Two Guns, One Small Compressor: Pressure Collapse, Orange Peel and Solvent Vapour

Arithmetic first. Demand when both guns spray = 2 × 200 = 400 L/min. Supply = 200 L/min FAD. Deficit = 200 L/min, i.e. the compressor can only feed one gun with nothing to spare.

Why the drop is so fast. The 50-litre receiver only buffers. Usable free air between the cut-out at 7 bar and the 3 bar the gun needs is roughly 50 × (7 − 3) = 200 L, which the deficit of 200 L/min consumes in about one minute. After that the system settles wherever the compressor output balances gun demand - here about 1.8 bar at the cap - and the motor runs continuously and hot.

Remedies, in order:

1. Spray one gun at a time (stagger the two painters) - instant, costs nothing, halves output but restores full pressure.
2. Hire or install a second compressor of equal size, feed both through a common manifold with non-return valves and a larger receiver; unequal sizes are workable but the small unit must not be the only supply at changeover.
3. Reduce consumption: smaller fluid tip and needle set for smaller work, lower fluid flow, shorter and larger-bore air hose (8 mm ID minimum, not a long coiled 6 mm line), avoid swivel fittings that throttle.
4. Service the compressor: clogged intake filter, slack belt, worn valves and hot thin oil all cut real FAD below the nameplate; gauge the drop across the regulator.
5. Only after supply is fixed, reset each gun regulator with the trigger held open (flowing pressure, not static) to the gun data sheet figure.

Do not simply raise the compressor regulator to 4–5 bar: the guns still cannot get the volume, and you over-pressurise the receiver and hose.

Orange peel. Atomisation depends on air velocity at the cap, not on fluid pressure. At 1.8 bar instead of 3.0 bar the air stream is too weak to shear the paint into fine droplets, so it lands as large droplets. Each droplet has too little solvent left, and surface tension pulls it into a bump before the film can flow level - the classic pebbled surface. The operator's natural reaction, opening the fluid needle or adding thinner to 'make it spray', makes it worse: a thicker wet film with a big-droplet structure cannot level, and over-thinning gives poor build and dullness. Correction: restore full air supply first, then reset fluid flow and viscosity to the datasheet range, check fan pattern on a test card, and keep gun distance at 15–20 cm at 90° to the surface with 50% fan overlap.

Dry gritty edges. The fan passes across a folded edge; particles that miss the flat get a second pass of air, lose solvent in flight and arrive semi-dry, so the edge collects a sandy overspray instead of a wet coat. It is also a thin film over a sharp radius. Correction: spray edges and returns first with a narrow fan and the gun angled to bisect the corner, then coat the flat and overlap onto the edge with a normal pass. Check the edge film with a comb gauge.

Both faults are symptoms of bad atomisation plus technique, not of bad paint - no change of brand or thinner will cure them.

Immediate actions. Stop spraying and shut the gun down. Headache, dizziness and a strong solvent smell in a closed booth are early signs of solvent narcosis (CNS depression); move the affected painter to fresh air, keep him sitting and warm, loosen clothing, monitor breathing; if consciousness is impaired or breathing is irregular, get medical help and do not leave him alone. Ban all ignition sources - no smoking, no grinding, no switched portable tools - because solvent vapour above roughly 1% of the lower explosive limit is a fire risk. Open the booth to full ventilation and only restart when the extractor runs.

Underlying controls. The booth extractor is local exhaust ventilation and is part of the control package, not an optional extra: interlock it with the spray gun so spraying is impossible with the fan off, and fit an airflow proving switch or manometer. Provide and enforce the correct respiratory protection (organic vapour cartridge with particulate prefilter), eye protection, coveralls and gloves; do not use compressed air to blow dust or solvent off skin or clothing. Keep the safety data sheets on site and train on solvent health effects.

Water in the line. Water comes from hot, humid air that cannot drop its moisture before the receiver. Fit an aftercooler plus a coalescing filter and a refrigerated or desiccant dryer, and drain the receiver and filter bowl at every shift, not once a week. Water in the gun causes fisheyes, blush and loss of gloss on the finished panel.

Noise complaint. Answer it with engineering and administration - silencer on the intake, anti-vibration mounts, relocate the compressor, restrict hours - never by switching off ventilation. Disabling a control to satisfy a neighbour converts a nuisance into an occupational health and fire hazard.