Surface preparation is the key to a good finish. Therefore, careful consideration must be given to the selection of surface treatment methods and paint matching.
It is also important to measure the surface finish by international standards, such as the Swedish standard SISO55900 or ISO8501.

Rust grade
Fundamental to the surface treatment standards are four different levels of rust:
Grade A The surface of the steel is completely covered with adhering scale, almost free of rust.
Grade B The surface of the steel has begun to corrode, and the oxide scale has begun to fall off in flakes.
Grade C The scale on the steel surface is corroded or scrapable, but slight rust spots are visible to the naked eye.
Grade D The oxide scale on the steel surface has been corroded and peeled off, and a large number of rust spots can be seen with the naked eye.
According to SISO55900, the surface preparation of these rust grades is wire brushing and sandblasting according to the following quality standards:
St——Wire brush rust removal standard St2, St3
Sa——Sand blasting and rust removal standard Sa1, Sa2, Sa2.5, Sa3
Sand blasting ——Sa
All thick rust layers, visible grease and dirt that should be chiseled before sandblasting should be removed. After blasting the rust, the surface should be clean and free of dust and debris.
Sa1 grade light sand blasting and rust removal
The surface should be free of visible grease, dirt, loose scale, rust, paint coating and impurities.
Sa2 level Thorough blasting and rust removal
The surface should be free of visible grease, dirt, scale, rust, paint coating and impurities, and the residue should be firmly attached.
Sa2.5 level very thorough sandblasting and rust removal
The surface should be free of visible grease, dirt, loose scale, rust, paint coating and impurities. Residue traces show only slight stains in the form of dots and streaks.
Sa3 grade sand blasting and rust removal until the steel is clean
The surface should be free of grease, scale, rust, paint coating and magazines, and the surface should have a uniform metallic color.
Wire brush rust removal——St
St2 Thorough hand and power tool rust removal
Surfaces should be free of visible grease, dirt, loose scale, rust, paint coatings and magazines.
Sa3 Very thorough hand and power tool rust removal
The same as St2, but should be treated more thoroughly than St2, and the metal substrate has a metallic luster.
These standards are of great help in surface preparation. Paint suppliers generally specify corresponding surface preparation standards for each type of paint.

Determine Requirements
The selection of surface treatment methods and equipment should consider the following factors:
1. What is the degree of surface damage?
2. Which equipment is more suitable?
3. What type of paint matching?
4. What is the surface structure?
5. What type of paint is applied?
6. What is the desired result?

Operating Features
How to do maintenance work, you should pay attention to the following points:
1. Remove rust and scale.
2. Remove zinc salts.
3. Remove grease and dirt.
4. Remove unnecessary paint, such as the old paint film is too thick and has no elasticity, or it needs to be replaced with a higher-level paint set, or the quality of the original paint set is poor.
5. Polish the sharp edges.
6. Wear the glossy paint surface.
7. Thoroughly remove dust.
8. Special treatment of aluminum and galvanized surfaces (degreasing and phosphating primer treatment)

How to remove paint and rust:

Removes loose paint and thick rust
It is ideal to use a scraper to remove loose oil and thick rust layer before sandblasting or wire brushing. are not economical.

Mechanical wire brush rust removal
Technically speaking, when sandblasting is not possible, wire brushing is a more appropriate method. However, if the processing methods and equipment are not used properly, the surface is prone to polishing. A wire brush with coarse bristles is recommended.

Hand Brush - Poor effect

Mechanical grinding 
This method is easy to polish the surface, so that the adhesion of the paint can not achieve satisfactory results. A grinding wheel or grinding wheel must be used for diagonal grinding to produce the roughest possible surface.

Mechanical descaling
This method is not recommended at all, as the surface will be gouged. Air and moisture can enter the cavity. resulting in rust. Due to the uneven surface of the paint film, even after applying several degrees of paint, the thickness at the end of the period is not uniform. In this way, the protection effect is poor, and it is easy to rust.

Pneumatic chisels and long scrapers
These are suitable tools for removing lumps of rust, thicker layers of loose paint, and other tools prior to the use of wire brushes, sanding wheels, or sandblasting equipment. Mechanical rusting is better than mechanical descaling, and in many cases considerably faster.
Needle chisel rust removal method
This method is ideal for small, inaccessible areas. Although not very effective in operation, it can handle surfaces that other equipment cannot. Be aware that using chisel-shaped needles is dangerous as they will damage the surface, thinner and blunt needles should be used, although they take a lot of time, they work well.
 

Steel surface after surface treatment
Abrasive blasting is the best method of surface preparation and can provide the ideal surface and roughness. The surface roughness of the correct sandblasted and derusted surface is uniform, and the upper and lower distances are 30-80 microns (1.2-3.2 mil/s)
Using a rotary wire brush will make the surface too smooth. However, using a grinder and a descaling machine to gouge the surface, so that the distance between the highest and lowest point of the paint film can be 5-600 microns (20-25 mil/s), so a large amount of paint is required to fill the depression.

In the table above The time taken for each preprocessing method is different, and these data clearly show the efficiency of working with different methods. It can be seen that sandblasting is the best method.

