Lathing and Plastering (C-35) · Lesson 3 of 6

Coat Counts and the Cure-Time Calendar

"Coat Counts and the Cure-Time Calendar" is lesson 3 of 6 in the Lathing and Plastering (C-35) Learning Hub. It opens with "How Many Coats the Base Requires" and works through 5 short steps, then a worked example you can check yourself against.

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Steps in this lesson

  1. How Many Coats the Base Requires
  2. Exterior Cement Plaster Waiting Times
  3. Interior Cement Plaster Waits Less
  4. Weather Limits Come First
  5. Build the Calendar and Check the Surface

The worked example in the course applies this lesson to a realistic job in 5 steps, from "Exterior brown coat" through "Weather check".

What you'll be able to do

  • Explain the main idea of "Coat Counts and the Cure-Time Calendar" in plain language you can use on the exam.
  • Apply this step with confidence: "Interior Cement Plaster Waits Less".
  • Apply this step with confidence: "Weather Limits Come First".
  • Apply this step with confidence: "Build the Calendar and Check the Surface".
  • Test yourself on the worked example and the quick-check quiz inside the classroom.
Cement Plaster Coats: Waiting Times Between CoatsEXTERIOR 3-COAT (CRC R703.7.5, CBC Table 2512.6)ScratchMonBrownWedFinishWed48 h min7 days minINTERIOR 3-COAT (CRC R702.2.2.2)ScratchMonBrownTueFinishThu24 h48 h min0123456789Days after the scratch coatWeather (CBC 2512.4): apply above 40°F (4°C) unless the work is kept above 40°Fduring application and 48 h after; protect from freezing 24 h after set.Exterior finish coat: day 9 (Wed of next week). Interior finish coat: day 3 (Thu).Second coat flat to 1/4 in under a 5 ft straightedge (CBC 2512.5)Source: 2025 CRC R702.2.2.1-R702.2.2.2, R703.7.4-R703.7.5; 2025 CBC Table 2512.6, §2512.4CC
The diagram for this lesson, exactly as it appears in the Learning Hub.

The first steps of this lesson

Step 1: How Many Coats the Base Requires

The 2025 California Building Code (CBC) 2511.1 requires interior gypsum plaster or cement plaster to be not less than three coats over metal lath or wire fabric lath and not less than two coats over other bases the chapter permits. CBC 2512.1 says the same for exterior cement plaster: three coats over metal lath, wire fabric lath or gypsum board backing as specified in 2510.5, and two coats over masonry or concrete. If the plaster is completely covered by veneer or other facing, or completely concealed by another wall, two coats are enough provided the total thickness is as set forth in ASTM C926. Exceptions exist for gypsum veneer plaster and for cement plaster specifically designed and approved for one-coat work. Note that the 2025 California Residential Code (CRC) R703.7.2 words the gypsum-backing case differently (two coats over gypsum backing), so check which code governs the building.

Step 2: Exterior Cement Plaster Waiting Times

CBC Table 2512.6 prints, for exterior cement plaster, a minimum moist-curing period of 48 hours for the first coat and 48 hours for the second coat, a minimum interval between first and second coats of 48 hours, and a minimum interval of 7 days between the second coat and the finish coat. CRC R703.7.5 agrees: in three-coat work the second coat goes on no sooner than 48 hours after the first, and the finish no sooner than 7 days after the second. For two-coat work the finish coat waits at least 7 days after the first coat. CRC R703.7.4 requires each coat to be kept moist at least 48 hours before the next, unless the application is installed in accordance with ASTM C926.

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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