An HVLP spray gun uses air to atomise the paint, so it will generally obtain a slightly better finish than basic airless; however the transfer efficiency will not be as good, resulting in more overspray, and the application rate will much slower than airless. In painting and decorating, where a mirror finish is not normally required, HVLP is only used for spraying small volumes of paint, or for spraying multi-colour (fleck) paints with a very large nozzle/needle set up. If airless can be used, it is always preferable to HVLP which is more complicated to set up and adjust, and can only spray relatively thin materials.
The original type of air spray gun is called a Conventional Gun and operates with much higher air pressure than HVLP, of the order of 50 -60 psi. from a compressor. It uses less airflow than HVLP but the tranfer efficiency of a Conventional Gun is very poor, resulting in the loss of at least 70% of the paint as overspray, so it may only be used in a compliant spray booth. An HVLP spray gun works with a large volume of air at up to about 10 psi, that carries the paint particles on to the work; the resulting transfer efficiency is about 50% in practice, although this can be as much as 65% in ideal conditions. There are two basic types of HVLP spray gun according to the source of compressed air. This can be either from a compressor in a fixed installation such as a spray booth, or from a smaller multi-stage air turbine that is easier to carry on site, and therefore better suited to painting and decorating. A compressed air HVLP spray gun cannot be interchanged with a turbine gun, since it contains a pressure reducing valve that is incompatible with the low pressure air from an air turbine. On the other hand, an air turbine gun can be adapted for use with a compressor by using an external pressure reducing valve. The operation of both types of HVLP spray guns is similar, with both adjustable paint and spray fan-width valves. As the gun trigger is pulled, the air valve opens first, then the paint valve. The paint valve, which consists of a nozzle (sometimes called a tip) with a matched tapered needle inside it, must be selected to suit the viscosity of the paint. An air cap surrounds the nozzle and must be matched to the nozzle/needle and the paint. The air cap provides the atomisation of the paint as well as compressing the paint spray into a fan shape; it is important that it is perfectly centralised around the nozzle, which is not always the case in cheaper designs.
The gun manufacturer should provide a table of nozzle/needle/air cap combinations for various paints. In general, the thinner the paint, the smaller the nozzle/needle. For spraying multi-colour (fleck) paints, atomisation is avoided in order to preserve the fleck particles, with the use of a very large nozzle/needle of the order of 2.8mm.
The paint supply is either mounted on the HVLP spray gun, or from a paint hose supplied under pressure from a remote pressure tank or low pressure pump. A gun mounted paint container can be under the gun in front of the trigger, when it is usually of 1 litre capacity and called a Suction Cup (sometimes called a Syphon Cup), or above the gun as a Gravity Cup that is usually smaller, about 700 ml. Some painters prefer a suction cup underneath the gun whilst others prefer a gravity arrangement, each type providing a different balance and view of the work. The gravity gun is gaining in popularity because of the claimed improved view. A gun supplied with paint from a hose has the best view and is much lighter than a gun with a full paint cup, but it requires more paint in the system.
Because an HVLP spray gun operates with low pressure air, typically at less than 10 psi, the passage of the air through the spray gun is usually insufficient to suck the paint from the paint valve (unlike a conventional high pressure air spray gun), so the paint needs to be under slight pressure. With a gun mounted paint cup, this is achieved by connecting the cup to the air supply, via a non-return valve. A remote pressure tank will supply the paint under pressure.
Some advanced designs of HVLP gun will generate enough suction to avoid having to pressure the paint cup, but these guns are not practical for the heavier materials used in painting and decorating and heavier industrial finishes.
Once the right nozzle/needle/air cap has been selected for the paint to be sprayed, the viscosity of the paint needs to be adjusted with a viscometer, although watching the rate of drips from a stirrer will work with practice. A suitable viscometer is the Ford #4 cup, which looks like a ladle with parallel sides and a calibrated hole in the bottom. It is dipped into the paint to fill it completely, then withdrawn and the time noted for it to empty, which should generally be 20 - 25 seconds. More than 25 seconds and more solvent should be mixed in. The gun manufacturer should give guidance on the viscosity adjustment. For portable use in painting and decorating, the air supply is a multi-stage air turbine. Although 2 stage turbines are available, for the small extra cost a 3 stage unit is much better, whilst the extra pressure from a 4 stage unit makes it much more flexible and available to spray heavier materials such as emulsion paint for retouching work as well as multi-colour.
Equipment for spraying multi-colour (fleck)
Equipment for spraying wood finishes
Air assisted airless spray equipment