Sandblasting and rust removal
Abrasive blasting can also cause problems such as dust and blasting material in the equipment. However, experience has shown that with the correct application of sandblasting and a reasonable work plan, in most cases, this surface treatment method will not be a problem.
It is important to observe the following:
1. Use the correct nozzle air pressure: 7-7.5kg/flat cm (100-107psi). The pressure drops below 4-4.5 kg/cm (57-65 psi).
2. Before sandblasting, use a pneumatic chisel or hard metal scraper to remove paint and rust. In some cases, the removal of coarse-grained contaminants prior to sandblasting can result in a 7% reduction in blasting material consumption.
3. Master the correct ratio of sandblasting material and air, and carefully read the instructions for use of the sandblasting equipment before use.
4. Use sharp and high-quality sandblasting materials. If you use defective products, dust will be generated and the effect will be poor.
5. Dust must be removed from the sandblasted surface. You can use an oil-free air compressor or a special vacuum cleaner, or sweep the sand before applying the first paint.
6. Sandblast a small area at a time and apply a ground-level primer to protect these areas. Temporary primer should dry quickly after application, and blasting should continue near the newly painted area.

Power of sandblasting and descaling equipment
The following is the power of sandblasting and rust removal equipment, but different types of equipment have different powers.
Sandblasted and derusted area:
About 20 square meters/hour
About 100 square meters/hour
Consumption of blasting material:
About 10 kg/m2 
About 250 jin/hour
About 2.5kg/cu.m.air
Compressed air consumption
About 2-2.3cu.m.ai/min
The above figures are based on equipment with a 6mm nozzle. With an 8mm nozzle, the consumption of sandblasting material and compressed air is doubled.
The correct blasting procedure should be:
1. Develop a detailed work plan.
2. Before sandblasting and rust removal, an air pressure chisel (air pressure long scraper) or a hard metal scraper must be used to remove lumps, rust and loose old paint.
3. Start sandblasting and rust removal. Make sure that the supply duct is the right size and that there is an adequate supply of compressed air. Clear dust and blast material before applying primer each time a small area is blasted. Continue blasting until the entire area is blasted and primed.
4, according to the paint matching construction paint.

best long-lasting protection

The best way to remove marine build-up, old paint box rust from the spread is sand blasting to Sa 2.5. Economical or environmental effects can be eliminated by sandblasting.
Problems with Contaminated Materials
The requirements for environmental protection have greatly affected the use of sandblasting to remove rust in dry dock, which will generate a lot of dust, pollute the air, and be harmful to the human body. Equipment must be protected, and removal of contaminated blasting material can add considerable expense.

Water blasting
This surface treatment method depends entirely on the energy of the elegant water jet to meet the requirements of surface cleaning.
Both low pressure flushing (low 340bar) and elegant flushing (340-700bar) have been used for many years and should both be considered a flushing procedure.
High-pressure water jetting (700-1700 bar) and very high-pressure jetting (17000 bar) have developed rapidly in recent times and can now achieve acceptable surface finish standards. Hydro jetting will soon become an internationally recognized surface treatment method.
In theory, Elegant Water Jet can remove all paint, and Ultra Elegant Water Jet can cut concrete and even steel. Both methods are dangerous and should only be performed by skilled and responsible personnel.
It is beneficial to environmental protection, the surrounding dust is reduced, and it is beneficial to the health of workers. Reduce the workload of removing pollutants No salt, no smoke, no sludge or dirt.
There are many kinds of water jet equipment, and the current equipment is suitable for small projects that require environmental protection. This kind of equipment is relatively bulky and consists of a spray gun and a rotating nozzle with one or two output ports. It uses 12-20 common colors of water per minute and that, and can only complete 8-15 square meters per hour. Therefore, and the shipyard needs thousands of dollars per day. Compared with the work of a square meter, the setting spray is not as efficient as sandblasting and rust removal. In addition, this surface treatment method does not change the roughness of the steel surface, and when a certain paint requires the steel to have an anchor chain-like appearance, the use of water jet is not suitable.
Low temperatures are also a limiting factor, as water freezes.

Flash Rust
Flash rust can occur when blasting is established as a surface treatment method. This is a disadvantage of hydrojetting, but with a full-scale low-finish epoxy coating, this phenomenon is reduced.
Flash rust occurs on metal surfaces within minutes to hours after the cleaning job is completed, and is a finish applied to a complete paint.
When applying on old, undamaged paint that is not heavily soiled, first rinse with clean water. When necessary, Eoh can add detergent. The surface should be thoroughly rinsed with clean water, and the paint applied on the traces of external equipment salt cannot reach the surface treatment on the complete paint.
The surface must be absolutely dry before the paint is applied, but the interval should not be too long, as the zinc salts will regenerate the damage on the surface.
If the old paint surface is hard and shiny, use sandpaper to roughen the surface.
When painting over old, complete, glossy paint, a one-time matt primer is generally required. But not necessary if the paint applied is acrylic or vinyl.
Heavily soiled areas should be treated with a water-soluble detergent, (but the applied paint should not be acrylic, vinyl or bituminous).
After ten minutes of treatment with detergent, the surface must be rinsed with clean water, and the treatment should be repeated if necessary.
Pretreatment of aluminum and galvanized steel 
When galvanized surfaces need to be painted, some measures must be taken to ensure satisfactory results. During electroplating and processing, surfaces can become stained with oil and other contaminants that must be removed prior to paint application. Even on new, unexposed parts, the surface must be scrubbed with a suitable detergent, and then rinsed thoroughly with clean water, especially alkaline detergents.

Dirt and zinc corrosion should be removed from old or exposed galvanized parts with water and a stiff brush. In some cases. Surface needs to be sanded. Remove stubborn substances.

Except for some epoxy paints, other kinds of paints, such as acrylic, vinyl, need to be applied once Phosphate primer under 10 microns to enhance adhesion.
Aluminum steel is protected in the same way as exposed galvanized metal. Phosphating primers are used on galvanized metals, but are not recommended for underwater applications.