Dew Point Calculator for Blasting and Painting
Enter the air temperature, humidity and steel temperature. See the dew point, whether you can blast and whether you can coat, and how much water your compressor is making today.
Updated November 2026
Contact thermometer on the steel, not an IR gun
From the product data sheet. 85% if you don't have it.
Dew point
Final blast
Go6.9°F over the line
Steel 3°C (5.4°F) above the dew point, humidity 85% or less (Jotun).
Coating
Go7.3°F over the line
Steel 5°F (2.8°C) above the dew point, humidity 85% or less (Carboline, Sherwin-Williams). Your data sheet governs.
Marginal means within 2°F (1.1°C) of the line: inside the rule now, but that much cooling of the steel puts you under it. One manufacturer, Jotun, says to prime within 4 hours of finishing the blast.
How much water is your compressor making?
Same air temperature and humidity as above. Compressing air squeezes out water; the aftercooler and separator catch most of it, and more condenses as the air cools in the hose.
Degrees over the air temperature. Doosan HP375/P425: 20°F. Sullair 900XH manual: 10 to 25°F.
Water out of the aftercooler
Water per hour
Dew point chart
Dew point in °F by air temperature and relative humidity. Add 5°F for the lowest steel temperature you can coat.
| Air °F | 30% | 40% | 50% | 60% | 65% | 70% | 75% | 80% | 85% | 90% | 95% |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 35 | 7 | 13 | 18 | 23 | 24 | 26 | 28 | 29 | 31 | 32 | 34 |
| 40 | 11 | 18 | 23 | 27 | 29 | 31 | 33 | 34 | 36 | 37 | 39 |
| 45 | 15 | 22 | 27 | 32 | 34 | 36 | 38 | 39 | 41 | 42 | 44 |
| 50 | 20 | 27 | 32 | 37 | 39 | 41 | 42 | 44 | 46 | 47 | 49 |
| 55 | 24 | 31 | 37 | 41 | 43 | 45 | 47 | 49 | 51 | 52 | 54 |
| 60 | 28 | 36 | 41 | 46 | 48 | 50 | 52 | 54 | 55 | 57 | 59 |
| 65 | 33 | 40 | 46 | 51 | 53 | 55 | 57 | 59 | 60 | 62 | 64 |
| 70 | 37 | 45 | 51 | 55 | 58 | 60 | 62 | 64 | 65 | 67 | 69 |
| 75 | 41 | 49 | 55 | 60 | 62 | 65 | 67 | 68 | 70 | 72 | 73 |
| 80 | 46 | 54 | 60 | 65 | 67 | 69 | 71 | 73 | 75 | 77 | 78 |
| 85 | 50 | 58 | 64 | 70 | 72 | 74 | 76 | 78 | 80 | 82 | 83 |
| 90 | 54 | 62 | 69 | 74 | 77 | 79 | 81 | 83 | 85 | 87 | 88 |
| 95 | 59 | 67 | 73 | 79 | 81 | 84 | 86 | 88 | 90 | 92 | 93 |
| 100 | 63 | 71 | 78 | 84 | 86 | 88 | 91 | 93 | 95 | 97 | 98 |
Data sources
- Dew point formula: Vaisala, Humidity Conversion Formulas. Magnus form with A = 6.116441, m = 7.591386, Tn = 240.7263, valid -20 to +50°C with a maximum error of 0.083%. Outside that range the calculator refuses instead of guessing.
- Final blast rule: Jotun, SteelMaster 1200HPE application guide: no blasting when relative humidity exceeds 85% or the steel is less than 3°C above the dew point; prime within 4 hours of finishing the blast; monitor conditions every 4 hours, or every one to two hours when they change.
- Coating rule: Carboline, Carbocrylic 3359 DTM product data sheet: do not apply when the surface is less than 5°F (3°C) above the dew point; humidity 85% maximum. Sherwin-Williams, Using the dew point calculator: a minimum of 5°F above the dew point; 70°F at 65% gives a 57°F dew point.
- Steel temperature: KTA-Tator, Emissivity and its application in the coatings inspection industry: an IR pyrometer on blast-cleaned steel can produce false readings and is not recommended.
- Psychrometers: ASTM E337 (DIN Media listing): the aspirated psychrometer is more accurate than the sling psychrometer.
- Aftercooler outlet: Doosan HP375/P425 spec sheet (aftercooler and water separator with 20°F approach temperature); Sullair 900XH manual (10 to 25°F over ambient).
