No Need to Panic: A Tale of Rust Streaking

I got a call the other day from a Kaser client with a question worth blogging about. The client is a retirement community, for whom we had powder coated quite a bit of railing five years prior, all in white.

If that raises red flags, it should: I’ve blogged before about how white powder coating is a difficult color to maintain, particularly on outdoor parts, due to the near-inevitability of rust streaking. But the client really wanted white, so we had blasted the railing very thoroughly, put it through rigorous pretreatment, and powder coated it with a primer and a top coat. Five years later, the coating was holding up well –for the most part, everything still looked really good.

However, one of the retirement community residents had “noticed a little bit of rusting”. Our client’s question to me was whether there was anything the maintenance team could do to stave off the rusting and prolong the life of the coating?

Often, if a part starts to rust within five years of powder coating, it’s because of a powder coating surface issue. Perhaps the coating was applied too thin, and the substrate wasn’t fully protected. Or perhaps the coating took a blow and chipped off in one area, allowing water direct access to the substrate. Left exposed to water too long, metal substrates will start to rust (if they’re steel) or corrode (if they’re aluminum).

We all know what rust looks like. Left untended, rust can eat its way through metal like a cancer, causing major structural failures. Rust can also devastate powder coating, causing it to flake off in chunks and leaving the metal exposed to the elements. If a powder coated part starts to rust, there’s no choice but to sand down the rusty pieces thoroughly (or blast the whole part, if it’s too far gone) and start the powder coating process from scratch.

I asked our client to describe what the “rusting” looked like, so I could try to determine if the issue was rusting of the substrate or rust streaking.

Rust streaking can – and does – occur even on parts that have been thoroughly blasted and very well coated. Water is persistent: if there’s a tiny crevice in the part, a seam that wasn’t sealed, or a weld that wasn’t fully welded, water will eventually find its way in. Even if the powder coating appears completely flawless to the naked eye, and even if the part is as thoroughly protected as it can humanly be, the tiniest pinhole will still allow water to come in contact with the substrate. The metal turns the water reddish, causing it to leave an orange, rusty-looking stain as it runs back down over the powder coating.

The part itself is not in any immediate danger, nor is the powder coating. Rust streaking is basically inevitable – it happens on virtually every powder coated, outdoor part, it just happens to be more alarmingly visible against white powder coating.

Caught early, rust streaking could be simply wiped off with a rag. The longer the orange water sits against the white powder coating, the more likely it is to cause a permanent discoloration. I advised her to use caulk, silicone, or a similar sealing agent on the spot she suspected the water was getting in. This will help prevent future streaking, and prolong the life of the coating.

In summary, rust is a serious, physical problem. Rust streaking, however, is a visual problem – it’s a cosmetic issue that doesn’t necessarily spell doom for your part, or your coating. The simplest fix is to choose a color other than white, that will help hide it better. If white is your choice, a rag and some caulk should help solve the problem. 




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