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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.