than less expensive paints.
This defect is indicated by small round imperfections in the top coat. The defect is caused by traces of silicone or oil on the surface prior to painting. The remedy is to thoroughly clean the surface and if spray painting, to ensure that there is an oil filter on the air line.
Flaking is the lifting of small-to-large sections of the paint and is due to poor adhesion and to the brittleness of the paint. The causes can be varied, for example the defect could be caused by efflorescence or the migration of soluble salts to the paint-media interface which can cause the paint to be forced off the surface. The paint may react with moisture and any traces of alkali to decompose the paint - this is called saponification. It may be due to failure to remove millscale from the steel before painting.
Floatation and Flooding
Floatation or floating occurs when a paint has been incorrectly formulated with two or more different colored pigments when one of the pigments floats to the surface giving different differences. On close examination the surface appears mottled with regular shaped cells.
Flooding is similar to floatation in that one of the pigments migrates to the surface when the paint is produced using two pigments with different densities.These defects are corrected mainly by better paint formulation.
This is the formation of a gas, usually by hydrogen, by the reaction of reactive pigments, like Zinc and Aluminum, with acidic materials in the resin. It can be overcome by better formulation or packaging the paint separate from the pigment and mixing the ingredients prior to application.
The growth of mould on a paint film causes severe discoloration. Mould is a plant growth that requires moisture, the presence of food and the correct temperature for growth. The defect can occur on most types of paint but is most prevalent in bathrooms, kitchens and exterior walls that are in shady positions. The paints that are most susceptible are soft oil-based paints or varnishes and emulsions, especially if they are low gloss where dirt can be trapped in the film.
Often the mould growth can be killed and color removed by washing with dilute sodium hypochlorite solution taking due care as this preparation is alkaline. Safety glasses and gloves have to be worn. Before repainting, susceptible surfaces should be prepared with anti-mould preparations, like sodium pentachlorophenate and by using either paints prepared with mould inhibiting pigments, like Zinc oxide, or by using high gloss finishes. In extreme cases it may be necessary to remove the high humidity in the room by using exhaust fans.
This defect gives the paint finish the rough appearance similar to the outside of an orange. The defect is found in spray painting and is generally due to having the wrong solvent. It can also be due to an incorrectly adjusted spray gun.The solution is to use the manufacturers recommended thinner and to adjust and use the gun correctly.
Peeling is simply another type of flaking where the amount of paint film removed is greater.
The defect is the appearance of small holes in dried paint film. The problem is most probably caused by too thick a coat trapping solvents into the film, or by air bubbles. The defect may be due to not cleaning the surface before painting, using the wrong solvents in spray painting or incorrect air pressure. The problem can be solved by addressing the above causes.
Poor flow can manifest itself in two ways: if the paint is too thick and will not flow out this will show up as a rough surface or orange peel where the surface resembles the skin of an orange; if the paint flows too much the result will be runs, sags and wave formation. This defect is remedied by proper formulation and when thinning the paint to use the right solvent and the correct amount.
Poor Hiding or Lack of Opacity
Opacity is the ability of a paint film, when applied to a given surface, to hide or obliterate the surface or the undercoat. Poor hiding power can be due to too thin a coat being applied or to the formulation using a poor quality pigment.
Runs and Sags
As the name suggests this is the formulation of runs or sages on the finished paint film.
The defect can be caused by a number of things:
- Too thick of a film.
- Too much thinners.
- Not allowing the first coat to dry before applying the top coat.
This is the separation of the pigments and occurs to a certain extent in all paints. It becomes a serious defect when the pigment is difficult to reincorporate into the paint by stirring.
The defect occurs due to the high densities of some pigments and can be accelerated by a drop in viscosity, the paint being stored at high ambient temperatures or by being subjected to vibration for example on long transportation by rail.
The control of settling lies in selection of suitable pigments and the addition of additives that increase the viscosity of the paint.
Slow drying occurs when the paint remains tacky for an extended period of time. This will result in the film picking up insects or dirt before it is hard and will make repainting difficult.
The main causes are:
- Too thick of an application of the paint when using air-drying paints. As these paints require oxygen to penetrate the film to produce dying, if the film is too thick oxygen will not penetrate.
- The viscosity of the paint is too high for the application. This can occur in cold weather and can be overcome by reducing the viscosity with the recommended thinners for the paint.
- The paint was applied at too cold a temperature. This will cause the chemical reaction that takes place to cause the film to cure slowly.
- High humidity due to rain or the like will reduce the evaporation of the solvent, the first step in drying.
- The surface to which the paint is applied is not clean and has traces of wax or paint removed on it.
- The coat to which the paint is being applied has not dried when the next coat is applied.
Contamination of many surfaces with water, soot, smoke, tannins and tobacco can result in color coming through the paint surface to cause stains. Stains caused by water will leave a tide mark and after drying, the paint around the stain can be removed and the surface repainted. If the surface may become damp again, remove the source of the water or paint with chlorinated rubber or a hard varnish.
Patches of soot or smoke should be removed before coating. Use of an insulating paint before the final coat can help. Nicotine should be removed with an alkaline cleaner (bleach) before coating. Remember to wash off all the alkali before attempting to paint.
Low viscosities may be simply due to incomplete stirring or the addition of too much solvent. The viscosity may decrease on standing in water-borne paints due to enzymic attack on the thickeners used. Modern latex paints use thickeners that are not readily attacked by bacteria. Changes in the orientation of the pigments (for example partial flocculation) may reduce the viscosity.
Wrinkling, Webbing, Frosting and Gas Checking
Wrinkling is the development of wrinkles in the paint film as it dries, usually due to the formation of a skin. Defects similar to wrinkling are webbing, frosting and gas checking. Webbing is the development of wrinkles, usually in a well defined pattern and if it occurs in an oven it is called gas checking. Frosting is the formation a haze which is due to fine wrinkles and it occurs in gas fired drying ovens.
The causes can be:
- Due to the paint's being applied too thick, especially with high oil-length alkyds, varnishes with wood oil and too much cobalt drier, enamels based on alkyd or phenolic resins with drying oils and black enamels containing bitumen.
- Stoving paints containing bitumen.
- Frosting may be due to the products combustion in the oven reacting with the surface of the film or may be due to high humidity.
- Too much cobalt drier.
Sometimes the wrinkle pattern may be induced into the paint to produce films that will hide surface defects.
Another type of defected related to wrinkling are crocodiling or alligatoring where the wrinkle pattern resembles the hide of one of these reptiles.