The Surface Five Degrees Above Dew Point Rule for Coatings

Why this matters

A coating applied to a surface carrying a film of condensation you cannot see bonds to the water, not the substrate. On steel it shows up as rust bloom under fresh paint within days, or as a coat that peels in sheets a season later. On wood and masonry it is blistering, poor penetration and early failure. The crew saw a dry surface. It was dry to the eye and wet at the molecular scale, because it was sitting a degree or two above dew point.

The common coatings rule exists to stop that: the surface stays at least 5 F above the dew point through application and early cure. This card explains what the rule is protecting, why the margin is 5 F and not zero, and when the data sheet's own figure replaces it. What dew point is and what it predicts in general is owned by 'What Dew Point Predicts'; the step-by-step for taking the readings is in How to Take a Dew Point and Surface Reading Before Coating.

What the rule is actually protecting

The dew point is the temperature at which the air in contact with a surface becomes saturated. A surface at or below it collects condensation. A surface a little above it does not form visible droplets, but it is not truly dry either: as relative humidity at the surface approaches saturation, a thin adsorbed layer of moisture builds on most materials, and on porous or contaminated surfaces (salts on steel, chalk on old paint, efflorescence on masonry) that layer builds earlier, because the contaminants draw water.

So the rule is not "no droplets." It is "far enough above saturation that the surface is reliably dry everywhere on the part, and stays that way while the film is vulnerable." Three things eat into a thin margin:

• Measurement error. A typical digital hygrometer is good to a few percent relative humidity, and in the conditions where this rule bites (60s F, 60 to 80 percent RH) an error of 3 percent RH moves the computed dew point by about 1 F. A typical infrared thermometer is good to about 2 F, more on shiny metal. Stack the two and a reading that says 3 F of spread may be zero.
• Variation across the part. The reading is one spot. The underside of a rail, a shaded flange, the face of a beam over cold ground, the bottom of a tank: these run colder than the spot you read. See related: Air, Surface and Material Temperature: Three Different Limits.
• Change over time. A spread measured at 10 a.m. is a snapshot. The surface is warming or cooling and the dew point is rising or falling, and the film is vulnerable for hours after the last stroke.

Five degrees is a working allowance for all three. It is a convention from industrial coating practice, not a law of physics, which is why the sheet can change it.

The readings, and how the call is made

Four numbers, measured at the work, not taken from a weather app:

1. Air temperature, in shade near the surface.
2. Relative humidity, at the same spot.
3. Dew point, computed from the first two by the meter or a psychrometric chart.
4. Surface temperature, at the coldest representative spot on the part.

Then spread = surface temperature minus dew point. At 5 F or more, the dew-point condition is met. Under 5 F, the coating holds. A surface reading equal to or below the dew point means condensation is present or forming, whatever the surface looks like.

Many specifications and sheets also cap relative humidity, commonly around 85 percent for many industrial coatings, with the sheet giving the figure. That is a separate gate; a reading can clear the spread and still fail on humidity.

Worked example: one steel rail, one day

A crew is recoating a steel handrail and four bollards at the east entry of a small commercial building. The sheet for the primer and finish reads, say: surface at least 5 F above dew point during application and until dry to touch, relative humidity 85 percent maximum, dry to touch in about 2 hours at 60 F. (Illustrative values; use the sheet.) The rail sits in morning shade, takes sun from about 9:30 a.m. to 3:15 p.m., and the overnight low was 49 F.

The lead's log for the day:

 CONDITIONS LOG - steel rail and bollards, east entry
 time   air F  RH %  dew pt F  surface F  spread F  call
 8:00    62     70     52.1      55         2.9    HOLD
 10:30   66     60     51.7      61         9.3    GO
 2:30    65     64     52.5      60         7.5    STOP
 4:30    63     72     53.8      58         4.2    no coat
 spread = surface minus dew point; gate 5 F; RH gate 85

8:00 a.m. The rail is still carrying the night: 55 F at the metal against 62 F air. Dew point at 62 F and 70 percent is 52.1 F, so the spread is 55 minus 52.1, which is 2.9 F. That is a hold, even though the rail looks dry. Humidity at 70 percent clears its own gate; it is the spread that fails. A crew that painted at 8 a.m. would have coated a surface whose real spread, after the meter errors above, may have been zero.

10:30 a.m. An hour of sun has warmed the steel to 61 F. The air is drier, 60 percent at 66 F, which puts the dew point at 51.7 F. Spread is 61 minus 51.7, or 9.3 F. Both gates pass. Coating starts.

