Least-Expensive Finish System

Q. I am an engineer for a consulting firm. One of our customers has a question about a paint finish system for steel. He is inquiring about the least expensive, most easily applied and most corrosion resistant finish system for steel parts. He needs to have good protection against the effects of seaside atmospheres. The parts will be exposed to temperatures ranging from -50ºF to 150ºF. Appearance is not an issue here, just cost and lasting corrosion protection. Since my knowledge of paint coatings is rather limited, I am asking for your help. M.S.

A. There are several paint finish systems that will provide corrosion protection to steel parts at the temperature range you mentioned. The finish systems listed below are applied by spraying but can also be applied by brushing. Since brushing is more labor intensive, it is probably more expensive than spraying.

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The following candidate finish systems are listed in descending order of cost and protection:

1. Zinc phosphate + epoxy primer + epoxy topcoat
2. Iron phosphate + epoxy primer + epoxy topcoat
3. Shot blast + wash primer + epoxy primer + epoxy topcoat

Although the aforementioned coatings are listed in descending order by degree of corrosion protection, there may not be too much difference in their costs. Zinc phosphate requires a minimum of five stages. Iron phosphate requires a minimum of three stages. Shot blasting is labor intensive, and the finish system requires an extra coat of paint.

Another approach would be the use of an inorganic zinc silicate primer applied to shot blasted steel as a replacement for the phosphate pretreatments. There are other enamels, such as certain polyurethanes and silicones that can also be used as topcoats, but they may be more expensive.