EXPERT TIP #37 – Coating Application and Dew Point
When applying protective coatings, temperature and humidity matter. If the surface temperature of a structure is too close to the dew point, condensation can form - leading to poor coating adhesion, blistering or premature failure.
Why it matters
A properly applied coating is your first line of defense against corrosion. Moisture on the surface of the structure to be coated, even invisible condensation, prevents coatings from bonding effectively.
Coating performance is heavily influenced by surface condition at the time of application, and “dry-looking” steel can still be at risk. When substrate temperature approaches the dew point, micro-condensation can form and lead to loss of adhesion, blistering and premature breakdown.
Coatings Matter to CP System Performance
A coating is the primary barrier to prevent corrosion; cathodic protection is the backup system to protect bare steel where coatings have been damaged or degraded. When a coating underperforms (poor adhesion, early failure), the effective exposed steel area increases, driving higher CP current demand for protection.
Checking for the Dew Point
Professional coating crews check dew point because it protects adhesion integrity, cure performance and long-term corrosion resistance. Dew point is not optional.
Dew point is the temperature at which air becomes fully saturated with water vapor (100% relative humidity). When air cools to this point - or a surface drops to this temperature - moisture can condense into liquid water (dew). A higher dew point simply means there’s more moisture in the air.
For coating applications, the dew point is a critical check because condensation on the substrate (even a thin, hard-to-see film) can prevent proper adhesion. If a coating is applied when the surface temperature is too close to the dew point, it can lead to premature failures such as blistering, peeling or cracking.
Most coatings should not be applied unless the steel surface is at least 5 degrees above the dew point. Temperature must be maintained during the coating curing process. Doing this simple check is one of the fastest, lowest-cost ways to prevent blistering, peeling, rework and warranty issues.
Use a digital hygrometer or sling psychrometer to compare against the chart values below. Remember - do not spray unless the substrate is at least 5°F above the dew point.
How Check the Dew Point
The dew point chart will help determine if the steel surface temperature is at least 5°F (3°C) above the dew point temperature, critical before applying the coating:
1. Measure ambient air temperature and relative humidity.
2. Locate both values on the chart to find the dew point.
3. Compare the surface temperature of your structure to the dew point value.
4. If the difference is less than 5°F, delay coating until conditions improve.
DEW POINT CALCULATION CHART
|
Relative Humidity |
Ambient Air Temperature (Fahrenheit) |
||||||||||
|
20 |
30 |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
120 |
|
|
90% |
18 |
28 |
37 |
47 |
57 |
67 |
77 |
87 |
97 |
107 |
117 |
|
85% |
17 |
26 |
36 |
45 |
55 |
65 |
75 |
84 |
95 |
104 |
113 |
|
80% |
16 |
25 |
34 |
44 |
54 |
63 |
73 |
82 |
93 |
102 |
110 |
|
75% |
15 |
24 |
33 |
42 |
52 |
62 |
71 |
80 |
91 |
100 |
108 |
|
70% |
13 |
22 |
31 |
40 |
50 |
60 |
68 |
78 |
88 |
96 |
105 |
|
65% |
12 |
20 |
29 |
38 |
47 |
57 |
66 |
76 |
85 |
93 |
103 |
|
60% |
11 |
19 |
27 |
36 |
45 |
55 |
64 |
73 |
83 |
92 |
101 |
|
55% |
9 |
17 |
25 |
34 |
43 |
53 |
61 |
70 |
80 |
89 |
98 |
|
50% |
6 |
15 |
23 |
31 |
40 |
50 |
59 |
67 |
77 |
86 |
94 |
|
45% |
4 |
13 |
21 |
29 |
37 |
47 |
56 |
64 |
73 |
82 |
91 |
|
40% |
1 |
11 |
18 |
26 |
35 |
43 |
52 |
61 |
69 |
78 |
87 |
|
35% |
-2 |
8 |
16 |
23 |
31 |
40 |
48 |
57 |
65 |
74 |
83 |
|
30% |
-6 |
4 |
13 |
20 |
28 |
36 |
44 |
52 |
61 |
69 |
77 |
Field Tips - “Is it safe to spray?”
Before blasting/priming/spraying:
1. Check air temp + humidity (digital hygrometer/psychrometer).
2. Use the chart (or meter) to find the dew point.
3. Shoot the steel with an IR thermometer (or contact probe).
4. Green light: steel is 5°F+ above the dew point.
5. Red light: if you’re within 5°F, stop and wait; you’re risking peel/blister/rework.
When condensation sneaks in:
· Early mornings/late afternoons (temperature swings)
· Shaded steel that stays cooler than the air
· After rain, washdown, fog or big humidity spikes
· When the steel feels “cold,” even if the air feels fine
If conditions are borderline, re-check more often (every 30–60 minutes). Dew point can change fast, especially at sunrise/sunset.
Need help choosing instruments, interpreting readings or tying coating quality back to CP performance? Call Farwest, our experts can help you get it right before you spray.
Farwest Corrosion Control Company
888-532-7937 FarwestCorrosion.com
Headquarters: 12029 Regentview Avenue, Downey, CA 90241
© 2026 Farwest Corrosion Control Company. All Rights Reserved.