In plastering, the 1st coat is called_____ and its thickness should be_____mm.

Discussion - Plaster Coats MCQ

A. Undercoat, 6-9

B. Floating coat, 6-9

C. Floating coat, 10-15

D. Undercoat, 10-15

Correct Answer: D. Undercoat, 10-15

🔬 Understanding the Coats of Plaster

For a high-quality and durable finish, plaster is typically applied in multiple coats, especially on new masonry walls. Each coat has a specific name and purpose.

The Plastering System:
• First Coat (Undercoat): Also known as the rendering coat. Its main job is to cover the rough, uneven surface of the brick or blockwork and provide a strong, level base for the subsequent coats.
• Second Coat (Floating Coat): Also known as the browning coat. This coat is applied over the undercoat to build up the thickness and create a truer, flatter surface.
• Third Coat (Finishing Coat): Also known as the setting coat. This is the final, thin layer that provides the smooth, decorative surface.

⚖️ Detailed Analysis of the Options

The question asks for the name and thickness of the first coat.

Identifying the First Coat

Based on the standard terminology, the first coat applied directly to the masonry is called the Undercoat or rendering coat. The "floating coat" is the second coat. This eliminates options (b) and (c).

Determining the Thickness

The first coat (undercoat) has the most important job of covering the irregularities of the raw wall. Therefore, it is the thickest of all the coats.
• A typical thickness for the undercoat is in the range of 10 to 15 mm. This is sufficient to fill the raked mortar joints and level out the surface.
• The second coat (floating coat) is usually thinner, around 6 to 9 mm.
• The final finishing coat is very thin, typically only 2 to 3 mm.

(d) Undercoat, 10-15

Why it's correct: This option correctly identifies the first coat as the "Undercoat" and provides the correct, standard thickness range of "10-15" mm for this initial, leveling layer.

💡 Study Tips for Plastering

Remember the Order: Undercoat → Floating Coat → Finishing Coat.
First Coat is the Thickest: The undercoat does the heavy lifting of leveling the wall, so it needs to be the thickest layer.
"Undercoat" is Under Everything: The name itself tells you it's the first layer, applied directly to the substrate.
Think of a Cake: The first layer of frosting (the crumb coat) is the undercoat—it fills all the gaps. The next layer is the floating coat, and the final decorative swirls are the finishing coat.