Home / News & Blog / Abrasive Blog / How to Achieve Perfect Surface Preparation with Sandblasting
Sandblasting for rust removal involves using compressed air to spray sand of a specific grain size onto the rusted surface of a workpiece. This process not only achieves rapid rust removal but also prepares the surface for subsequent processes such as painting, coating, or electroplating.
To make the metal surface have good cleanliness and roughness, the metal structure surface must be pre-treated by rust removal. There is a saying in the anti-corrosion industry that “70% rust removal and 30% painting”, which shows the importance of rust removal. Given the particularity of metal structures, such as large volumes and the construction environment being open-air, sandblasting is still used for rust removal at this stage. Sandblasting rust removal control is mainly reflected in the following aspects:
Preliminary Control
Before sandblasting, the rust grade of the metal structure substrate surface should be assessed following the standards specified in “Rust Grades and Preparation Grades of Steel Surfaces Before Application of Paints and Related Products”. A thorough inspection is conducted to remove welding slag, spatter, and other attachments. Surface grease and soluble contaminants should also be cleaned. Additionally, any unnecessary welded or connected components should be properly handled.
Abrasive Control
The sand used for sandblasting and rust removal requires hard, angular, dry (water content <2%) particles, and no soil or other impurities; quartz sand is preferred, and coarse river sand is also acceptable; at the same time, factors such as sand transportation conditions, unit price, and recycling capacity should be considered; the particle size of the sand should be 0.5-1.5 mm, and it must be dried before screening and stored in a shed or indoors. The sieve size is: coarse sieve 40-48 holes/cm2 (particle size 1.2 mm), fine sieve 372-476 holes/cm2 (particle size 0.3 mm). After the rust removal plan is determined, the abrasive can be selected from river sand, quartz sand, or a mixture of the two. In the reinforcement of Gangnan Reservoir, after pre-spraying comparison, the best rust removal effect is achieved by mixing quartz sand and river sand at a ratio of 1:1.
Tool Control
During sandblasting operation, the air compressor pressure is 6.0×105～6.5×105Pa, and the pressure amplitude is 0.5×105～1.0×105Pa. The general sand bucket pressure is 4.5×105～5.5×105Pa, and shall not be less than 4.0×105Pa. Before sandblasting, check whether the manufacturer of the pressure vessel holds a production license issued by the relevant department, whether the protective tools, safety belts (ropes), and oxygen supply devices worn by the sandblasters are safe and reliable, and the wear of the nozzle. When the orifice diameter increases by 25%, it is advisable to replace the nozzle.
Environmental Control
Given that sandblasting and rust removal of metal structures is an outdoor operation, attention must be paid to dust control and environmental protection during construction. Additionally, the ambient temperature, humidity, and metal surface temperature at the construction site should be measured daily, and the dew point for the day should be calculated with proper records maintained. Construction must not proceed when the relative humidity exceeds 85% or when the metal surface temperature is less than 3°C above the dew point.
Process Control
The compressed air used for sandblasting must be treated by a cooling device and an oil-water separator to ensure that it is dry and oil-free; the oil-water separator must be cleaned regularly. The distance from the nozzle to the surface of the base steel material should be 100 to 300 mm. The non-sandblasting parts should be shielded and protected before sandblasting. The angle between the spray direction and the normal line of the base steel surface should be 15° to 30°. After sandblasting and rust removal, before the next process, if it rains or other conditions that cause the surface of the base steel to be wet, wait until the environment reaches the construction conditions, and then use dry compressed air to blow dry the surface moisture before construction. If sandblasting is required again, the abrasive requirements cannot be reduced to avoid reducing the roughness. When sandblasting, the nozzle should not stay in one place for a long time. Sandblasting operations should avoid sporadic operations, but the spraying area should not be too large at one time. The time interval requirements between the coating or thermal spraying process and the surface pretreatment process should be considered. For parts that cannot be sprayed by the spray gun, manual or power tools should be used to remove rust.
