Timber preservation methods: arrange the methods in decreasing order of effectiveness (most to least) among pressure impregnation, dipping, and brushing/spraying.

Introduction / Context:
Timber preservatives extend service life by resisting decay, insects, and weathering. Different application methods yield different penetration and retention, which determine long-term performance.

Given Data / Assumptions:

• (1) Dipping: immersion for a short period.
• (2) Brushing or spraying: surface application only.
• (3) Pressure impregnation: preservative forced into wood under pressure.
• Assume standard softwood with typical permeability; surface preparation adequate.

Concept / Approach:
Effectiveness correlates with preservative penetration depth and retention. Pressure processes (e.g., full-cell or empty-cell) achieve deep, uniform penetration and high retention. Dipping offers some penetration in outer layers. Brushing/spraying largely coats the surface with minimal penetration.

Step-by-Step Solution:
Rank pressure impregnation highest due to controlled penetration and retention.Place dipping next; limited penetration but better than mere surface coating.Place brushing/spraying last for mostly surface protection.Thus, decreasing effectiveness: 3 > 1 > 2 → sequence 3, 1, 2.

Verification / Alternative check:
Codes and practice guides consistently endorse pressure treatment for structural and ground-contact applications, with brushing reserved for temporary or maintenance coatings.

Why Other Options Are Wrong:

• 1,2,3 or 2,1,3 overrate surface methods relative to pressure treatment.
• 3,2,1 incorrectly places brushing above dipping.

Common Pitfalls:

• Assuming heavy-brushing equals pressure penetration; it does not.
• Ignoring wood species permeability; while it affects absolute results, the ranking remains.

Final Answer:
3, 1, 2