In architectural painting practice, which primer/base is most traditionally used on timber before finishing coats? Select the most appropriate historic/standard choice used on wood.

Introduction / Context:
Before finish coats are applied to wood, a compatible base/primer is used to seal pores, improve adhesion, and provide opacity. Historically, building practice relied on certain pigments for the base on timber. The question tests recognition of that conventional choice (not modern health regulations).

Given Data / Assumptions:

• Focus is on traditional/common practice in civil engineering texts.
• Substrate is timber, not metal or plaster.
• Primers must provide sealing, adhesion, and a good key for subsequent coats.

Concept / Approach:

White lead (basic lead carbonate) was historically the most commonly specified base/primer for timber because it offered excellent coverage, flexibility with oil vehicles, and good sanding properties. While modern practice often replaces it with safer titanium dioxide systems, exam standards referencing classic materials usually identify white lead as the canonical answer for timber bases.

Step-by-Step Solution:

Verification / Alternative check:

Older specifications and handbooks list white-lead priming on joinery before puttying and finishing coats.

Why Other Options Are Wrong:

Red lead: metal anti-corrosive primer. Zinc white and titanium white: used in modern primers but not the classic “most commonly used” in traditional texts. Shellac alone lacks hiding power and build of a pigmented base.

Common Pitfalls:

Answering by today’s environmental regulations rather than traditional specification context.

Final Answer:

White lead