Quality Control
After the sandblasting is completed, the sandblasting rust removal area should be fully inspected first, and the cleanliness and roughness of the base steel surface should be inspected second. The focus should be on inspecting the areas that are not easy to spray, and the requirements for the areas that are rusted by hand or power tools can be appropriately lowered. When inspecting the cleanliness and roughness of the base steel surface, it is strictly forbidden to touch it with hands; second, it should be carried out under good diffused sunlight or artificial lighting conditions with equivalent illumination to avoid missing inspections. After sandblasting rust removal, the surface cleanliness of the metal structure should reach Sa21/2, and it should be compared with the photos provided in “Rust Grade and Rust Removal Grade of Steel Surface Before Painting”, and there should be at least one comparison point for every 2m2. After sandblasting rust removal, the surface roughness of the metal structure should reach Ry60~100μm. According to the technical requirements, the corresponding roughness sample should be taken and compared after magnification with a magnifying glass of at least 7 times, and there should be at least one comparison point for every 2m2. When using a surface roughness meter for direct measurement, there must be at least one evaluation point for every 2m2 of surface. The evaluation length is 40mm, and 5 points are measured within this length range. The arithmetic mean is taken as the surface roughness value of this evaluation point.
1. Surface Preparation:
Clean the steel surface thoroughly to remove grease and oil using a cleaning agent before sandblasting；
Ensure the surface remains free of grease and oil during and after the blasting process.
2. Environmental Conditions:
The ambient temperature must be at least 3°C higher than the atmospheric dew point；
Relative humidity should not exceed 85%；
Sandblasting operations are prohibited during rain, condensation, or other unsuitable weather conditions.
3. Abrasive Requirements:
Use cast steel shot and cast steel sand that meet national standards, with a particle size of 0.7–1.0mm；
Abrasives must be dry, clean, and free from grease contamination；
For optimal efficiency and quality, mix abrasives on-site with a recommended ratio of S330 steel shot to G25 steel grit at 3:7.
4. Equipment and Process:
Adjust the travel speed of the shot blasting machine to achieve the required surface roughness and cleanliness；
Ensure compressed air pressure for sandblasting is no less than 0.6 MPa；
Protect treated surfaces to prevent secondary contamination；
Blow off dust from inspected, rust-free surfaces before applying paint.
5. Coating Application:
Apply epoxy zinc-rich paint as soon as possible, ideally within 4 hours after blasting；
Conduct surface treatment and painting in sections after completing inspections.
6. Surface Quality Standards:
Achieve specified surface cleanliness and roughness using angular or round sand with a particle size of 0.3–0.8mm；
Ensure abrasives are free from organic contaminants.
7. Effective Sandblasting Techniques for Rust Removal
Rust is a very common problem for iron workpieces, and it is often difficult to eradicate it. This is when sandblasting can be used to remove rust. Sandblasting uses compressed air to spray sand of a certain particle size onto the rusted surface of the workpiece through a sandblasting gun, which can not only achieve rapid rust removal but also prepare the surface for painting, spraying electroplating, and other processes. After the workpiece is treated with sandblasting, the surface can meet the requirements of cleanness and a certain degree of roughness, thereby improving the bonding strength between the covering layer and the parts. Common methods for sandblasting rust removal include dry sandblasting, wet sandblasting, dust-free sandblasting, and high-pressure water sandblasting:
Dry Sandblasting: This method involves completely dry operations, requiring workers to wear full sandblasting gloves and suits. However, it generates a significant amount of dust on-site, making it less environmentally friendly and potentially harmful to the health of operators.
Wet Sandblasting: Wet sandblasting produces no dust on-site, but the moisture involved can cause the workpiece surface to rust. To prevent this, rust inhibitors containing trisodium phosphate and sodium carbonate (concentration 1–15%), emulsifiers, or soapy water are added to the water. An air compressor is then used to dry the surface, ensuring it remains rust-free for a short time. A protective primer is applied afterward for maintenance.
High-Pressure Water Sandblasting: This method involves adding soapy water and passivating agents to the water and is commonly used for large-scale rust removal projects, such as ship hulls, oil tanks, oil compartments, and boilers. It is highly efficient but requires a higher level of expertise from operators.
Dust-Free Sandblasting: Conducted within a sealed system, this method includes processes like sand feeding, sandblasting, and sand recovery, all performed in a closed environment with continuous recycling. This prevents dust from spreading during operations, making it a very clean rust removal method. Additionally, it eliminates concerns about rusting of the workpiece that may occur in wet or high-pressure water sandblasting, and it offers strong reproducibility.
Sandblasting is a highly effective method for rust removal, offering various techniques to suit different project requirements. Each approach has unique advantages and considerations, from dry and wet methods to advanced dust-free and high-pressure water sandblasting. Proper control of abrasives, tools, and environmental factors ensures optimal results while maintaining safety and environmental protection. By adhering to best practices and using suitable techniques, sandblasting removes rust efficiently and prepares metal surfaces for subsequent processes, enhancing durability and finish quality.
