Compressed Air for Spray Painting Applications

Spray painting's pickiness about compressed air quality exceeds many people's expectations. Same air source equipment, supplied shop air tools for years without a hitch, hook it up to the paint booth and start scrapping parts. Reason is simple. Air wrenches don't care if there's a fraction of a milligram of oil in the air. Spray guns care.

Painting

Spray Booth Air Quality

Oil in Compressed Air

Compressed air carrying oil sprayed onto workpiece, paint surface develops a very distinctive defect: small round craters, rim slightly raised around each one, like a fish eye. Industry calls them fisheyes. Also called craters. Oil droplet lands on that tiny area of workpiece surface, wet paint can't spread, gets repelled outward in all directions. Dries and you've got a pit.

Old timers glance at a paint surface and know what's going on. Fisheyes are oil. White haze is water. Roughness is particles. Each has its own look.

To keep oil content under control, ISO standard Class 1 is 0.01 mg per cubic meter. Class 2 is 0.1 mg per cubic meter. Automotive paint, aerospace components, these demanding applications must achieve Class 1. General hardware spray painting, Class 2 is acceptable, but margin is thin. Equipment condition fluctuates slightly and you might exceed limits. Playing it safe, configure for Class 1. Doesn't cost much more.

Oil-flooded screw machine with refrigerated dryer, followed by three-stage filtration: coarse filter catches large particles and liquid oil-water, precision filter handles oil mist aerosols, activated carbon adsorber handles gaseous oil molecules. Gaseous oil passes right through coalescing filter elements. Only activated carbon can grab it. This stage can't be skipped. Oil-free machine approach is simpler. No oil source means no oil to deal with. Refrigerated dryer plus two stages of particle filtration and you're done.

Moisture Problems

Moisture causes two types of defects on paint surfaces.

Blushing. Entire paint film looks hazy, foggy. Most obvious on clear coats. Like a layer of frost. Water vapor gets trapped in paint film during spraying. During curing it tries to escape, disrupts internal paint film structure. Light goes in but can't get out. Turns milky white.

Blistering is more annoying than blushing because it doesn't show up immediately. Looks fine right after spraying. Few hours later or even next day, bumps start rising. Moisture trapped between paint layer and substrate, or sandwiched between two coats, slowly pushes up a bubble. Especially common during high humidity seasons and when paint is sprayed thick. By the time you notice, that batch might already be packed and ready to ship.

Controlling moisture looks at pressure dew point. Requirement is 20 to 25°F below paint booth ambient temperature. Shop at 70°F, pressure dew point needs to be 45°F or even lower. Standard refrigerated dryer can do around 37°F. Paint booth with AC temperature control, basically enough. Outdoor work or shops with poor temperature control, need desiccant dryer. Refrigerated dryer dew point isn't low enough to hold up.

Particles Embedded in Paint Surface

Large particles form obvious raised bumps. Can feel them by touch. Must sand and respray. Fine particles individually aren't noticeable. Too many and overall texture turns rough. Metal particles embedded don't show at first. After a while rust bleeds through. Yellow-brown spots on paint surface. Can't spot-repair these. Only option is strip everything and redo.

Particles per ISO standard Class 2 is 1 micron particle size upper limit, 1 mg per cubic meter concentration limit. One more 0.01 micron terminal filter element before use point, sufficient for most applications.

Filtration

Multi-Stage Treatment

Piping

Aluminum System

Piping Gets Overlooked More Than Equipment

Air source treatment equipment configured perfectly, air travels tens to hundreds of feet through piping, can get dirty all over again.

Galvanized steel pipe is cheap. Common in general industrial air applications. But using this for paint lines is planting a landmine. Galvanized layer wears out in few years. Steel underneath starts rusting. Rust flakes peel off, ride the airflow downstream. Upstream filter filters upstream air. Rust peeling off pipe inner wall, it can't do anything about. Stainless steel pipe or aluminum alloy pipe solves this problem at root. Inner wall stays smooth forever. Aluminum pipe is light, convenient connections, good value.

Old factory renovations frequently make this mistake. Compressor replaced with new one. Dryer and filters all upgraded. Just didn't change the piping. Then paint surface particle problems never go away.

Corrosion

Old Galvanized Pipe

Supply

Dedicated Line

Horizontal pipe runs need slope. 1/8 to 1/4 inch per foot is enough. Let condensate flow by gravity to drain points. Low points get drain valves. Automatic is best. Manual works too but someone needs to drain regularly. U-shaped traps absolutely cannot exist. Condensate pools at bottom with no way out. Keeps accumulating. One moment it suddenly surges out and sprays all over the workpiece. Branch pipes to paint booth, tap from top of main header. Water and crud accumulated at main header bottom won't directly flow into branch pipe.

Terminal Filtration at Paint Booth

Running a separate pipe to supply the paint booth, isolated from shop's other air-using equipment. This is basic configuration. But just isolating isn't enough.

Branch pipe end, before entering paint booth, install one more terminal filter element. Position as close to spray gun as possible. This element's precision is often higher than the main header filter. Specifically catches secondary contamination in piping. Pipe walls may have deposits. Long distance pipe runs may recondense water droplets. No matter how clean treatment upstream, going through this stretch can get dirty again. Terminal filter element is the last line of insurance.

Element gets dirty and filtration efficiency drops. When installing, mark installation date and planned replacement date on the housing. Change when due. More precise method is watching differential pressure. Pressure gauge across filter or built-in differential indicator. Pressure drop hits threshold, time to change. More accurate than looking at calendar. Some elements have three-month theoretical life but differential exceeds limit in two months. Some go six months and differential is still fine. Totally depends on operating conditions.

Spray Gun Air Consumption Determines System Capacity

HVLP spray guns save paint and save air. Mainstream choice. One gun roughly 8 to 15 CFM. Inlet pressure 25 to 30 PSI. Traditional high pressure guns use more air. 12 to 20 CFM or even higher.