To be honest, there wasn't much good information about this topic on the Internet. At a Website for Toyota pickup truck owners this topic was touched upon in a general discussion about whether it was OK to powder coat coil springs. Some posters said it was fine to blast springs, some said it wasn't and some admitted they just didn't know for sure. The bottom line was that some of the Toyota enthusiasts had done it and gotten away with it, but no one had any hard data or expert advice to go by.
Another discussion at www.shotpeener.com included a highly technical article called "Shot Peening Techniques for Springs" written by Kumar Balan, who is a Product Engineer with Wheelabrator Group Equipment's Process Design & Specification Conformance Department. After reading this article, we still didn't know for sure if blasting would ruin a coil spring, but we formed an opinion that if the heat created during a shot peening process didn't hurt springs, blasting probably wouldn't either. But we still weren't sure.
Since powder coaters blast and prime parts before powder coating them, we asked Jeff Maney at Statewide Custom Powder Coating of New London, Wis., what he thought. The Toyota Website had suggested that powder coating temperatures of 200 degrees were too low to affect the integrity of coil springs. Maney said he wasn't sure whether or not it was OK to powder coat springs.
Next, we decided to ask old car spring guru Mike Eaton the question. Eaton is the owner of Eaton Detroit Spring Company and a true expert on both coil springs and leaf springs, as well as complete suspension. Here is what he said:
"Great topic. I'll do a full blown article on this subject for our news letter. But in the meantime, blast away. After springs are formed and heat treated, we shot blast them with steel balls .060 in diameter to 'stress relief' them. A spring that is stress relieved will last up to 10 times longer than one that is not.
"We recondition many leaf springs for high end restorations and one of the very first things we do is shot peen them to clean the surfaces. This does absolutely no harm to the springs.
"We see problems with springs powder coated at temperatures that are too high. I addressed powder coating spring in three online articles on our Web site: www.eatonsprings.com/techquestions/powdercoatingandchromingsprings.htm and www.eatonsprings.com/powdercoating.htm and finally www.eatonsprings.com/techquestions/leaflinerpowdercoatubolttorque.htm.
"Spring steel begins to anneal — or soften — at 400 degrees, which is well above what any media blasting cabinet can develop, but within the range of unknowledgeable powered coaters."
Most restorers finish their coil springs black with a rust paint such as Eastwood's Rust Encapsulator or POR 14 as seen on this Corvette chassis.
Eaton said that his custom made leaf springs are shipped in bare metal , but notes that a film of metal dust is left on the surface from the shotpeening process. This film protects the surface from rust as long as the springs stay dry.
The leaf springs that Eaton manufactures in volume and all coil springs are dip coated in black paint. Eaton uses high-quality, high-temperature engine enamel. He warns that it is important to let paint dry completely between coats. He adds that springs can be powder coated as long as it's done at below 350 degrees. "Expect to spend over $100 for powder coat or $20 for paint," he says.
This restorer did a beautiful job finishing his 1969 Camaro Z/28 coil springs with Eastwood's Cast Iron Gray detailing paint.
This 1969 Camaro Z/28 chassis was finished to the Nth degree during a four-year-long every-nuts-and-blots restoration.
Since we do strictly mechanical work on old cars at our shop, we deal with lots of coil springs. Years ago we used to use paint stripper to take off flaky paint (a common form of cosmetic deterioration caused by the spring steel flexing under cheaper production line paints). After the paint flaked off, rust always formed. So next we would attempt to scrape or sand all the rust off the coils. This was not easy, since it is hard to get at certain surfaces of a coil spring. Finally, we would paint the spring black (also hard to do on hard-to-get-at surfaces.)