The compressor is usually the real constraint
An HVLP gun trades pressure for volume, and the volume has to come from somewhere. Guns of this type are quoted with an air consumption figure in cubic feet per minute at a working pressure, and that figure is continuous rather than intermittent, because spraying a panel means holding the trigger down.
Compressors are rated for delivery, and the honest number is what they actually deliver at pressure rather than the headline displacement. A compressor that cannot keep up shows it in the finish: pressure drops as the tank empties, atomisation coarsens through the pass, and the result is orange peel or a dry, sandy edge on the last part of the panel.
Tank size buys you continuity rather than capacity. A large tank lets a marginal compressor finish a panel before the pump has to catch up, which is workable for occasional work. For continuous spraying, the delivery rate is what matters and there is no substitute for enough of it.
| Symptom | Usual cause |
|---|---|
| Coarse finish that worsens through the pass | Compressor cannot sustain the air demand |
| Orange peel | Too little atomisation, material too thick, or gun too far out |
| Runs and sags | Too much material, gun too close, or moving too slowly |
| Dry, sandy edges | Gun too far from the work, or air pressure too high |
Nozzle size follows the material, not the gun
The nozzle and needle are a matched set, and the size is chosen from the viscosity of what you are spraying rather than from the gun or the job. Thin materials need a small orifice; thick ones need a larger one or they will not atomise at all.
As a general guide, around 1.3 mm suits basecoat and clearcoat, 1.4 to 1.5 mm suits single stage and heavier topcoats, and 1.7 to 1.8 mm suits primer and high build materials. A gun sold with one setup will usually take others from the same family, and having a second nozzle set is what makes one gun cover primer and topcoat properly.
Viscosity is measurable rather than a matter of judgement. A flow cup and a stopwatch give a repeatable number, and paint datasheets state the viscosity the material should be sprayed at. Thinning to the stated figure removes most of the guesswork about whether a poor finish is the gun or the mix.
Fan, fluid and air: setting the gun up
Three controls do different things and they interact. The fan control changes the pattern from round to a tall oval. The fluid control limits how far the needle opens, which sets how much material comes out. The air control, where the gun has one, sets the pressure at the inlet.
Set them in that order and test on card or masking paper rather than on the job. A correct pattern is an even oval with material distributed consistently, not heavy in the middle and thin at the ends, and not split into two lobes. A split pattern usually means a partly blocked air cap horn, which is a cleaning problem rather than a setting one.
Then set the distance and keep it. HVLP guns generally work around 150 to 200 millimetres from the surface, held perpendicular and moved parallel to it. Arcing the wrist through the pass changes the distance across the panel, which is what leaves a stripe of heavier coating in the middle and dry edges at each side.
Cleaning is what decides how long a gun lasts
A spray gun is a set of very small precise passages, and dried paint in any of them changes the pattern permanently. Cleaning is not maintenance to be done when convenient; it is part of using the gun, done immediately after each session while everything is still wet.
Strip it as far as the manufacturer intends, clean the passages with the correct solvent for the material sprayed, and use the soft brushes supplied rather than anything metal. Never put a drill bit, a wire or a pin through an air cap orifice: those holes are precisely sized, and opening one out even slightly ruins the pattern for good.
Reassemble with a light smear of gun lubricant on the needle packing and the trigger pivot, and store the gun with the fluid control backed off so the needle is not sitting under spring pressure in its seat. A gun looked after this way stays consistent for years; one left to dry out once often never sprays evenly again.
