Stream-abrasive processing

in other words sand-blasting, blast cleaning – the operation of propelling, with compressed air water or centrifugal force, a stream of abrasive material onto the cleaned surface. This method gives the most optimal preparation of surfaces for painting.  

Types of processing: open or closed circulation of abrasive stream, dry (wet) under water shield, medium-pressure (0.3 – 0.5 MPa), high-pressure (1.0 – 1.2 MPa), hydro-dynamic (10 – 200 MPa), centrifugal rotary wheels.
Many factors influence stream-abrasive processing, the most important are: pressure of compressed air, inclination angle of the abrasive material, the distance between the nozzle and surface, type and shape of the nozzle, type, measure and shape of the abrasive material.

The stages of steel surface corrosion and the degree of surface preparation before application of paint are defined by the PN-ISO 8501-1 standard.

They are defined by description of surface appearance after cleaning and photographic patterns typical for specific examples. Surface preparation for stream-abrasive processing is marked with letters ‘Sa’.
Stages of surface preparation according to the PN-ISO 8501-1 standard at stream-abrasive processing.

• Sa l – rough stream-abrasive processing – on the viewed surface there can be no oil, grease, dust, poorly adhering: rolling scale, rust, painting layers and external contamination (water soluble salts, welding remains).
• Sa 2 – thorough stream-abrasive processing – on the viewed surface there can be no oil, grease, dust, larger signs of rolling scale, rust, old painting layers and external contamination. All the residual contamination adheres strongly. The surface is grey metallic.
• Sa 2 1/2 – more thorough stream-abrasive processing – there can be no oil, grease, dust, rolling scale, rust, painting layers and external contamination. The surface has almost uniform metallic colour, so called ‘almost white metal’. There can only be signs of contamination in the form of dot or stripe shaped shadows.
• Sa 3 – stream-abrasive processing to visually clean steel – there can be no oil, grease, dust, rolling scale, rust, painting layers and external contamination. The surface has uniform metallic colour, so called ‘white metal’.

Manual-mechanic cleaning

carried out through: brushing, hammering and/or grinding with manual and mechanical equipment. It should be limited to the cases where stream-abrasive processing is impossible. In that way rust, scale or old layers can be roughly removed. After such cleaning, surfaces are smooth and less adhesive than rugged surfaces.
Manual cleaning and cleaning with mechanical equipment is marked with letters ‘St’

• St 2 – there can be no oil, grease, dust, poorly adhering rolling scale, rust, painting layers and external contamination on the viewed surface; the surface shows metallic polish.
• St 3 – requirements as for St 2 with such a difference that every surface should be cleaned until it acquires definitely metallic polish (from its metallic basis).