ITI/NCVT Painter General (AITT Theory) key terms

The lowest temperature at which a paint or thinner gives off enough vapour to form an ignitable mixture with air near its surface. Solvents with low flash points need flame-free storage and ventilation.

Wearable gear such as respirators, gloves, goggles and coveralls that shields a painter from solvent vapour, dust, skin contact and eye splashes. It is the last line of defence after ventilation and safe work practice.

An enclosed, ventilated cabin where spraying is done so that overspray, solvent vapour and dust are extracted and filtered. It protects the painter and keeps the wet film free of airborne contamination.

Fine paint particles that miss the target and dry in the air before landing. Overspray causes a rough, gritty surface on nearby work and wastes material, so gun distance and pattern are controlled.

A numbered abrasive made of fine steel strands, used for rubbing down between coats and for cleaning. Coarser grades cut faster; finer grades give a smoother key for the next coat.

The number that states how coarse an abrasive paper or cloth is; a low number is coarse and a high number is fine. Grit is chosen by the stage of work, from stripping to final flatting.

A flexible-bladed hand tool used to press stopping or putty into dents, cracks and nail holes and to scrape it level. The blade must be kept clean and square so the filler beds firmly.

A measuring instrument with a sliding jaw and a vernier scale that reads outside, inside and depth dimensions to a fraction of a millimetre. Painters use it to check film thickness of dry test panels.

A precision instrument with a screw spindle that measures small thicknesses, such as sheet metal or a dry paint film on a panel, to hundredths of a millimetre. The thimble scale is read with the barrel scale.

A flat gauge with notched teeth pressed into a freshly applied coating; the highest tooth that touches wet paint gives the wet film thickness. Wet thickness multiplied by the volume solids gives dry thickness.

The thickness of the cured paint film, measured with a magnetic or eddy-current gauge on metal. It controls coverage, durability and freedom from defects such as mud cracking.

A small bare or thin spot in a coating where the substrate is not fully covered. Holidays break the barrier and start local corrosion, so they are found by visual check or a high-voltage detector.

Surface preparation in which abrasive is driven by compressed air against steel to remove mill scale, rust and old paint and to leave a rough profile. The profile helps the primer key mechanically.

The roughness profile left on a metal surface by blast cleaning, measured as peak-to-valley height. Too little profile weakens adhesion; too much can let the primer sag into valleys and fail.

The chemical attack in which alkali, often from fresh masonry or mortar, reacts with oil in a paint and turns it into a soft soapy layer. Alkali-resisting primer or a waiting period prevents it.

A white powdery salt deposit that appears on plaster, brick or concrete when moisture brings salts to the surface and evaporates. It must be brushed off and the damp source cured before painting.

The first coat, formulated to bond to the prepared substrate and to give protection and a key for later coats. Primers are matched to the surface, such as zinc phosphate for steel or alkali-resisting for plaster.

The coat applied over the primer to build film thickness, hide the surface and provide a smooth, slightly flat key for the gloss finish. Its colour is usually close to the finishing coat.

A volatile liquid added to a paint to reduce its viscosity for brushing or spraying and to control drying. The correct type and amount matter, because wrong thinner causes poor flow or film defects.

A measure of a liquid's resistance to flow, which decides how easily paint brushes or sprays and how well it levels. It is adjusted with thinner and checked with a flow cup.

A small cup with a standard orifice used to time how long a measured volume of paint takes to drain out. The result in seconds indicates viscosity and guides thinning for spraying.

A method in which paint is forced at high hydraulic pressure through a small tip and atomised without compressed air. It gives high output and less overspray but a coarser pattern than conventional air spray.

A machine that supplies compressed air to a spray gun or blast equipment. Its delivery capacity and pressure must match the gun, and its air must be filtered and dried to avoid water and oil in the finish.

A defect in which the dried film has a dimpled surface like orange skin, caused by poor atomisation, too high viscosity, wrong thinner or spraying too cold or too fast. It is cured by flatting and recoating correctly.

Small craters in a wet film where it pulls away from a contaminated spot, usually oil or silicone from the gun line or the surface. Cleaning the air supply and the substrate prevents it.

Downward flow of a wet coat that leaves thick curtains or runs after drying, caused by over-thinning, a heavy coat or spraying too close. Correct gun distance and thinner coats avoid it.

A network of deep cracks in a coating, usually from applying too thick a film or too many coats at once so the surface dries before the lower layers. It is repaired by stripping to sound film.

Rubbing down a dried coat with fine abrasive and water or oil to remove brush marks and gloss and to give a key for the next coat. It is followed by cleaning off the slurry before recoating.

Filling dents, cracks and holes with a stiff putty or filler so the surface is level before priming. Stopping must be fully dry and sanded flush, or it shows through the finish.

A decorative technique in which paint is combed, dragged or overlaid to imitate wood grain on plain surfaces. It needs a ground coat, a transparent graining colour and a protective varnish.

A thin, semi-transparent glaze of oil or water paint applied over a ground coat and worked with a brush, rag or comb to produce a broken decorative effect such as graining or stippling.

A cut-out sheet through which paint is applied to reproduce a design or lettering on a surface. Bridges in the cut hold the sheet together, and the brush is used nearly dry to avoid bleeding.

The attribute of colour that names it, such as red, yellow or blue, and places it on the colour circle. Hue is independent of how light or dark or how strong the colour is.

The lightness or darkness of a colour, from near black to near white, regardless of its hue. Matching value is often more important than matching hue when tinting a paint.

The strength or saturation of a colour, meaning how pure or greyed it appears. A low-chroma colour is muted by adding its complement or a neutral tint.

Two hues opposite each other on the colour circle, such as red and green or blue and orange. Mixed together they neutralise each other and produce greys and browns.

Corrosion that occurs when two different metals are in electrical contact in the presence of an electrolyte, so the less noble metal corrodes. Insulating the joint or choosing compatible metals prevents it.

A primer heavily loaded with zinc dust that protects steel by acting as a sacrificial anode, so the zinc corrodes before the steel. It is used on blast-cleaned structural steel in severe environments.

The area that a given volume of paint will cover at a stated film thickness, usually expressed in square metres per litre. It is calculated from volume solids and used to estimate paint quantity.

The percentage of a paint's volume that remains as dry film after the solvent evaporates. It links wet film thickness to dry film thickness and fixes the coverage rate.