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In trampoline design, the most common springs have a total elongation of about 60% of rest lengh. E.g. a 10" spring can be stretched to 16" indefinitely without deformation. These springs are usually zinc plated, chrome plated. But rust is a problem. 10 year old springs often show significant rust and pitting.

In competition grade trampolines, they will use music wire springs (high carbon steel) These springs are typically designed with a lower spring constant, higher pre-tension, and can stretch to twice their rest length. Taht same 10 inch spring can stretch to 20 inches.

But high carbon steel rusts readily.

Given the degree of flexing, what coating systems exist to prevent the springs from rusting in an outdoor environment.

Spring stretch in a trampoline is not uniform. On rectangular tramps, the side springs in the middle third stretch most. On a round tramp, the springs in line with the fabric grid stretch the most.

My suspicion is that greater flex causes cracks in the protective layer, allowing water to get in.

If this is correct, then a thinner plating may well be more effective. I've seen "yellow zinc" plated springs. Don't know if that is a real benefit or a gimmick. But plating is an alternative.

Parts are also zinc dipped. This, I udnerstand produces a thicker coating, that for a static object provides better protection.

Paint is a common protective coating. Painting the inside of a coil spring would be tricking, especially as it needs to be kept under tension while painting and drying. I would expect taht this has to periodically redone, particularly at the points of contact at the frame and mat ring

Powder coating is an option. usually more durable than paint, but I don't know if powder coats are flexible.

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  • $\begingroup$ Silicone rubber certainly meets the elongation requirements. Might not be cost effective, and some points of contact with the springs may have to be bare metal, fully encased in the stuff - coating parts of the frame as well, and making spring replacement more difficult. $\endgroup$
    – Abel
    Commented Jun 11 at 15:36
  • $\begingroup$ Only put the springs out when the weather is clement. $\endgroup$
    – Solar Mike
    Commented Jun 11 at 17:01
  • $\begingroup$ Installing springs takes about 30 seconds per spring. Depending on design, there are 80-140 springs. $\endgroup$ Commented Jun 12 at 4:37

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You need a paint that is incompletely polymerized, so it acts like rubber. You can find such paint in hardware stores- it is used to dip-coat tool handles. Its cheapest composition is plasticized vinyl but silicone rubber can also be used to dip coat metal parts if the right solvent is mixed in with the rubber.

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  • $\begingroup$ Silicone caulk can be thinned with acetone. Not clear that that after the solvent evaporates that the material is still water proof. Could test by mixing a water soluble dye that is also acetone soluble. See how fast the dye moves out when the spring is in a circulating water bath. $\endgroup$ Commented Jun 12 at 4:42
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Tramp springs take a beating. You need decent thickness and a hard coating. Nickel seems to be the common choice. For high-end tramps, I guess you would spec an electroless nickel coating over electro polished springs.

Customers typically send parts for deburring and micro finish improvement prior to plating. We also electropolish music wire springs for cycle life improvement. https://www.ableelectropolishing.com/resources/frequently-asked-questions/can-able-electropolish-carbon-steel/

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Rubber substances are your likely solution.

Silicone caulk would be an accessible, but labor-intensive choice with exceptional elongation capabilities. It tends to be thick; rather than thinning it, a mold to squeeze out the excess followed by trimming might work (it has poor adhesion to some plastics so making a mold shouldn't be too bad). This may be a bit labor intensive and never be cost-effective, and may not be appropriate for the contact surfaces between frame and spring.

Vinyl for tool grips is thin but does not stretch as much as silicone once cured.

Urethane requires additional curing, but is also available as a powder coat at some facilities. It may need high quantity to be cost-effective.

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