Painting and Decorating (C-33) · Lesson 5 of 6

Weather Window: When You May Coat Steel (Caltrans §59-1.03B)

"Weather Window: When You May Coat Steel (Caltrans §59-1.03B)" is lesson 5 of 6 in the Painting and Decorating (C-33) Learning Hub. It opens with "Weather Decides When You May Paint" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. Weather Decides When You May Paint
  2. Solvent-Borne Paint and Blast Cleaning
  3. Water-Borne Paint Has a Tighter Window
  4. The Dew Point Rule
  5. Keep the Clock Running Between Steps

The worked example in the course applies this lesson to a realistic job in 5 steps, starting with "Record and plan".

What you'll be able to do

  • Explain the main idea of "Weather Window: When You May Coat Steel (Caltrans §59-1.03B)" in plain language you can use on the exam.
  • Apply this step with confidence: "Water-Borne Paint Has a Tighter Window".
  • Apply this step with confidence: "The Dew Point Rule".
  • Apply this step with confidence: "Keep the Clock Running Between Steps".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Weather Window for Coating SteelSOLVENT-BORNE / BLAST CLEANINGSurface or air below 36 F: stopSurface or air above 100 F: stopHumidity above 85 percent: stopSteel under 5 F above dew point: stopWATER-BORNE PAINTSurface or air below 51 F: stopSurface or air above 100 F: stopHumidity above 75 percent: stopSteel under 5 F above dew point: stopTIMEAIR / STEEL FRH %DEW PT FSTEEL − DEWSOLVENTWATER7:30 am49 / 4780434NONO10:00 am62 / 6070528YESYES1:00 pm88 / 104355450NONO3:30 pm80 / 8078719YESNOSource: 2025 Caltrans Standard Specifications §59-1.03BCC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: Weather Decides When You May Paint

Cold, damp mornings are the classic reason coatings fail, and the 2025 Caltrans Standard Specifications put numbers on it for structural steel coatings in §59-1.03B: paint only thoroughly dry surfaces during favorable weather, and repair or replace paint damaged by weather. The numbers below are what the State prints for state highway structures; on a private job the contract and the manufacturer's data sheet govern, but this is a California specification you can show a client. Four readings control the day: air temperature, steel surface temperature, relative humidity and dew point.

Step 2: Solvent-Borne Paint and Blast Cleaning

Do not blast clean or apply solvent-borne paint if any one of these exists: atmospheric or surface temperature below 36 degrees F; atmospheric or surface temperature above 100 degrees F; or relative humidity more than 85 percent (§59-1.03B). One failing reading is enough to stop. Note that the test applies to the surface temperature too, so steel baking in the sun can be over 100 F while the air is cooler.

The remaining 3 steps, the worked example and the quiz are in the course. This is a preview. The full lesson, with the step-by-step explanations, a worked example and a quick-check quiz, is inside the classroom.

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