- Water cross-checks: Atlas Copco, What is condensate (75 hp, 75°F, 75% humidity: 280 liters, 75 gallons, a day) and Aftercooler (up to 70% of the moisture drops out there). CAGI figures via Fluid Power World: 1,000 ft³ of 80°F, 60% air holds 0.121 gallons of water; this calculator gets 0.113, about 6% lower.
- Elevation: NWS pressure altitude formula, solved for station pressure. Water math: dry air 287.055 J/(kg·K), Vaisala mixing ratio constant 621.9907 g/kg, 1 liter of water taken as 1 kg.
This page does math on published rules. It does not replace the coating data sheet, the job spec or the inspector.
How to check conditions before you blast
Three readings decide it: air temperature, relative humidity and the temperature of the steel itself. The first two give the dew point, the temperature where water condenses out of the air. If the steel is at or under it, the surface is wet even when it looks dry.
Measure the steel, not the air
The rule is about the surface, and steel does not have to be at the air temperature, so the air reading alone does not answer it. Use a contact thermometer on the surface; KTA-Tator warns that an IR gun on blasted steel can read wrong because the roughness changes how the steel gives off heat.
Blasting and coating have separate rules
The coating rule is the one on the product data sheet, most often 5°F above the dew point with a humidity cap. Jotun also puts a rule on the final blast: steel 3°C above the dew point and humidity no higher than 85%. The calculator shows both so you know whether to start the blast and whether you can follow it with paint.
Check again as the day changes
Jotun recommends checking conditions every 4 hours, and every one to two hours when the weather is moving. A Marginal reading is less than 2°F from a Stop, so it is the one to check again first.
Where the compressor water goes
Compressing air leaves it holding more water than it can carry once it cools. The aftercooler cools it and the separator drains what drops out; the rest rides down the hose as vapor and keeps condensing as the hose cools toward the air temperature. The compressor section shows both numbers. Size your air for the job with the nozzle and compressor calculator.
Common questions
How far above the dew point does steel have to be to blast or paint?
At least 5°F (3°C). Carboline's data sheet says not to apply "when the surface temperature is less than 5°F (3°C) above the dew point", and Sherwin-Williams says no coating should go on unless the surface is "a minimum of 5°F above" it. For the final blast, Jotun's rule is steel at least 3°C above the dew point and relative humidity no higher than 85%. Measure the steel, not the air.
What is the dew point at 70°F and 65% humidity?
About 57.7°F. Sherwin-Williams uses this example and rounds it to 57°F. The steel has to read at least 62.7°F before you coat, and 63.1°F for a final blast under Jotun's 3°C rule.
Why does the calculator use 3°C for blasting and 5°F for coating?
They are the same rule, rounded in different units: 3°C is 5.4°F and 5°F is 2.8°C. Jotun writes its blasting rule in Celsius and Carboline and Sherwin-Williams write their coating rule in Fahrenheit, so the calculator uses each one's own number. The difference is under half a degree.
What humidity is too high to blast or paint?
Jotun says not to blast when relative humidity is over 85%. Carboline Carbocrylic 3359 DTM lists 85% as its maximum for application. Other coatings set their own limit, so enter the one on your product's data sheet; the data sheet governs, not this page.
How soon after blasting do I have to prime?
It depends on the coating maker and the spec. Jotun's application guide for SteelMaster 1200HPE says to apply the primer within 4 hours of finishing the blast. That is one manufacturer's rule, not a universal one; check your product data sheet and the job spec.
Can I use an infrared thermometer on blasted steel?
KTA-Tator says an infrared pyrometer set to a fixed emissivity "can produce false readings and is not recommended" on blast-cleaned steel, because roughness changes the emissivity by an unknown amount. Use a contact surface thermometer on the steel.
Sling psychrometer or aspirated psychrometer?
ASTM E337, the standard method for measuring humidity with a psychrometer, says "the aspirated psychrometer is more accurate than the sling (whirling) psychrometer." Either way, read air temperature and humidity where the work is, then enter them here.
How much water does a blast compressor make?
A 375 cfm compressor at 100 psi on an 85°F day at 70% humidity drops about 2.5 gallons an hour once the aftercooler brings the air to 100°F: about 20 gallons in an 8-hour shift, 71% of the water it pulls in. Another 3 gallons a shift condenses if the air cools to 85°F in the hose. Atlas Copco's own example is a 75 hp compressor at 75°F and 75% humidity making 75 gallons (280 liters) a day.
Keep building the job
Checking the weather before every job?
BlastBid shows the weather forecast on your scheduled jobs and keeps notes and photos with every one, from the estimate to the invoice.
$79/mo after trial · Cancel anytime