General guidelines on preparation of steel panels by blast-cleaning

Preparation of steel panels by blast-cleaning is not intended for cold-rolled steel sheet, but can be required for hot-rolled steel to remove rust, mill scale, etc. For such purposes, the following general guidance is given. For additional information, consult ISO 8504-2.

The selection of abrasive size and type should be based on the hardness and surface condition of the steel to be cleaned, the type of blast-cleaning to be employed and the surface profile to be produced. For general testing purposes, the abrasive used should be angular or sub-angular and be of a material harder than the steel to be cleaned. Suitable abrasives include steel grit, aluminium oxide, garnet, and copper or coal slag.

The particle size of the abrasive used may conveniently be between 0,5 mm and 1,2 mm. Other abrasives and sizes may be specified for particular tests. Requirements for blast-cleaning abrasives are given in ISO 11124 and ISO 11126. It should be noted that the use of abrasive containing free silica is subject to regulations in many countries.

Before blast-cleaning, remove any visible deposits of oil or grease by solvent cleaning, aqueous cleaning or any other suitable method. These deposits, if not removed, will contaminate the abrasive, resulting in contamination of subsequently blast-cleaned panels. Any other surface imperfections, such as sharp edges or burrs, should also be removed.

Clean, dry compressed air should be used for nozzle blasting. Moisture separators, oil separators, traps or other equipment might be necessary to meet this requirement. Any of the methods of blast-cleaning described in Clause 5 of ISO 8504-2:2000 can be used to achieve a blast-cleaned surface. Compressed-air abrasive blast-cleaning and centrifugal abrasive blast-cleaning are effective for this purpose. It should be noted that some methods are less effective than others and can require more time to achieve the required surface.

Consult ISO 8504-2 regarding the effectiveness of the various methods.

It should be noted that, when steel is cleaned with a wet abrasive, it can rust rapidly. It might be necessary to add rust inhibitors to the water to temporarily prevent rust formation. Suitable inhibitors include sodium nitrite, chromic acid and sodium dichromate. Some inhibitors can interfere with the performance of certain coating systems.

For best results, direct the abrasive at angles of 70° to 90° to the surface being cleaned and from a distance that ensures the abrasive is travelling at the optimum speed. The optimum speed and distance are somewhat dependent on the type of abrasive blast-cleaning used.

Blast-cleaning should be continued until the surface shows a blast-cleaned pattern completely free from any visible contamination or discoloration and corresponding to preparation grade Sa 3 as defined in ISO 8501-1:1988. Panels prepared as above should have a surface roughness Rz (maximum height of profile) of not greater than 30 % of the recommended dry film thickness of the coating to be applied. After dry abrasive blast-cleaning, remove dust and loose residues from the surface by vacuum, by brushing or by blowing off with a stream of compressed air which is clean and dry. After wet abrasive blast-cleaning, rinse the surface with fresh water to remove loosely adhering residue. Dry the surface using compressed or heated air prior to application of paint. Unless otherwise agreed, the panels should be painted as soon as possible after blast-cleaning, and certainly within four hours. Do not contaminate the cleaned panels.

Blast-cleaned panels are susceptible to rusting if they are not coated immediately after cleaning. To minimize the risk of rust formation, it is not advisable to blast-clean panels unless the temperature of the panel is at least 3 °C higher than the dew point of the surrounding air.