A workshop buys a compressor. The carton says 3 HP, the tank holds 100 litres, and the price is right. Two weeks later the half inch impact wrench bogs down after four wheel nuts, the orbital sander runs for eight seconds and then whines to a stop, and a customer's bonnet has come back covered in fisheyes.

Nothing on that compressor is broken. It was never able to feed those tools, and the number that would have warned the buyer was not the number printed largest on the box.

Compressed air looks like the cheapest utility in a workshop. It is air, after all. In practice it is the one that most often limits what the workshop can take on. This guide covers the specification that decides capability, how to match a compressor to the tools it feeds, why the tank is a buffer rather than a supply, what the water in the line does to your paint, how to set up a spray gun, and the safety a pressure vessel demands.

CFM at a Stated Pressure Is the Only Number That Matters

An air tool does not consume horsepower. It consumes a volume of air per minute at a working pressure. Feed it that volume and it performs as rated. Feed it less and it slows, stalls, or gives a fraction of the torque stamped on the plate.

That volume is quoted as CFM, cubic feet per minute, where one CFM is about 28.3 litres per minute. The load-bearing half of the specification is the pressure at which that flow was measured, conventionally 90 psi, or 6.2 bar. A machine rated at 12 CFM at 6.2 bar and one rated at 12 CFM at 2 bar are not comparable, and only one will run a sander.

There is a second trap in the paperwork. Some compressors are advertised on displacement: the swept volume of the piston multiplied by its speed, calculated on paper. What reaches your tool is free air delivery, the measured output after losses to valve leakage, blow-by past the rings, heat and re-expansion in the cylinder clearance. It is always the smaller number, and on a small single-stage machine it is meaningfully smaller. If a sheet quotes a big CFM with no pressure beside it, assume displacement.

Horsepower tells you very little. It describes what the motor draws, not what the pump delivers, and peak horsepower figures on small machines are a marketing convention rather than a continuous rating. When a claim looks generous, read the motor plate instead of the carton: voltage and full load current tell you the real electrical input, and a single phase motor on a domestic socket cannot continuously deliver anything like the shaft power some stickers imply.

Ask one question in the shop. How many CFM does this deliver at 6.2 bar? If the answer is a horsepower figure, a tank size, or a CFM with no pressure attached, you have not been told what the machine can do. Every air tool worth buying states its own consumption at a pressure, so buying is making those two numbers meet.

Match the Compressor to the Tool, Not to the Tank

Every air tool has a rated consumption, printed on the tool or in its manual, and it is the starting point for every decision that follows.

The next question is duty. Some tools work in bursts: an impact wrench runs two seconds per nut, a nailer fires and stops, a blow gun is used in flicks. Between bursts the compressor refills the tank. Others draw continuously for minutes at a time: a spray gun across a panel, an orbital sander on a door, a blast cabinet. Continuous tools break undersized compressors, because there is no pause in which to catch up.

For an intermittent tool you can size close to the rated consumption and let the tank absorb the peaks. For a continuous tool, size free air delivery at least a third above the rating, and preferably half again. That headroom is not waste. It stops the machine running flat out all day, which is how pumps overheat and how oil carries over into your air.

If two people will ever work at once, add the consumptions together. A workshop where a sander and a spray gun can run in the same minute needs a compressor sized for both, or a rule that they never run together. The rule will be broken.

Air demand is set by the tool, not by the tank Typical consumption in CFM at 6.2 bar. Check the plate on your own tool before buying. Brad nailer Blow gun, inflator Impact wrench 1/2 in Die grinder HVLP spray gun Orbital sander 150 mm Blast cabinet 2 3 5 8 12 13 20 0 5 10 15 20 CFM at 6.2 bar (90 psi) Intermittent duty Continuous duty
Illustrative. Typical published ranges, shown to make the gap visible: the tools that run continuously are the ones that decide the size of compressor a workshop actually needs.

Tank Size and Duty Cycle Are Two Different Promises

A bigger tank does not make more air. The pump makes air, and the tank stores a little of it.

What a tank buys you is time. A large receiver lets an intermittent tool draw a burst that exceeds the pump's output, then refills while the tool is idle. It also reduces how often the motor starts, and frequent short cycling is a common cause of early motor failure. What a tank cannot do is add capacity for continuous work. Run a sander from a 200 litre tank fed by a pump making 6 CFM and you get a slightly longer run before it dies, then the same problem.

