HVLP Spray Gun PSI Settings: Starting Pressures and the Cap Number They Really Produce
Set 26 psi at the gun inlet with the trigger pulled and you will read about 9 to 10 psi at the air cap on a typical 1.3 mm gravity gun — and cap pressure is the number that atomises and the number that makes the gun legally HVLP. Most coatings hold a usable window five to nine psi wide at the inlet. Below it you get peel and sags; above it, dry spray.
By the Paint Desire Editorial Team
Two numbers, and people confuse them constantly. Inlet pressure is what your regulator shows at the gun handle with the trigger pulled. Cap pressure is what the air is doing at the horns where it actually meets the coating, and it is roughly a third of the inlet figure on most gravity HVLP guns.
For a typical 1.3 mm gravity gun, 26 psi at the inlet lands you around 9 to 10 psi at the cap. That 10 psi ceiling is not arbitrary — it is the legal definition of HVLP, and it also happens to be about where atomisation stops improving and bounce-back starts eating your material.
Set inlet pressure with the trigger pulled, always. A static reading with the gun closed is meaningless — it will show 30 psi and drop to 22 the moment air starts moving, because every foot of hose, every quick-coupler and every filter is a pressure drop you are not seeing.
The log below gives a starting inlet figure per coating, the cap pressure it actually produced when checked with a test cap, and the band either side of the start point where the film stayed acceptable. That band is the Spray Window, and its width tells you how much room for error the combination gives you.
Two gauges, two different numbers
A test cap is an air cap fitted with two gauge ports, one at each horn. You screw it on in place of your normal cap, pull the trigger, and read what the air is actually doing at the point of atomisation. It costs less than a decent respirator cartridge and it is the only way to know whether your gun is doing what its badge says.
Most people never buy one. Fair enough — but then the inlet numbers in any settings chart are a guess about your specific gun, your specific hose and your specific coupler, and they can be out by six or seven psi at the cap.
Hose is the biggest single variable. A 3/8 in ID hose at 25 ft loses very little. A 1/4 in ID hose at 50 ft with two quick-couplers can eat eight to twelve psi at flow. If your gun feels starved, swap the hose before you touch the regulator — you are probably not short of pressure, you are short of volume.
The little ball-type regulator that screws onto the gun handle deserves a specific opinion: most of them are junk. They read pressure upstream of their own restriction, so the number on the dial is not the number entering the gun, and the cheap ones restrict flow badly enough to starve a 1.4 mm tip. If you use one, use it as a fine-trim knob with a proper regulator at the wall, or fit a digital in-line gauge instead.
Set it in this order or you will chase your tail
- Reduce and strain the coating first, and check it on a cup. Pressure cannot fix a viscosity problem and every attempt to make it try ends with the gun blamed for the coating's fault.
- Open the fan control fully, then set the fluid knob. Back the fluid knob all the way in until it seats, then count turns out — that is a repeatable number you can write down. Two to three turns is typical for a 1.3 mm.
- Set inlet pressure with the trigger pulled and the fan open. Write down what the gauge reads at flow, not at rest.
- Shoot a test pattern on masking paper at your intended distance, one quick burst, then a full stripe. Look for a symmetrical ellipse with a soft top and bottom and no heavy edges.
- Adjust the fluid knob before touching pressure. Nine times in ten, a wet, running pattern is too much fluid rather than too little air, and a dry, sandy edge is too little fluid rather than too much air.
- Only then move pressure, and move it two psi at a time. One psi at the inlet is roughly a third of a psi at the cap and you will not see it on the panel.
- Re-check after the first full coat. Coating warms in the cup, viscosity drops, and a setting that was perfect at the start of a kitchen can be laying too much film by door twelve.
The pressure log
All rows shot on a 1.3 or 1.4 mm gravity HVLP gun with a 3/8 in ID hose at 25 ft, at 68 to 74°F and 40 to 50% RH, onto vertical test panels. Cap pressures were read with a two-port test cap at flow. The band column is the inlet range across which the film stayed acceptable on a vertical surface — horizontal panels widen every band by roughly two psi on the high side, because you can get away with a drier film when gravity is not working against you.
Coating Fluid tip Start inlet PSI at the trigger Measured cap PSI Fluid knob (turns off seat) Distance (in) Overlap Inlet band in window (psi) Window width (psi) Waterborne basecoat 1.3 mm 26 9.5 2.5 6 75% 23-29 6 Solvent basecoat 1.3 mm 24 9.0 2.25 6 75% 21-28 7 2K clearcoat, medium reducer 1.3 mm 28 10.0 2.75 6 50% 26-31 5 2K clearcoat, medium reducer 1.4 mm 26 10.0 3.0 7 50% 24-29 5 2K primer surfacer, 4:1:1 1.8 mm 29 10.0 3.5 7 50% 27-33 6 Single-stage urethane 1.4 mm 27 10.0 2.75 6 50% 25-30 5 Pre-catalysed lacquer 1.3 mm 22 8.5 2.0 7 50% 18-27 9 Conversion varnish 1.4 mm 26 9.5 2.5 7 50% 23-29 6 Waterborne acrylic cabinet coating 1.4 mm 27 10.0 2.75 7 65% 24-30 6 Dewaxed shellac, 2 lb cut 1.2 mm 18 7.0 1.75 6 50% 15-24 9 Spray dye or wiping stain 1.0 mm 15 5.5 1.5 6 50% 12-20 8 Start at the figure in column three, verify the film, then use the band in column eight as your permission to move. If your gun reads a very different cap pressure at the same inlet setting, trust the cap number and adjust the inlet to match. The width column is the honest one. A 2K clearcoat gives you five psi of latitude at the inlet — under two psi at the cap — which is why clear is the coat that separates people who spray from people who own a spray gun. Lacquer gives you nine, and shellac gives you nine, and that is why both are excellent coatings to learn on.
