Coating Long Tubes

Question:

I'm a manufacturing engineer who was recently hired on to look at paint problems. I've been involved in the powder coating industry for about five years, most of which dealt with painting product for indoor use. My new company manufactures and paints outdoor tubing products ranging from 1– 16 inches in diameter, 5–60 ft in length and 3–11 gage in thickness. We paint over aluminum, mild steel and galvanized using TGIC powder or sometimes liquid. The current painting process in place is as follows: Parts are hung and run through a steel shot blaster, blown off with an air wand, painted and cured. As you can guess, we have an adhesion problem with some of our product in the field but no color problems. We are looking at extending the warranty on these products by possibly installing a wash system but we really have no baseline information that says how long our paint currently lasts. What would be the best test to determine this? I've heard salt spray would be a good test but how does salt spray hours translate to years?

Also, if we were to install a wash system, what type of system would you recommend for washing such long parts of varying substrates? E. F.

Answer:

I'll answer your last question first. Salt spray exposure is an accelerated test. Because of its varying effects on different coating systems, it is best used as a comparative test of their corrosion resistance. After 50 years in the organic coatings business, the best translation I can give you is 300 hours salt spray exposure is equivalent to 300 hours salt spray exposure.
If your production rate requires it, you can install what you call a “wash system,” a conveyorized power spray pretreatment system. For outdoor products, you should use a zinc phosphate pretreatment formulated to handle steel, aluminum and galvanized steel. This will require at least five stages and perhaps as many as seven or more. A five-stage system would be clean, rinse, phosphate, rinse and final rinse (seal). Optional stages would be pre-cleaner, conditioner and DI rinses. On the other hand, if your production rate does not require a conveyorized system, you could use a multistage high-pressure hand-power washer to pretreat the tubes. If you use an immersion system, the tanks must be able to hold the longest part.