- With reference to surface preparation for the painting of a vessels hull in dry-dock, list the advantages and disadvantages of EACH of the following methods:
(a) abrasive blasting;(5)
(b) hydroblasting.(5)
(a) Abrasive Blasting (5 marks)
Description:
A process that uses compressed air to propel abrasive particles (e.g. grit, sand, or steel shot) at high velocity to clean the hull’s surface to bare metal.
| Advantages | Disadvantages |
|---|---|
| 1. Produces an excellent surface profile (usually Sa 2½ standard) for good paint adhesion. | 1. Generates large amounts of dust and debris, requiring full containment and cleanup. |
| 2. Removes all rust, scale, and old coatings effectively. | 2. Health hazard — airborne dust can contain toxic materials (e.g. old antifouling paint residues). |
| 3. Can reach difficult areas and complex shapes effectively. | 3. Can cause surface contamination or embedment of grit particles if not properly cleaned after blasting. |
| 4. Provides a uniform, roughened finish ideal for primer application. | 4. High equipment and labour cost, requiring trained personnel. |
| 5. Rapid cleaning rate, suitable for large hull areas. | 5. Noise and environmental pollution (dust, waste grit) — strict safety controls needed. |
(b) Hydroblasting (5 marks)
Description:
Also called water jetting, this method uses high-pressure or ultra-high-pressure water jets (up to 2500 bar) to remove loose rust, scale, old paint, and marine growth without abrasives.
| Advantages | Disadvantages |
|---|---|
| 1. No abrasive material used — eliminates disposal of spent grit, reducing environmental impact. | 1. Does not produce a surface profile as rough as abrasive blasting — may affect coating adhesion if not specified for water-blasted surfaces. |
| 2. Produces a clean, contamination-free surface with no embedded particles. | 2. High equipment cost and requires specialised operators. |
| 3. Reduces dust and air pollution, improving safety and working conditions. | 3. Surfaces must be dried immediately after blasting to prevent flash rusting. |
| 4. Can be used where abrasive blasting is restricted by environmental regulations. | 4. Less effective on heavy corrosion or thick coatings. |
| 5. Suitable for spot-cleaning or maintenance work while afloat or in dock. | 5. Large volumes of wastewater must be collected and properly disposed of. |