Airless spray tips explained: the three digits

Reading the number

Airless spray tips from the major makers share a three-digit numbering system. Take a 517 tip:

• The first digit, 5, is half the width of the spray fan in inches, measured with the gun held 12 in from the surface. A 5 means a fan about 10 in wide.
• The last two digits, 17, are the orifice size in thousandths of an inch. A 17 means a 0.017 in hole.

So a 313 sprays a 6 in fan through a 0.013 in orifice, and a 621 a 12 in fan through 0.021 in. The two halves of the number are independent: the orifice controls how much paint comes out, the fan width controls how wide it is spread.

The orifice: how much paint flows

The orifice is the part that matters most. Its size sets the flow, in gallons per minute, at a given pressure. Typical flows for water at 2,000 psi, the standard rating condition in manufacturers’ charts:

Paint is thicker than water and flows somewhat less through the same orifice, so these are upper figures. The spray tip calculator and the sprayer usage calculator use this table.

Matching the orifice to the paint

Thin materials atomise through small holes; thick ones need large holes to flow at all. The usual ranges:

Too small an orifice for the paint and the pattern breaks up into “tails”, heavy stripes at the top and bottom of the fan, or the tip clogs. Too large and the paint goes on too heavy, runs, and wastes material. The paint’s data sheet often gives a recommended tip range; it is the authority.

The fan: how wide

The fan width is a matter of the surface, not the paint. Wide fans cover walls and ceilings quickly; narrow fans put paint on trim, spindles and doors without coating everything around them.

• 4 in (2xx tips): spindles, balusters, narrow trim.
• 6 to 8 in (3xx, 4xx): doors, cabinets, window frames.
• 10 in (5xx): walls and ceilings, the common default.
• 12 in and wider (6xx and up): large walls, exteriors, commercial work.

The flow does not change with the fan: a 517 and a 317 put out the same paint, the 517 spreads it over a wider stripe, so the coat is thinner per pass. Painters often keep two tips with the same orifice and different fans, one for walls and one for trim.

Matching the tip to the pump

Every sprayer has a maximum flow, in gpm, and a maximum tip size it can supply while holding pressure. A tip whose flow exceeds the pump’s rating cannot be kept at spraying pressure, and the pattern collapses.

The thinning ratio calculator works out a thinning mix within the label limit. Wet film, dry film and volume solids explains what thinning does to the film you leave on the wall.

Pressure

Turn the pressure up only until the fan is even, with no tails, then a notch more. Pressure above that does not improve the finish; it adds overspray, wears the tip and the pump faster, and wastes paint. If you find you need more pressure over time to get a clean fan, the tip is wearing.

Tip wear

Paint is abrasive, and the orifice wears larger with use. A worn tip sprays a narrower fan with heavy edges and puts out more paint than its number says, so you use more paint and get a worse finish. Signs that a tip is worn:

• The fan is narrower than when the tip was new.
• Tails appear that pressure no longer removes.
• You are using noticeably more paint for the same area.

Replace the tip. It is the cheapest part of an airless system and the one that wastes the most paint when worn. Paints with heavy fillers, such as elastomerics and block fillers, wear tips fastest.

Fine-finish tips

Several makers sell fine-finish, low-pressure tips, often marked with an extra letter or a different colour, designed to atomise well at lower pressure. They give a smoother finish with less overspray on trim, doors and cabinets, and they read the same way: the digits still mean half the fan width and the orifice in thousandths. A fine-finish 310 or 312 is a common choice for enamels on doors and cabinets. Because they run at lower pressure, they also wear more slowly and are easier on the pump.

Reversible tips and guards

Modern tips are reversible: a clog is cleared by turning the tip 180° in its housing and spraying briefly onto scrap, then turning it back. The guard around the tip is a safety device that keeps fingers away from the high-pressure stream, which can inject paint through skin. Keep it on.

How much paint and time

Once the tip is chosen, the flow tells you how fast the paint goes. At 0.31 gpm, 14.29 gallons, enough for 2,000 sq ft of interior walls at two coats with overspray, passes through a 517 in about 46 minutes of trigger time, or about 1 h 17 min of spraying at a realistic 60% trigger time. The sprayer usage calculator does this arithmetic, and spraying versus rolling covers when the speed is worth the overspray.

Common mistakes

• Reading the first digit as the fan width. It is half the fan width.
• Choosing a tip for speed rather than the paint. The orifice is set by the material.
• A tip too big for the pump. It cannot hold pressure.
• Raising the pressure to fix a bad fan. Usually the tip is worn; replace it.
• Spraying trim with the wall tip. Use a narrow fan with the same orifice.
• Removing the tip guard. Injection injuries are serious; keep it on.