Steel [Metal] Preparation
(For purposes of this Technical Note, painting and coating are interchangeable. Also, this Technical Note deals with carbon steel, stainless steel, and other ferrous metals only. Preparation of non-ferrous metals is dealt with separately.)
The life of a coating depends as much on the degree of surface preparation as on the subsequent coating system. Surface preparation, therefore, should receive thorough consideration. The primary functions of surface preparation are to clean the surface of material that will induce premature failure of the coating system and provide a surface that can be easily wetted for good coating adhesion.
All coating systems will fail eventually. However, most premature coating failure can be attributed to inadequate surface preparation or lack of coating adhesion.
Typical contaminants that should be removed during surface preparation are:
- Corrosion by-products (also soluble salts)
Rust will be stressed in this Technote. But, since long-term performance is the prime objective, anything that interferes with or lessens adhesion is of concern. Most preparation methods will remove dirt, grease, and oil.
The presence of soluble salts will significantly change the chemistry of the steel environment and rust is one of the places where these salts can hide.
Additionally, mill scale is erratic in its effect upon the performance of coatings. Tightly adhered or intact mill scale does not have to be removed for mild atmospheric exposure. If, however, the steel surface is to be coated with primers with low wetting properties or exposed to severe environments such as chemical exposures and immersion in fresh or salt water, then removal of mill scale is necessary.
You can rest assured that you will be called upon to install polyurea in conditions where other systems have failed. Polyurea has some very attractive features that will capture the attention of an Owner. Its general chemical resistance and speed of return to service will be attractive. In such cases the difference between polyurea fulfilling its function and another system that has failed may well be the expertise of the preparation, rather than the properties of the coating itself.
There is no trade off between cost of proper preparation and the cost required to do a job over again.
Greatly expanded discussion of these and further topics can be found in the full PDF download.