2:30 p.m. The rail and the first two bollards are done, the last of them finished at 2:00. The lead reads again before the last two bollards. Air 65 F, 64 percent, dew point 52.5 F; surface 60 F; spread 7.5 F. On the application gate alone, it is a go. But the rule says "through application and early cure," and early cure on this sheet is about 2 hours to dry to touch. A coat started at 2:30 and finished by 3:00 is vulnerable until about 5:00. The sun leaves the rail at 3:15, steel loses heat fast once it does, and the hourly forecast shows humidity climbing into the evening. The spread will be falling through the whole cure window.

The lead stops there and leaves the final two bollards for the next morning's 10:30 window. The coat finished at 2:00 reaches dry to touch around 4:00.

4:30 p.m. The check confirms it: air 63 F, 72 percent, dew point 53.8 F; surface 58 F; spread 58 minus 53.8, which is 4.2 F. Under the gate. Had the crew coated the last two bollards from 2:30 to 3:00, that coat would now be half an hour short of dry to touch and sitting below the margin, with the spread still falling.

What the log proves. Every reading is written down with its spread. If a bollard fails next year, the shop can show which coats went on at what spread, and that the last coat stopped on a reading rather than on a feeling.

A passing spread on a surface that is already wet

The spread tells you whether new condensation will form. It does not tell you whether the water that formed overnight has left. Dew that collected at 5 a.m. is still sitting in the inside corner of a bollard cap, under the lip of a rail bracket and in the pores of a masonry footing at 10:30, even though the steel is now 9 F above dew point. Evaporation takes time, and shaded pockets and horizontal ledges hold water longest.

So the spread is one gate and the surface condition is another. Before coating on a morning after dew, rain or a hosed-down cleaning, the crew wipes a dry cloth across the low and shaded spots and looks for darkening, and checks porous substrates with a moisture meter where the sheet sets a moisture limit for them.

Salts make this worse. Steel exposed to de-icing salt or coastal air carries salt that draws water at humidities well below saturation, so a salted rail can be damp at a spread that would leave clean steel dry. On those surfaces the surface preparation and any salt test the specification calls for come first; the dew-point rule assumes a clean surface and is not a substitute for cleaning one.

When the data sheet's figure overrides the rule

The 5 F convention is a default for when the sheet is silent or says "above dew point" without a number. Where the sheet states its own figure, the sheet governs, and it can move either way.

• A metric margin. Many sheets written in metric say 3 C above dew point. That is 5.4 F, slightly more than 5 F. Use 5.4, not 5.
• A larger margin. Some products that are especially sensitive to moisture at the bond line, or a specification on a critical job, set a wider spread. A commercial specification can be stricter than the manufacturer; the stricter one governs.
• A moisture-tolerant product. Some coatings are formulated to be applied to damp surfaces. Their sheets say so explicitly and describe what "damp" may mean (no standing water, no running water, for example). Absent that explicit language, assume the product is not moisture-tolerant.
• Cure requirements that go further. Some sheets require the margin to hold for longer than dry to touch, or require a minimum temperature for a stated cure period. Then the cure part of the rule is set by the sheet, and the crew plans the last coat against it.

What changes the answer

• Interior work in a conditioned building. The dew point inside is set by the building, not the weather, and is usually low enough that the rule passes easily, except on surfaces cooled from behind: chilled-water piping, a cold slab, an exterior wall in winter.
• Cold, dry weather. On a winter day the dew point may sit far below the surface and the spread passes by a wide margin, while the surface temperature itself fails the product's minimum. The spread is one gate of several, not a go by itself.
• Steel in direct sun on a hot day. The spread passes with room to spare; the risk shifts to the maximum surface temperature many sheets state, and to solvent flashing off too fast.

How to verify you got it right

• The log for the day shows every reading with a computed spread and a call, and no coat was applied on a row whose spread was under the governing margin.
• The last coat's start time plus its early-cure time lands before the forecast or trend takes the spread under the margin.
• The margin used is the one on the sheet if the sheet gives one, and 5 F only if it does not.

References

• See related: What Dew Point Predicts
• See related: How to Take a Dew Point and Surface Reading Before Coating; The Coating and Sealant Conditions Log SOP
• See related: Weather as a Work Condition: The Rule for Every Trade
• Industrial coating practice for surface preparation and application conditions (AMPP, formerly SSPC and NACE, publishes the guidance most project specifications draw on)
• Manufacturer product data sheets (the governing margin, humidity cap and cure requirements)