Cleaning and Painting of Structural Steel for Buildings
The CISC would like to address the concept being promoted by some paint contractors that more sophisticated coating systems for structural steel in buildings are better.
Steel buildings require NO paint when the steel is hidden behind drywall and suspended ceilings. The humidity in such buildings is below the threshold limit for corrosion of steel to occur (Clause 6.6.2 of CSA Standard S16-01 and S16-09). Buildings that have exceptionally high humidity such as swimming pools and water treatment plants are the exceptions and should be treated as exterior exposed steel.
Steel exposed to view is generally painted for appearance. A one-coat paint system, such as performance specification CISC/CPMA 1-73a, is sufficient for standard warehouse structures that will not be top coated. Since the building environment is controlled no corrosion occurs once the building is enclosed. These buildings perform adequately throughout the country. One-coat systems are referenced in Clause 28.8.6 of CSA Standard S16-01, 28.7.3.3 in S16-09.
Steel exposed to view that will be top coated for appearance requires a prime coat for adhesion. A fast dry primer, such as CISC/CPMA 2-75, is sufficient to provide the necessary base. To ensure that this system will perform for longer periods a greater degree of cleanliness is required by the specification. Consultants must ensure that the finish coats are compatible with the primer. Alkyds are acceptable but epoxies are not. Once the building is enclosed no corrosion occurs. The many discount department stores and food chains scattered throughout the country use these types of finishes very successfully.
Structural steel that is exposed to view and the elements on the exterior of buildings requires more thorough cleaning and finishing to ensure long-term performance. Higher degrees of cleanliness along with better quality multi-coat paints should be considered under these circumstances. Epoxy systems over compatible primers are usually most suitable. Urethanes should be used when wear is a consideration.
Tender documents should include the following information to ensure good quality coating systems:
a) identification of the members to be painted
b) a specification for the degree of cleanliness required to ensure performance such as SSPC Surface Preparation Standards
c) compatible primer, intermediate and finish paints and if applicable
i) the manufacturer’s product identification
ii) the average dry film thickness per coat
We recommend that you review the painting with your local fabricator to ensure that the most suitable system is chosen for your specific application. Fabricators are committed to providing the most economical structures for your specific application.
The industry continues to stress that paints should only be applied where absolutely necessary