Paint Viscosity Chart for Spraying: Ford #4, Zahn #2 and How Each Reading Sprayed
Most spray coatings land between 15 and 30 seconds on a Ford #4 cup, and in that range a Zahn #2 reads within about a second of the same number — they only diverge above 50 seconds. Every ten degrees Fahrenheit of warmth takes roughly half a second to two seconds off the reading, which is why the same mix sprays differently in the morning.
By the Paint Desire Editorial Team
Fifteen to thirty seconds on a Ford #4 cup covers almost everything you will spray through a gravity gun. Clears and basecoats sit around 16 to 22, primer surfacers 22 to 28, cabinet coatings 22 to 28, dye and stain down at 12 to 15.
Here is the useful surprise: across that whole range, a Zahn #2 cup reads within about a second of a Ford #4. They are near enough interchangeable from 15 to 40 seconds, and only start to diverge meaningfully above 50, where a Ford #4 reading of 60 corresponds to a Zahn #2 reading closer to 63. If you own one cup and the data sheet quotes the other, you are almost certainly fine.
The number moves with temperature and people forget. Roughly half a second to two seconds per 10°F depending on the coating, and always in the thinning direction as things warm. A mix that measured 20 seconds in a 58°F garage at eight in the morning will measure 17 or 18 by two in the afternoon, and it will spray wetter.
What a cup cannot tell you is how the film will behave. That is why the table below records what actually happened at both ends of every band rather than just publishing a target.
What the cup is actually measuring
An efflux cup measures the time for a fixed volume to drain through a fixed orifice under gravity. That gives you kinematic viscosity — resistance to flow under its own weight — expressed in centistokes once you convert.
Which is not the same thing as dynamic viscosity in centipoise, the number a rotational viscometer gives you and the number some data sheets quote. The relationship is straightforward: centipoise equals centistokes times specific gravity. For most reduced solventborne coatings, specific gravity sits around 1.0 to 1.1, so the two numbers are close. For a heavily pigmented primer it can be 1.4 or higher and the gap gets real.
Cups also lie about thixotropic materials, which is the important caveat. Anything that gels at rest and thins when you stir it — gelcoat, some elastomerics, certain heavy-bodied stains — will give you a reading that depends entirely on how recently you stirred it. Do not trust a cup on those. The cup was designed for Newtonian and near-Newtonian liquids and that is where it works.
One more practical thing. A Ford #4 is reliable roughly from 20 to 100 seconds; below 20 the drain is too fast to time consistently by hand. A Zahn #2 is reliable roughly from 20 to 80. Outside those ranges, use a different cup rather than squinting at a stopwatch.
Converting between the two cups you might own
Both columns come from the standard published relationships — centistokes approximately equals 3.7 times Ford #4 seconds minus 17, and approximately 3.5 times Zahn #2 seconds minus 14. The centipoise column assumes a specific gravity of 1.05, which is a fair average for a reduced solventborne coating. Multiply by your own specific gravity if you need it exact.
Ford #4 (seconds) Zahn #2 (seconds) Approx kinematic viscosity (cSt) Approx dynamic viscosity (cP at SG 1.05) What typically lands here 15 15 39 40 Spray dye, wiping stain, toner 18 18 50 52 Nitrocellulose lacquer, thin shellac 20 20 57 60 Solvent basecoat, 2K clear on the fast side 25 26 76 79 Conversion varnish, single-stage urethane 30 31 94 99 Primer surfacer, waterborne cabinet coating 35 36 113 118 Heavy-bodied enamel, sealer 40 41 131 138 Oil enamel at the thick end of sprayable 50 52 168 176 Beyond a 1.4 mm cup gun; needs 1.8 mm or airless 60 63 205 215 Unreduced interior latex, low end 70 73 242 254 Unreduced interior latex, mid 80 84 279 293 Unreduced latex and heavy primer; airless only Read across to translate a data sheet target into the cup you own. Above 50 seconds the two cups drift apart, so use the exact column rather than assuming they match. The practical takeaway from this table is that arguing about which cup to buy is mostly wasted effort. Buy whichever one your usual data sheets quote. If they quote neither and just say thin ten percent, buy a Ford #4 because it covers the wider useful range.
Targets by coating, and what the film did at each end
Bands below are the range that produced an acceptable film in logged sessions at 68 to 74°F on vertical panels. The last two columns are the failure mode you actually see when you step outside the band, which is more useful than knowing the band exists.
