Surface Preparation

The effective lifetime of a coating applied on to a substrate depends to a large extent on how thoroughly the surface is prepared prior to painting. Most premature paint failures are attributed to improper surface preparation.

Surface preparation consists of primary surface preparation and secondary surface preparation. The primary surface preparation aims to remove mill scale, rust, corrosion products, and foreign matter from a steel surface prior to application of a shop-primer or paint.

The secondary surface preparation aims to remove rust and foreign matter, if any, from a steel surface that has been already coated with a shop-primer or paint, prior to application of anti- corrosive system. All rust, rust scale, heavy chalk or deteriorated coatings must be removed by a combination of solvent or detergent washing, hand or power tool cleaning or abrasive blasting. Glossy areas of sound previous coatings need not be removed but should be mechanically abraded or brush blasted to create a surface profile which increases coating adhesion.

Cleaning
Cleaning involves the cleaning of oil/grease, dirt, soil, salts and other contaminants from the surface of substrate by the use of solvents, solvent-vapour, alkali, emulsion or steam.

Hand Tool Cleaning
Hand tool cleaning is one of the oldest processes for preparing surfaces prior to painting. Hand tool cleaning is used only for removing loosely adhering paint or rust. The hand tools include scrappers, abrasive pads, chisels, knives and chipping hammers. The common processes of hand tool cleaning are -

Wire brushing
Wire brushing is a conventional method not suitable for the removal of mill scale, but suitable for the preparation of weld seams.

Chipping
Chipping is usually done in combination with wire brushing. It is suitable for local repairs with conventional or some specific paint systems.

Power Tool Cleaning
Power tool cleaning involves pneumatic or electrically operated tools for cleaning operations. It is very rapid compared to hand cleaning methods. It provides a duplication of hand tools in power driven equipment, such as sanders, grinders, wire brushes, chipping hammers, scalers, and Flame Cleaning Flame cleaning involves de-rusting by use of high temperature flame (oxy-acetylene or propane and oxygen). It is very efficient in removing mill scale but removes rust to a lesser extent. This method is restricted because of safety hazards.

Water Jet Cleaning
Water jetting uses water of sufficient purity and quality at high or ultra high pressure to prepare a surface for recoating. It can be used to clean steel, non-ferrous metals and other hard surfaces. It generally removes loose paint, chemical contaminants, loose rust and scale, grease and other material not tightly bonded to the surface. Four types of surface preparations using water are given below: Low pressure water cleaning: Cleaning performed at pressures less than 34 MPa(5000 p.s.i)
High pressure water cleaning: Cleaning performed at pressures from 34 to 70 MPa (5,000 to 10,000 p.s.i)
High pressure water jetting: Cleaning performed at pressures from 70 to 170 MPa (10,000 to 25,000 p.s.i)
Ultra high pressure water jetting: Cleaning performed at pressures above 170 MPa (25,000 p.s.i

Abrasive Blast Cleaning
Abrasive blast cleaning involves the impingement of a high kinetic energy stream of abrasive (such as sand, grit or shot) onto the surface to be prepared. It may either be hand operated by jet or automatically by impeller and is the most effective method for removal of mill scale, rust and old coatings, but not oil or grease. Four common grades of blast cleaning are:
White metal blast cleaning: (Swedish standard - Sa 3)
Near - white metal blast cleaning: (Swedish standard - Sa 2½
Commercial blast cleaning: (Swedish standard - Sa 2)
Brush off blast cleaning: (Swedish standard - Sa 1)

Wet Abrasive Blast Cleaning
Wet abrasive blasting may be performed with low or high pressure fresh water to which a relative small amount of abrasives is introduced, and in some cases inhibitors are added to prevent flash rusting (however, as a general rule it is recommended not to use inhibitors when cleaning areas are to be immersed during service).
This reduces the amount of airborne dust and sand. It is necessary to rinse the surface after blasting to remove sand and debris.
Surface Preparation - Non-Ferrous Substrate Surface should be dry and clean. Any visible oil/grease should be removed. Cleaned surface should be abraded or sweep blasted using low pressure and non - metallic abrasives, then primed with a coat of etch primer.

Galvanised Steel
Degreasing to remove any oil/grease. Any white zinc corrosion products should be removed by high pressure fresh water washing or fresh water washing with scrubbing. Even if sweep blasting is done, water-washing is recommended to ensure removal of soluble zinc salts.

Stainless Steel
Stainless steel surface does not require any specialized surface pretreatment prior to coating. These surfaces should be free from oil, grease, dirt and other foreign materials by chemical cleaning. The development of a surface profile on stainless steel is highly recommended to assure good coating adhesion. A profile depth of between 1.5 and 3.0 mils is suggested for most coating systems. Because stainless steel is a very hard metal, abrasive blasting is recommended to impart a continuous surface profile.