Notice that every high-solids coating has a narrow window and every thin, fast-flashing coating has a wide one. That is not a coincidence. Thin coatings flow out and hide small errors; high-solids coatings record them.
Reading the window width column
Width is the single most useful number here and nobody sells on it. A nine-psi window means you can be substantially wrong about your hose, your regulator, your ambient temperature and your reduction, and the panel still comes out fine. A five-psi window means all four of those have to be roughly right at the same time.
For a first-timer, choose the wide-window combination even if the finish ceiling is lower. A pre-cat lacquer at nine psi of latitude will teach you gun distance and overlap in an afternoon. A 2K clear at five will teach you nothing except that spraying is hard, because you will not be able to tell whether the defect came from your pressure, your reduction or your hand speed.
Production sprayers work the narrow windows happily because everything else in their setup is controlled — booth temperature, filtered air, the same reducer every time, the same gun. Take any one of those controls away and the narrow window bites.
What failure looks like at each end
Both ends of the window produce defects, and they look nothing alike, which is genuinely useful — the panel tells you which direction to move. Under-pressure failures are wet: too much fluid arriving as droplets too large, flowing together into peel or running outright. Over-pressure failures are dry: droplets shrinking, flashing in flight, and landing partly cured.
Coating class Four psi below the window Four psi above the window The tell on the panel First correction to try Waterborne basecoat Mottled metallic, patchy hide, slow flash Dry, chalky, flake sits wrong and the flip dies Metallic uniformity under a bright light Move two psi at a time; change hand speed before reduction Solvent basecoat Runs at panel edges, dark blotching Tiger stripes and dry overlap bands Stripes running parallel to your pass direction Increase overlap to 75% before touching pressure 2K clearcoat Heavy peel, sags on vertical, solvent pop risk Dry spray edges, dull matte bands, dieback Gloss falls off toward the ends of each pass Drop the fluid knob a quarter turn first 2K primer surfacer Ropey texture the topcoat cannot flow out Dry, powdery deposit that will not fill Texture visible from three feet Slow the reducer grade; primer is reducer-sensitive Pre-catalysed lacquer Sags at rails and inside corners Cobwebbing off the cap, blush in humidity Fine strings on the masking paper Back off pressure two psi and warm the material Waterborne acrylic cabinet coating Runs on stiles, long flash, prints under stacking Rough matte film, poor inter-coat adhesion Run the back of a fingernail over the edge Raise ambient temperature before raising pressure Spray dye or stain Floods, runs, drips off the bottom edge Mottles, stop marks at every trigger release Uneven colour where the passes met Reduce fluid, never raise pressure on dye Diagnose the direction from the defect before you touch anything. Wet defects mean go up or reduce fluid; dry defects mean come down or slow the material. Temperature and hose move the whole window
Every band in the log belongs to roughly 70°F. Warm the shop to 85°F and the coating thins in the cup, which means the same inlet pressure now atomises a thinner material and the whole window shifts down two to four psi. Cool it to 55°F and everything shifts up, and you will be tempted to add pressure when what you actually need is a faster reducer and a warmer room.
Humidity does something different. It does not move the pressure window much, but above about 70% RH a fast reducer will blush a solventborne coating regardless of what your gauge reads. That is a chemistry problem wearing a pressure problem's clothes.
Hose length and diameter shift the inlet numbers directly. Swapping from 3/8 in at 25 ft to 1/4 in at 50 ft can cost you eight psi at flow, which means every start figure in the log needs adding to. If you change hose, re-verify the cap pressure once and write the new inlet figure on a piece of tape stuck to your regulator. It takes five minutes and saves an argument with yourself every time you set up.
Altitude matters and almost nobody mentions it. Above about 4,000 ft the air is thinner, atomisation gets slightly worse at the same cap pressure, and most sprayers compensate with a touch more reduction rather than more air.
Turbine guns do not have an inlet PSI
A turbine produces pressure as a consequence of stage count and hose diameter — there is no regulator in the sense a compressor user means. A three-stage puts roughly 4 to 5 psi at the cap, a four-stage roughly 6 to 7, a five-stage 8 to 9. What you adjust is the fluid knob, the fan, the tip size and the reduction.
Which makes the numbers in the log unusable as inlet figures on a turbine, but very usable as cap-pressure targets. Find the cap PSI column, match it to what your turbine actually produces, and you know immediately whether your machine can serve that coating without help.
The practical consequence: a three-stage turbine is a stain, dye, shellac and thin-lacquer machine. Ask it to shoot a 2K clear at 10 psi of cap pressure and it simply cannot, and no amount of reduction gets you there without wrecking the film. Buy stages for the coating, not for the badge.
The three settings people never write down
Fluid knob turns off seat. Everyone remembers pressure and nobody remembers this, and it changes the film more than pressure does. Seat the knob, count out, write the number on tape on the cup.
Distance. Six inches for a 1.3 mm gravity gun, seven for a 1.4, and it is measured to the surface, not to where your elbow feels comfortable. Every inch past that is a measurable drop in transfer and a measurable rise in dry edge. Most people drift out to nine or ten inches without noticing, particularly on the far side of a panel.
Overlap. Fifty percent is standard, seventy-five for metallics and for anything where uniformity matters more than build. If you cannot see your last pass — and on a clear you often cannot — put a light behind you at a glancing angle so the wet edge shows. Guessing overlap is the single most common cause of stripes that get blamed on pressure.