There are three types of sprayed metal coating systems. These options are bare metallic coating, sprayed-metal-plus-sealer or sprayed-metal-plus-sealer-plus-topcoat.
The bare metallic coating simply corrodes in preference to the underlying steel. This may be the longest maintenance interval, but once the coating is consumed, the steel has to be blast cleaned and re-metalized.
The sealed-sprayed-metallic coating is often the most economical and is the preferred system of the three metalized coating options.
There are two schools of thought on the use of sealers with sprayed-metal coatings. The first is to apply the sealer after metalizing and allow it to preserve the weight of galvanic metallic coating. No subsequent maintenance is planned for decades. Eventually the sealer fails, but in the meantime less metal is consumed by oxidation than if the metallic coating is left bare. Once the sealer is gone, the metallic coating corrodes slowly over time protecting the steel. The sealer extends the life of the coating for the relatively low cost to apply the sealer.
The second school of thought is to periodically apply more sealer as the sealer and eventually the metallic coating deteriorate. This minor periodic maintenance, re-application of sealer about every ten years or so, can preserve the metallic coating indefinitely, and the need to blast clean to bare steel may be eliminated.
The addition of one or more layers of paint "topcoats" is not necessary for corrosion control. It is usually a matter of aesthetics or the need for additional abrasion resistance. The topcoated system also has the shortest maintenance interval, since the paint will require more frequent maintenance than either of the other options. The use of paint directly over an unsealed sprayed-metal coating should be avoided.
Why insist on sealing?
Consider the successful use of sealed metalizing in Canada, the UK and the U.S. Navy. Salt attack on metalized bridges in Canada caused them to conclude that sealing is necessary to resist chloride attack. Decades of experience in the UK results in their sealing but not necessarily painting over sprayed-metal. All metalized coatings used by the U.S. Navy are sealed.
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