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Maintenance Interval
Recently, several organizations have contacted us regarding the use of un-sealed-sprayed-metal coatings, and the use of paint over sprayed-metal coatings. So, we refer to a past issue of the Newsletter Metalizing For Corrosion Control.
From "Metalizing For Corrosion Control", January 1999
Metalized coatings may be bare-sprayed-metal, sprayed-metal-plus-sealer, or sprayed-metal-plus-sealer-plus-paint topcoats. Generally, the maintenance intervals of the three coating options are, ranked from the longest interval to the shortest interval, as follows:
Bare sprayed-metal (Assumes no subsequent maintenance until blasting cleaning and re-metalizing is needed.)
Sprayed-metal-plus-sealer (Periodic touch-up with sealer may extend the coating life indefinitely.)
Sprayed-metal-plus-sealer-plus-paint (Paint coating will require touch-up.)
The natural porosity and slight roughness of the metal coating tend to hold the paint better than even blast cleaned steel. The sealers or topcoats, will last longer when applied over sprayed-metal than when applied directly to steel. Even though the sealer deteriorates, the underlying metal coating continues to protect the steel.
The Association of Metal Sprayers, (Birmingham, England) information sheet 2 comments on the use of sealers and topcoats over sprayed metal coatings:
The metal sprayed coating is the primary protective scheme and, apart from restoring colour or surface smoothness there is no need to renew the sealant upon deterioration, since it has fulfilled its original function, and the coating is then relying upon the basic protection afforded by the sprayed metal deposit.
When maintenance of the sealer is required, AMS recommends cleaning down and brushing to remove products of deterioration, then over-coating with the same type of sealant as that originally used. Maintenance of the sprayed-metal-plus-paint systems may be more involved than this, and the Association makes a very important, but seldom recognized point:
Multi-coat paint systems are particularly vulnerable to breakdown on the underside of horizontal surfaces where moisture is retained. In the process of breakdown it has been shown that multi-coat paint systems act as a poultice, absorbing the moisture and maintaining permanently corrosive conditions, thus accelerating the attack on the metal beneath. Better performance is usually obtained with metal coatings which have no more than a sealant applied.
Painting over un-sealed metalizing is not recommended. Special circumstances such as extreme pH, chemical attack, or the need for additional abrasion resistance, may be reasons for painting over sealed-sprayed metal.
NOTE: In most cases the sprayed-metal-plus-sealer systems are the most economical option.
It is important to consult an experienced metalizing contractor when planning the application or maintenance of a thermal sprayed or metalized coating.
References
British Standard 5493, para. 6.6.6, p.6 and Notes to table 2,(d) p. 21.
AMS information sheet 2, "Sealing and Painting of Sprayed Aluminum and Zinc Coatings"
SSPC-CS 23.00 "Guide for Thermal Spray Metallic Coating Systems, June 1, 1991.
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