Surface Preparation of Metals

GENERAL CONSIDERATIONS: A properly prepared surface is essential to achieve performance from any coating system. Since the cost of surface preparation can easily exceed the cost of coating materials, it is important to be certain that the minimum satisfactory degree of surface preparation be specified and that coatings capable of satisfactory service over less than ideally prepared surfaces be used where possible.

The most recent advances in coating technology have been incorporated into Permite’s maintenance finishes, and the preparation recommended, in some cases, is less extensive than previously thought compatible with good coating performance. The preparation selected should be adequate to remove any contamination which affects adhesion or which will cause corrosion in the presence of moisture. For example, the presence of oil or grease is almost certain to result in lack of adhesion, as can rust and scale, which can also cause pinholes or poor coverage. Soluble salts such as chlorides and sulfates can combine with traces of moisture resulting in underfilm corrosion.

Contaminants such as oil, grease, and other chemicals must be removed before abrasive blast cleaning or they may be driven into the surface and become almost impossible to remove later.

Oil and grease can be removed with solvent soaked rags. Rags should be changed frequently to avoid recontaminating an already clean surface. A final rinse should be made with clean solvent. An alternate procedure is high pressure water or steam with detergent or solvents added.

Salts and other water soluble contaminants can be removed by thoroughly rinsing with clean water. Occasionally, added chemicals will be required to neutralize the surface. Consult Permite in case of doubt regarding the best combination of materials and procedure for surface cleaning.

PREVIOUSLY PAINTED SURFACES present a unique problem. No specific instructions can be given to apply to all cases. Determination of the degree of preparation required is primarily a matter of judgment based on experience and common sense; however, a few guidelines can be followed:

An existing coating that has good adhesion and integrity requires little preparation other than removing dust, dirt and any other surface contamination plus reducing gloss to assure adhesion. Gloss can be removed by sanding, “brush blasting,” or by wiping with a solvent blend that will soften the surface. The proper solvent blend for this operation will vary with the coating; consult Permite for recommendations.

To determine the condition of an existing coating, a careful visual examination is necessary; a knife should be used to probe at spots of possible failure. Look for cracks, checks, blisters, flaking, peeling, heavy chalking, or any other obvious abnormalities. Check at numerous spots to determine if there is good adhesion. Look for corrosion under blisters and other areas of doubtful adhesion.

When the existing finish is 75-80% sound, the areas of failure can be cleaned to a sound surface and spot primed before coating. If more than 25% of the existing finish has failed, it is generally more economical to remove the finish entirely and treat the surface as a new one.

The performance of the new coating is strongly dependent upon the integrity of the existing finish; it is important that the evaluation of the existing surface be objective.

METAL SURFACES can be prepared by a variety of procedures depending upon the type of service to be encountered and the coating to be used. The Steel Structures Painting Council has standardized these procedures through a series of specifications described below (the actual specifications, as well as visual standards, are available from the Steel Structures Painting Council, 440 Fifth Ave., Pittsburgh, PA, 15213):

SSPC-SP1, Solvent Cleaning has been described earlier under “General Considerations.” It should be considered a part of all previously described cleaning procedures, since oil or grease can be driven into the surface to be cleaned if not removed before blasting. The solvents used are flammable. Precautions should be observed pertaining to sparks and open flames. Adequate ventilation or fresh air supply should be provided for applicators. Follow supplier’s material safety data sheets.

SSPC-SP2, Hand Tool Cleaning usually consists of wire brushing, scraping, chipping, or sanding. It is generally used for spot preparation of an existing coated surface which is, for the most part, in sound condition.

SSPC-SP3, Power Tool Cleaning is an extension of SSPC-SP2 in that power tools are used to perform the same operations, but more effectively. It is sometimes used in conjunction with abrasive blasting, particularly to remove heavy mill scale or slag.

SSPC-SP5, White Metal is the ultimate in abrasive blast cleaning. It is costly and only used where the most severe conditions are encountered. SSPC-SP5 is limited to tank linings, immersion service and for some zinc rich primers where the maximum electrical conductivity is required for galvanic protection. All rust, mill scale, existing coatings, etc., must be completely removed.

SSPC-SP6, Commercial Blast is the most widely specified abrasive blast preparation. It is generally accepted when a clean surface is required. This method requires removing all rust, mill scale, dirt, and other surface contaminants. The surface is usually a gray color, and some “shadows” (due to a tightly-adherent previous coating) are tolerated.

SSPC-SP7, Brush Blast is not usually recommended for severe service. It is sometimes used to remove existing finishes which have severely deteriorated, or to remove dust, dirt, loose rust, or other poorly adherent contaminants. Brush blast is relatively low cost.

SSPC-SP10, Near White is a step beyond SSPC-SP6 and only very light shadows from rust, mill scale, oxides, or paint are permitted. The specification requires at least 95% of each square inch of the surface to be free of all visible discoloration; the remainder is limited to the light discoloration described above.

SSPC-SP11, Power Tool Cleaning to Bare Metal. This specification differs from SSPC-SP3 Power Tool Cleaning in that power tools are used to clean to bare metal and produce a profile. Tools included are: non-woven abrasive wheels and discs, coated abrasive discs, rotary flap, and needle guns. This is the preferred alternate to sandblasting.

SSPC-SP12, Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating. This specification uses standard water jetting discharged from a nozzle at pressures greater than 10,000 psi to prepare a surface for coating or inspection. Waterjetting can be useful in areas where environmental regulation are stringent on air pollution prevention (sandblasting dust control) and in reducing hazardous waste. Waterjetting can be a very useful means, however, it does not produce an etch or profile of the magnitude currently recognized by the coatings industry. Rather, it exposes the original abrasive-blasted surface profile if one exists.

In addition to specifying a particular cleaning procedure, the coating manufacturer will specify a surface profile. The profile selected is generally a compromise between the profile produced by the fastest cleaning procedure and the amount of paint required to cover the resulting profile. The use of relatively large particles for abrasive blasting will shorten the time required to achieve a clean surface. However, since a minimum of one mil DFT of primer above the maximum surface profile is usually required, it is evident that some balance between ease of cleaning and coating material cost must be reached.

To assist in maintaining control over surface profile, the following chart is offered to show the approximate relationship between particle size of the abrasive and the resulting profile:

In addition to the procedures specified by SSPC, there are techniques based on high pressure water, with or without chemical additives, steam cleaning, and the use of water with abrasives. All of these have some advantages for specific conditions.

The surface preparation chosen should be the result of consultation between the client, the coating applicator, and the coating's manufacturer.