Duty cycle is the honest statement of how much of an hour the machine may run. Small direct drive oil free compressors are commonly rated at around fifty percent, or thirty minutes per hour. They are quiet, cheap and genuinely useful for inflation, nailing and short bursts. Belt driven cast iron pumps turn more slowly, run cooler, are properly lubricated, and are typically built to run continuously.

If the work is spraying, sanding or blasting, the belt driven machine is not an upgrade. It is the correct category, and buying the other one is buying a consumable.

What an undersized compressor does under a continuous load Receiver pressure while a sander or spray gun runs without pause. Conceptual shapes only. Minimum pressure this tool needs Below this line the tool is still running, but it is not doing its job 9 bar 7 bar 5 bar time under continuous load Pump sized above the tool's demand: cycles and holds Pump sized below it: drifts down
Illustrative. A correctly sized machine cycles between cut-in and cut-out and never falls below working pressure. An undersized one never catches up, so the finish and the torque quietly deteriorate through the job.

The Water in Your Air Line Is Not Optional to Deal With

Ambient air carries water vapour. Compressing it does not remove that water, it concentrates it, because the same moisture is packed into a small fraction of the original volume. Compression also heats the air, and hot air carries moisture happily. Then the air sits in a steel receiver and cools, the vapour condenses, and you have liquid water in the bottom of the tank and a fine mist travelling down the line.

On the coast, around the lake and through the long rains, this happens fast and continuously. Even in dry Nairobi air it happens daily, and what that water does is expensive. It flashes into wet paint and produces fisheyes, blushing and craters that no technique will fix. It rusts air tools from the inside and washes the lubricant out of them. It corrodes the receiver, which is the failure you cannot see.

A moisture trap is therefore not an accessory, and one trap at the compressor outlet is rarely enough alone. Let the air cool before you try to trap the water, because a trap on a hot line passes vapour that condenses further along. Run the main line with a slight fall away from the compressor, drain the low point, and take every branch off the top of the line rather than the bottom so gravity works for you. Then fit a filter and regulator close to the point of use, and add a desiccant or coalescing stage for spraying.

Dry air is part of the finish, not an accessory to it. A filter regulator and a daily tank drain cost less than one ruined respray. Wet air at the gun produces fisheyes and craters that look exactly like contamination on the panel, so the workshop strips and repaints a surface that was never dirty.

Spray Guns: What the Letters Actually Mean

Guns are described by how they atomise and by how the paint reaches the nozzle.

A conventional gun atomises with high air pressure at the cap. It gives very fine atomisation and heavy overspray, so a large share of what you paid for ends up in the air rather than on the panel. HVLP means high volume, low pressure: cap pressure stays low, around 0.7 bar, while the volume of air moved is high. Softer atomisation puts more material on the work and less into the room, but it needs a compressor able to supply that volume continuously, which is where undersized machines are exposed. LVLP guns chase similar transfer efficiency on less air, which suits a smaller compressor, at the cost of being fussier about viscosity.

Feed matters too. A gravity cup drains almost completely and suits fine finishing, a suction cup holds more and suits volume work, and a pressure pot feeds heavy material on long production runs.

Then the fluid tip, chosen for the material rather than for the gun. Thin materials such as basecoats and clears usually sit around 1.3 to 1.4 mm. Primers and heavier single stage finishes want roughly 1.7 to 1.8 mm. High build primers, textured coatings and wood sealers need 2.0 mm and above. Pushing thick material through a fine tip produces the orange peel most people blame on their technique.

Setting a Gun Up So It Sprays the Same Way Twice

Most bad spray jobs are a setup problem, and setup is a procedure rather than a feel.

Set the pressure at the gun, with air flowing. A reading taken with the trigger closed tells you nothing about what the cap sees while you spray, because every metre of hose and every fitting drops pressure once air is moving. Fit a small gauge at the gun inlet, pull the trigger with no fluid, and set the pressure the gun maker specifies for that cap.

Check the material next. A viscosity cup costs very little and removes the largest variable in the job. Thin to the maker's stated seconds at the stated temperature, not until it looks right in the pot.

Then set the gun in a fixed order. Open the fan control fully, open the fluid control fully, spray a single burst onto masking paper held vertically, and read the pattern. It should be an even elongated oval. Heavy in the middle and light at the ends usually means too little air or too much fluid, and split into two heavy ends usually means the reverse. Crescents and tails on one side point to a blocked air cap horn, which cleans with solvent and a soft bristle, never with wire.