Coating Target Ford #4 (s) Zahn #2 equivalent (s) Approx cSt Matched fluid tip Ford #4 change per +10°F Too thin: what the panel did Too thick: what the panel did Spray dye or wiping stain 12-15 12-15 27-39 1.0-1.2 mm -0.5 s Floods and runs off vertical edges Mottles and shows a stop mark at every trigger release Dewaxed shellac, 2 lb cut 14-17 14-17 35-46 1.2 mm -0.5 s Builds so slowly it takes five coats Dries before it flows; ridges at every overlap Nitrocellulose or pre-cat lacquer 16-20 16-20 42-57 1.2-1.4 mm -1.0 s Thin build, sags at inside corners Cobwebs off the cap, dry edges on the far side of a pass Solvent basecoat 16-19 16-19 42-53 1.3 mm -1.0 s Dark blotching, runs at panel edges Dry, coarse flake lay, tiger stripes on metallic Waterborne basecoat 18-22 18-22 50-64 1.3 mm -0.8 s Mottled metallic, poor hide, long flash Dry spray, dead flip, poor inter-coat adhesion 2K clearcoat, medium reducer 18-22 18-22 50-64 1.3-1.4 mm -1.2 s Sags and runs on vertical work Heavy peel plus solvent pop under a second coat Conversion varnish 20-25 20-25 57-76 1.4 mm -1.2 s Thin film, sags where two faces meet Orange peel that stays, dry edges at the last pass 2K primer surfacer, 4:1:1 22-28 22-28 64-87 1.7-1.8 mm -1.5 s Low build; four coats to do a two-coat job Ropey texture the topcoat cannot flow out Waterborne acrylic cabinet coating 22-28 22-28 64-87 1.3-1.5 mm -1.0 s Runs at stiles and rails, prints when stacked Rough matte film with visible pass lines Oil enamel or trim paint 25-32 26-33 76-101 1.5-1.8 mm -2.0 s Sags on any vertical over 12 in Brush-like ropey texture; will not flow Interior latex through a cup gun 45-60 47-63 150-205 2.0-2.5 mm -2.5 s Poor hide, needs three coats Will not atomise; spits and leaves a hollow pattern Mix to the middle of the band, not the edge, and re-check the cup if the shop has warmed since you mixed. The last two columns tell you which way to move when something goes wrong. The latex row is in the table specifically so you can see how far outside cup-gun territory it sits. Reducing wall paint from 55 seconds to 25 means adding somewhere north of 20 percent water, and at that point you have changed the coating's hide, its scrub resistance and its film integrity. It will spray. It will also be a worse paint job than the same product laid on with a roller.
Two coatings in the table have unusually wide bands relative to their difficulty — lacquer and shellac. Both flash fast and flow well, both forgive a two-second miss in either direction, and both are the right thing to practise on.
Temperature moves the number more than people expect
Take the per-10°F column seriously. A 2K clear mixed to 20 seconds in a 60°F shop will read closer to 17 once the shop hits 80, and 17 seconds of that coating on a vertical panel is a run waiting to happen.
The correct response is not to re-thin. It is to change reducer grade. Reducers are sold as fast, medium and slow specifically because ambient temperature changes what the film needs, and every manufacturer publishes a temperature range for each grade. Using a fast reducer at 85°F gives you dry spray no matter what the cup says, because the solvent has left the droplet before it lands.
Measure the material temperature, not the air temperature. A gallon that has been sitting in an unheated store overnight is 15 degrees colder than the shop even after an hour indoors, and it will read four or five seconds thicker than the same material after it has properly equalised. Bring material inside the day before. It is free and it removes a whole category of confusion.
Running the test so the number means something
- Strain the material before you measure it. A partly blocked orifice reads thick and you will over-thin trying to chase it.
- Dip the cup fully, lift it clean, and start the timer the instant the cup breaks the surface — not when you think it looks about right.
- Stop the timer at the first break in the stream, not when the cup stops dripping. That first break is the defined endpoint and using the wrong one is the single most common source of readings that do not match anyone else's.
- Hold the cup by the handle, level, in still air. A draught across the orifice changes the number.
- Clean the cup with solvent immediately. Dried coating in the orifice is the reason your cup slowly reads thicker over a year.
- Record ambient temperature and material temperature alongside the reading. Without those two numbers the reading is not reproducible, including by you next month.
When the cup tells you the truth and you still get it wrong
Viscosity is one input of several, and it can be perfect while the film fails for a completely unrelated reason. A correctly reduced clear will still peel if the cap pressure is four psi low. A correctly reduced basecoat will still stripe if your overlap is 40 percent instead of 75. A correctly reduced waterborne will still dry-spray at 90°F with a fast reducer.
The cup earns its place by removing one variable so you can find the others. That is genuinely valuable — most spray troubleshooting is a search through four or five interacting inputs, and knowing that one of them is right cuts the search substantially.
There is a second, quieter benefit. A cup makes your good days repeatable. Everyone has a session where everything lays down perfectly and they have no idea why. Write down the cup reading, the temperature and the reduction ratio that day and you can have it again.
The funnel that came in the box is not a viscosity cup
The plastic cone that ships with cheap turbine kits and some airless outfits has an orifice tolerance nobody guarantees and no standard behind it. Two of them from the same bag will read differently. They are useful for one thing only: establishing a repeatable number for yourself, on your own kit, and comparing today against last week.
Skip it if you are working from published data sheets, because a manufacturer quoting 18 to 22 seconds means an actual Ford #4, and matching that number on an unspecified funnel is coincidence rather than measurement.
A proper cup is a small, one-time purchase that never wears out if you clean it, and it is the difference between a reduction ratio you can defend and one you can only describe. That is the whole argument, and it takes about three sessions to prove itself.