PAINT FAILURES : The most important and most common paint failures occur as a result of poor or insufficient steelwork, preparation or application. Some of these faults are revealed during or shortly after application but some only appear after a certain period in service. The most common failures are:
- Insufficient film thickness is often the result of non-systematic application and inadequate checks with a wet film gauge.
Sags/runs occur when the paint is applied too thickly or too much thinner has been added to the paint. This is probably because the specification has, not been followed. Occasionally, faults are also found in the paint. The inspector must note the production number in the daily log. Sags/runs should be repaired immediately with a brush
Dry spraying is normally a result of poor application or difficult weather conditions. The most common application fault is too great a distance between the spray gun and the structure. High temperature combined with low relative humidity will also contribute to dry spraying as the solvents evaporate en route from the gun to the object. Strong wind or strong ventilation also contributes to an increased risk of dry spraying.
The Pinholes often occur on porous substrates, for example zinc silicate. On these substrates, it is important to apply a thin layer of paint, normally called-a tie coat or by using the mist coat - full coat technique. Pinholes are also found if there is too strong ventilation during application.
Blistering is normally an adhesion-related problem and is due to poor cleaning before application of the paint. The most common cause of blistering is the application of the paint to a substrate contaminated with salt (osmotic blistering). After blisters have formed, they burst and the underlying unprotected material begins to rust if rust formation has not already started. Osmotic blistering occurs on exposure under water or in areas with heavy condensation. In particular, the salts sodium chloride and ferric chloride, and welding smoke, cause blistering, Other causes of blistering can be dust or grit on the surface (reduces adhesion),voids between the steel and the paint or trapped air in the paint film.
Rusting occurs after a blister in the paint film bursts. The failure will occur most quickly where the paint film is too thin. Particularly susceptible areas are sharp edges, rough welds and places which are difficult to access for application. If rusting occurs after a very short time without prior blistering, there will be an opening through to bare steel, i.e. a pinhole.
(vii) Cracking occurs after a certain ageing of the coating. The causes can be: The top coat is harder than the coats underneath Excessively thick system combined with temperature variations Excessively fast curing of two-component systems Excessively thick zinc silicate gives “mud cracking”. There are various degrees of cracking. Cracks can either form in the top coat only or throughout. The time before the fault occurs can vary. Mud cracking occurs, immediately after application but cracking occurs only after a certain time.
(vii) Flaking is normally the result of a poorly cleaned substrate (oil, grease) or the paint being applied onto condensation or surfaces with amine sweating. Paint will frequently flake off from areas with blistering or cracking and occurs where adhesion is poorest. viii) Chalking is an ageing problem. The binder is degraded by UV radiation from the sun and the pigments appear as dust on the surface. The paint’s ability to resist chalking will vary according to the binder used. Epoxy will chalk in sunlight after just a few months whereas polyurethane will retain its gloss for many years.
(x/xi) Wrinkling/Lifting WRINKLING Appearance: -- Small wrinkles through or partly through the paint film. Caused by—Skin drying of the paint film, which is usually applied too thick
LIFTING: Appearance :Small wrinkles through the paint film Caused by Softening and raising or swelling of a previous coat by the application of an additional coat Not Normally when overcoating Alkyd Lifting is often caused because the solvents in the new coat is too strong for the previous coat.
BLOOMING: Formation of dull patches on the painted surface is known as blooming. The primary cause of this defect is poor quality of paint and improper ventilation.
FADING: When there is a gradual loss of colour from the painted surface, it is known as fading. The main cause of this defect is the reaction of sunlight on pigments of paint.
FLASHING: Presence of glossy patches on the painted surface is known as flashing. The cause of this defect is mainly due to poor workmanship, cheap paint or weather actions.
GRINNING: If the thickness of the final coat of paint becomes very thin, the background can be seen clearly. This is known as grinning. Poor workmanship is the main cause of this defect.
SAPONIFICATION: Formation of soap patches on the painted surface is termed as saponification. Chemical action of alkalis is the cause of this defect.