Spray at a consistent distance, roughly a hand span from the surface, moving parallel to the panel rather than arcing your wrist, and overlap each pass by half. Then write the settings down, so the next job starts from a number instead of starting over.

The setup order that makes a gun repeatable Work left to right. Skipping a box is what makes the next panel different from the last. 1. Dry air Trap and regulator before the gun 2. Set pressure At the gun inlet, trigger open 3. Check fluid Viscosity cup, stated seconds 4. Test pattern One burst onto masking paper 5. Log it One change, then write it down Change one thing, spray one test, look at it. Changing two settings at once tells you nothing.
The sequence that turns spraying from a knack into a procedure. Recording the pressure, tip size and thinning ratio for each material is what lets a second person match the first person's finish.

A Compressor Tank Is a Pressure Vessel, So Treat It as One

The receiver on your compressor stores a serious amount of energy. Steel that has been thinning from the inside for years fails without warning, and it does not leak politely first.

Drain the condensate every day the machine is used. Open the valve at the bottom of the tank while some pressure remains, so the air pushes the water out, and let it run until only air comes. In humid conditions that is a real quantity of water, not a few drops. A workshop that skips it for a year is running a rusting vessel without knowing, because corrosion works from the inside where no external check will show it.

Never weld, braze or patch a compressor tank. A repair alters the metallurgy in the heat affected zone and leaves a vessel of unknown rating that nobody has tested. A tank showing rust perforation, deep pitting or a drain rusted solid is scrap. Replace it.

Test the safety relief valve periodically by pulling its ring so it vents, and replace it if it does not seat cleanly. Never adjust it, never block it, and never raise the pressure switch above the tank's stamped rating to squeeze out performance. Keep the machine level, out of standing water, with the belt guard in place.

Sized correctly, drained daily and fed dry air, a compressor is one of the few workshop machines still earning fifteen years from now. Sized on horsepower and never drained, it is a two year consumable with a paint problem attached. We stock compressors, air tools, spray guns, hoses and filter regulator sets in our workshop and industrial tools range, with current figures on the price list and larger plant on the machinery page.

Key Takeaways

  • Buy on free air delivery at a stated pressure, conventionally 6.2 bar. Horsepower and tank size tell you nothing about capability.
  • Intermittent tools can be sized close to their rating. Continuous tools such as spray guns and sanders need a third to a half more than they consume.
  • The tank is a buffer, not a supply. For continuous work, duty cycle and pump type matter far more than litres of storage.
  • Compressing air concentrates moisture. Cool the air, drain the low points, take branches off the top of the line, and filter at the point of use.
  • Drain the tank every working day, test the relief valve, and never weld or patch a receiver. A corroded tank is replaced, not repaired.

Frequently Asked Questions

What size air compressor do I need for a spray gun?

Start from the gun's own rated air consumption, which is printed on the gun or in its manual and quoted at a pressure. Because spraying is continuous rather than intermittent, size the compressor's free air delivery at least a third above that figure and preferably half again. A belt driven machine that can run continuously is the right category for spraying, and a small direct drive compressor rated at fifty percent duty is not.

Is CFM or horsepower more important on an air compressor?

CFM at a stated pressure, every time. Horsepower describes what the motor draws, not what the pump delivers, and peak horsepower claims on small machines are a marketing convention rather than a continuous rating. An air tool consumes a volume of air per minute at a working pressure, so the only comparison that means anything is free air delivery quoted at the same pressure, conventionally 90 psi or 6.2 bar.

Why is there water in my air line?

Because compressing air concentrates the water vapour already in it rather than removing it, and compression also heats the air so it holds that moisture until it cools. Once the air cools in the steel receiver and the line, the moisture condenses into liquid water. In humid coastal and lakeside conditions this happens quickly and continuously, which is why a filter regulator and a daily tank drain are not optional.

How often should I drain an air compressor tank?

Every day the machine is used. Open the drain valve at the bottom of the tank while some pressure remains so the air pushes the water out, and let it run until only air comes. Condensate corrodes the receiver from the inside where you cannot inspect it, so the drain habit is the single most important maintenance task on the machine.

What is the difference between HVLP and conventional spray guns?

A conventional gun atomises with high pressure at the air cap, which gives very fine atomisation and a great deal of overspray. HVLP holds cap pressure low, around 0.7 bar, while moving a high volume of air, so softer atomisation puts more material on the work and less into the room. HVLP needs a compressor that can supply that volume continuously, which is where undersized machines are exposed.