Before a final decision is made, the overcoat system should be tested for adhesion to the existing paint system. After ensuring the existing system is dry and clean, it is best to use ASTM D5064, Standard Practice for Conducting a Patch Test to Assess Coating Compatibility, as a guide in conducting adhesion patch tests. ASTM D3359 Method A is best used to evaluate adhesion of the new system to the old. The overcoat system should not wrinkle, lift, or show any other adverse response to the existing system.
Another method for determining if an aged coating system is sound enough to accept overcoating is the ‘recoatability test’ recommended in CSSBI Fact Sheet 4:
- Clean and otherwise prepare several small test areas representative of the entire surface to be repainted.
- Apply a coat of the desired repaint per the manufacturer’s instructions. Allow each test area to dry per the manufacturer’s instructions.
- After drying using about 200 mm [8 in] of gray “duct” tape for each area to be tested, firmly smooth about 75 to 125 mm [3 to 5 in.] of the tape onto the repainted areas. Rapidly pull off the tape, attempting to remove the recently applied air-dried coating.
- Unsatisfactory adhesion/compatibility is indicated if the new coating is removed with the tape.
- If an unsatisfactory test occurs, it may be necessary to conduct a different or additional cleaning procedure, apply an intercoat adhesion primer, or select a different type or different manufacturer’s repaint coating.
- Repeat the “recoatability test” until satisfactory results are obtained.
Conclusion
Whether or not overcoating a metal building is a feasible alternative to complete removal and repainting depends on factors such as the condition of the existing paint system, the amount of corrosion present, and the number of old coatings and their adhesion to the substrate/between coats. Surface preparation of existing coatings and geographic exposure conditions are other major considerations in determining whether overcoating is practical or if complete removal is required. Environmental service conditions are especially important in Canada, where freeze/thaw cycling during winter months causes significant stress on overcoat systems. With the availability of new high-performance coatings products for overcoating metal building components, specifiers can achieve enhanced protection against corrosion, extended colour and gloss retention, and energy savings.
Gil Giles has worked in the paint and coatings industry for more than a decade in various functions, such as coatings sales, specification writing, and inspection. Giles currently serves as an independent representative with Tnemec Company Inc., assisting architects, engineers, and applicators with protective coatings and linings in New Brunswick, Nova Scotia, Prince Edward Island, and Newfoundland. He is a NACE III coating inspector, is certified by the Master Painters Institute (MPI), and holds a bachelor’s degree in commerce from Dalhousie University. Giles can be reached via e-mail by contacting ipaint@nb.sympatico.ca.
