What pressure you should spray your HVLP gun at depends on gun type, tip size, paint viscosity, and manufacturer recommendation, but general guidelines help achieve best atomization and finish. HVLP stands for high volume low pressure and is designed to operate at low air cap pressure around 10 PSI at air cap for compliance, but inlet pressure at gun handle is higher. For most gravity feed HVLP automotive spray guns, recommended inlet pressure is 26 to 29 PSI for basecoat and clearcoat with trigger pulled, measured with air gauge at gun inlet, which results in about 10 PSI at air cap. For primer with larger tip like 1.7 to 2.3 mm, you may need 28 to 32 PSI inlet. For fine detail and metallic basecoat, use lower end around 24 to 26 PSI to reduce mottling. For clearcoat, 27 to 29 PSI gives good flow and gloss. Always check gun manufacturer spec, Black Widow HVLP recommends 26 to 29 PSI, Devilbiss around 22 to 26 PSI, Iwata around 16 to 20 PSI at cap. Set compressor regulator to 90 PSI then adjust gun regulator. Test spray pattern on cardboard, you want even oval with no heavy center or tails. Too low pressure causes orange peel and poor atomization, too high causes overspray and dry spray. So start at 26 to 29 PSI inlet for base and clear with HVLP and adjust slightly.
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