Steel-Preparation of the Substrate

Key Points Note

Surface preparation includes degreasing, rust removal, and rust prevention. Flexible approaches are necessary depending on each case. 

1. About Metal Surface Preparation 

Types of Metals: Metals are classified into ferrous and non-ferrous metals. The hardness of steel varies with carbon content, dividing it into mild steel (carbon content below 0.25%) and hard steel. Most of the targets for painting are mild steel. 

Importance of Surface Treatment: Since steel is protected with rust preventive oils and mill scale, it is necessary to completely remove these before painting. 

Process of Surface Preparation: Metal surface preparation includes degreasing, rust removal, and rust prevention. Degreasing is done by soaking in an alkaline solution (about 8wt% NaOH) at 60°C for about 15 minutes, followed by washing with tap water. 

2. Process for Removing Mill Scale 

Removing Mill Scale and Red Rust: First, remove the mill scale with acid pickling, followed by water washing and grit blasting. 

Acid Pickling Process: 

(1) Immerse in about 15% sulfuric acid solution (50°C, for about 15 minutes). 

(2) High-pressure washing with tap water. 

(3) Rinse in a 50°C water tank (tap water) for 20 seconds. 

(4) Dry with air blow. 

(5) Perform grit blasting. 

(6) Paint within 4 hours after blasting. 

Post-Treatment Condition: The surface becomes activated after treatment, and painting is done within 4 hours depending on environmental conditions. If time exceeds, re-blasting is necessary. This process completely removes the effects of welding. 

3. Overview of Surface Preparation 

Meaning of ‘Keren’: In Japanese, ‘Keren’ (derived from ‘clean’) refers to blasting work, where abrasive materials are rapidly struck against the steel surface. Type 1 Keren means performing blasting work by rapidly striking abrasive materials against the surface of the object to be coated. 

Types of Blasting: Shot blasting uses tiny iron balls, grit blasting uses small pieces of steel, and sand blasting uses alumina particles. 

Processing Non-Ferrous Metals: For non-ferrous metals, degreasing and film-forming processes are suitable. 

Approach for Repainting: Decide whether to completely or partially strip the old paint film, and determine the type of Keren work accordingly. Decisions are made case by case, considering the budget. 

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