Before painting on the steel, careful surface preparation is of the utmost importance. And it gains unexpected protection effect. First step of surface preparation is aims to remove mill scale, rust and foreign matter from steel surface prior to application of a shop primer or primer. The secondary surface preparation aims to remove rust an foreign matter, if any, from steel surface coated with a shop primer or primer prior to application of the anticorrosive paint. A steel surface can be rast-removed in the following different ways.
Blasting:
A corroded or "dirty" steel surface can be very rapidly and effectively cleaned by abrasive blasting. Using a medium(air is most common), abrasive particles(grit, sand etc.)are propelled at high speed through a nozzle to impact on the surface removing corrosion and contamination. A grit size in the range 0.3-1.5mm, has proved to be the most effective in terms of production rates and in achieving specified standards specified standards especially when repairing pitted surface. The rate of cleaning using mineral slags(grit) is usually greater than using sand.
Spot Blasting:
This localized abrasive cleaning is often carried out on the outside of the hull of a vessel where patch corrosion and yield surfaces cleaned to Swedish Standards. In practice there are some precautions which need to be taken in order to prevent subsequent breakdown:
l The surrounding paint film can be undercut by the abrasive particles and the edges around the blasted patch loosened from the steel surface. If this occurs, the loose edges must be removed by thorough scraping of feathering, using a rotary disc.
l The surrounding paint will be peppered by spray abrasive particles and the protective coating system it is necessary to treat the area of damage around the blasted patch. Damage can also occur in the area between patches if the jet of abrasive particles is played across the surface. Blasting should be discontinued whilst moving from one patch to the next. Any damage which is sustained in this way should be made good as described above.
l It is recommended that whenever possible the patches to be blasted should be defined by "chalking in " the boundaries.
Grit Blasting:
When large areas of a vessel's hull or tanks are cleaned by grit blasting a variety of steel surface conditions will be found. Previously coated or superficially corroded steel can be readily cleaned to Sa2.5. However cleaning heavily corroded or pitted surface is more difficult and Sa2.5 may not be practically achieved. The effectiveness of various types and particle sizes of abrasives has been examined in carefully controlled practically trials and the most efficient grit particle size for corroded steel is found to be mineral slag with a range of 0.3-1.5mm.
Sweep Blasting:
Sweeping blasting is the treatment of a surface of the sweeping of a jet of abrasive across the surface. Its effectiveness depends on the mature and condition of the surface, the type and particle size of the abrasive and above all he skill of the operator.
High Pressure Fresh Water Cleaning:
The operation consists of directing a high pressure fresh water jet at the surface. As with sweep blasting the effect will depend on the nature and condition of the surface and also on the pressure of water. Distance of the nozzle from, and angle of the jet to the surface, will also have an effect. Usually for removing surface contamination or weed fouling, pressures in the range of 140-210kg/sq·cm are employed shell fouling may resist the water jet. Pressures as high as 350kg/sq·cm are used when weak or poorly adhering coatings to be removed. Even at these high pressures firmly adhering pain will not be damaged.
Hydro Blasting:
For hydro blasting, abrasive is introduced in to the fresh water stream. It can be used for the removal of tenacious and tough coatings and tough coatings and corrosion scale to give satisfactory results. There are various types of equipment on the market which vary in the method of mixing the abrasive and the water. Normally this method is restricted to pressures up to 210KG/sp·cm.Please select abrasive and adjusts pressure carefully.
Power Rotary Wire Brushing:
The brushes used on this type of equipment consist of steel bristles which bend in contact with the surface. As a result rust scale is often polished rather than broken away from the surface.
Rotary Power Disc:
This method is effective in producing a surface suitable for the application of most types of coating. The silicon carbide pads will effectively abrade the rust scale from the surface of the steel substrate.
Mechanical Chisels:
These instruments, usually air-driven, consist of either vibrating needles or chisel edges which pound the surface to break away corrosion scale.
Manual Treatments:
The term "slicing" refers to manual scraping using a sharp blade implement, together with wire brushing and chipping. It is a traditional treatment.
Surface Preparation Degrees:
When specifying a precise degree of derusting of a steel surface prior to painting, the company uses the Swedish Standard SIS055900; this indicates the following rust grades:
A: steel surface covered completely with adherent mill scale and with little, if any rust.
B: steel surface, which has begun to rust, and from which the mill scale has begun to flake.
C: steel surface on which the mill scale has rusted away or from which it can but scraped, but with little pitting visible to the naked eye.
D: steel surface on which the mill scale has rusted away and on which considerable pitting is visible to the naked eye.
The Swedish Standard indicates six preparation degrees, three of which are used in our specifications:
St3: Very thorough scraping and wire brushing, machine brushing grinding etc. The treatment shall remove loose mill scale, rust and foreign matter thoroughly. After cleaned by a vacuum cleaner, dry compressed air or a clean brush, the surface should have a pronounced metallic sheen.
Sa2.5: Very thorough blast cleaning, mill scale, rust and foreign matter shall be removed to the extent that the only. Traces remaining are slight stains in the form of spots or stripes. Finally, the surface is cleaned with a vacuum clean dry compressed air or a clean brush.
Sa3: Blast-cleaning to pure metal. mill scale, rust and foreign matter shall be removed completely. Finally, the surface is cleaned with a vacuum cleaner, clean dry compressed air or a clean brush; it shall then have a uniform metallic color

Equivalents: Contrastive table for Standard-of Derusing in countries :