When preparing painting surfaces, touch all the bases

What makes a good paint job? Certainly, a well-specified coating system is one part of the equation. Another part is proper application of the coating. But don't forget what's underneath it all. It is estimated that up to 80% of all coatings failures can be attributed to inadequate surface preparation. Like a strong foundation, proper surface preparation lays the groundwork for coating performance.

The best results from any coating system are achieved when the substrate is properly prepared. Mildew, oil, grease, dust, dirt, loose rust, peeling paint or other contamination must be removed before painting to ensure proper adhesion. Premature paint failures such as delamination and blistering can result from improper surface preparation.

Methods of surface preparation include solvent or detergent cleaning, detergent washing, high-pressure water blasting, power-tool cleaning, hand-tool cleaning, acid etching, wet abrasive blasting, and dry abrasive blasting. Preparing plant surfaces for painting requires knowledge of these methods, as well as knowledge of the substrate and the environment in which the coating system must perform, so the best method can be chosen for the circumstances.

"When it comes to surface preparation, getting to the bottom of it all — including the bottom line — means asking the right questions. These include:

• What is the substrate?
• Is the substrate new?
• Is it previously coated or uncoated?
• Is it rusted or deteriorated?
• Are there harsh chemicals or corrosive agents present in the environment?
• Can desired surface preparation equipment be used given the machinery present in the environment?

Using abrasive blasting methods around mechanized conveyors and computerized tracking systems, for example, can damage moving parts and bearing mechanisms, so other surface preparation methods should be considered.

Economics is another factor that affects surface preparation choices. Cost per square foot for surface preparation can range from approximately 45¢ to well over $1.50. In circumstances where surface preparation costs are prohibitive, surface tolerant coatings are available that can reduce surface preparation requirements.

The majority of metal surfaces in industrial plants are aluminum, galvanizing and ferrous metal. They all require protection to keep them from corroding in aggressive environments. Standards for surface preparation for various substrates are available in the form of joint standards issued and approved by NACE International (formerly National Association of Corrosion Engineers) and SSPC (The Society for Protective Coatings).

Aluminum, for example, is often used for railings, equipment supports, structural steel for roofs and ceilings, and elevated walkways in plants. Before painting aluminum, all oil, grease, dirt, oxides and other foreign contaminants should be removed per SSPC SP-1 Solvent Cleaning. This standard specifies the use of water, solvents or alkaline cleaners to remove foreign matter from the surface of the substrate. Rags and the cleaning solution should be changed frequently so that deposits of oil and grease are not spread over additional areas during the cleaning process.

Galvanized metal typically used for guardrails, handrails, catwalks, platforms and other plant surfaces should be allowed to weather a minimum of six months and prepared per SSPC-SP1 Solvent Cleaning. When weathering is not possible or the surface has been treated with chromates or silicates, first solvent clean per SSPC-SP1 and apply a test area. Allow the coating to dry at least one week before checking adhesion. If adhesion is poor, brush blasting per SSPC-SP-7/NACE No. 4 is required.

When blasting is the required surface preparation for steel, abrasive size, air pressure and distance from the surface determine the depth of profile and cleaning rate. BlastingSurface profile, it increases the surface area of the substrate and provides an irregular surface, both of which promote adhesion by mechanical bond. A surface profile that is too deep may leave exposed steel at the peaks, resulting in pinpoint-rusting above the coating.

Two types of abrasive are available. Steel shot is round and provides a peened profile. Grit is irregular in shape with jagged edges. It provides better cutting action than shot and imparts an angular profile. In many cases steel grit and shot are mixed together to achieve a surface profile acceptable for coating.

Previously painted surfaces should be evaluated before coating using ASTM-D3359 Method A X-Cut Tape Test to ensure the existing coating is acceptable for overcoating.

Properly prepared concrete surfaces should have the texture of medium-grit sandpaper and should be tightly adherent, not powdery. Surface preparation of concrete is addressed in International Concrete and Repair Institute (ICRI) Technical Guideline No. 03732 Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays. This guideline outlines the proper procedure for surface preparation using various methods, and provides visual standards to determine if the preparation standards are achieved. The preferred method of concrete surface preparation, particularly for concrete floors, is shot blasting with vacuum equipment that recycles the spent shot.

New concrete should be cured for a minimum of 28 days at temperatures of above 70°F. Concrete must be as free of excessive moisture as possible, keeping in mind that the moisture content of concrete seldom falls below 15%. All grease, dirt, paint, oil, tar, glaze, laitance, efflorescence, loose mortar and cement should be removed prior to coating. Hollow areas, bug holes, honeycombs and voids should be opened and filled. Fins, form marks, protrusions and rough edges should be ground.

Surface preparation for concrete includes dry abrasive blasting, shot blasting, mechanical scarification, or suitable chemical cleaning. Detailed descriptions of the above preparation methods are described in ICRI Technical Guideline No. 03732. Remember that different methods of preparation impart different depths of surface profile, so it is important to select the method of surface preparation that matches the thickness of the coating system that will be applied.

Surface preparation methods, environmental conditions, and supporting documentation (surface profile readings, replica tape, visual documentation, etc.) should be recorded through inspection reports. Documentation of these results should be retained for future reference. Taking the time to properly prepare surfaces for painting is always a good call.

• Proper surface preparation can extend the life of an industrial paint job; improper preparation can shorten that life.
• Asking the right questions about the condition of the surface will help match the right paint to the job.
• There are different techniques for surface preparation, and different materials must be used depending on whether the surface is metal or concrete.