Strictly speaking, plastic can’t rust. By definition, rust is iron oxide, the combination of iron and oxygen. But plastic can combine with oxygen and oxidize. It’s not rust by stretch definition, but it’s the same concept. Here’s why it can happen, and what you can do about it.
Other metals can oxidize as well. Oxidation is a common reason for chips on credit cards failing. Scrape off the oxide, and the chip starts working. Oxidation also was what caused Nintendo cartridges to malfunction, which was why people blew into them. But treating the oxidation is much more effective in the long term.
Plastic can oxidize as well, and when it does, it usually discolors, often turning yellow. It generally happens over a period of years or decades, and lighter colored plastics are more prone to it than dark colored plastics. We don’t notice it as much as we used to since electronics are usually made of black plastic now, and have been for more than a decade. But when they were off white or gray, they did discolor with age, usually due to exposure to certain types of light and heat.
It is possible for actual rust to end up on plastic as well, if a piece of metal is right up against it and rusts over time. Some of the rust ends up on the plastic. The plastic didn’t rust, the rust stained the plastic. You can usually clean that off with a mild abrasive, such as baking powder or a magic eraser, or with a mild acid like vinegar.
But what about treating discolored plastics?
Retrobright is controversial, but it generally reverses the yellowing in aged plastics. We’ve been doing it for years without understanding how it works, but the current understanding is that we are bleaching the plastic back to its original color. The most popular method is to put the plastic in hydrogen peroxide and set it out in the sun for a few hours. The amount of time depends on the formulation of the plastic and how yellow it was, but generally a few hours in the sun will reverse the aging.
It is possible to just set the plastic out in the sun and reverse the yellowing. This is usually slower than using peroxide, and it doesn’t always work, but some plastics can de-yellow in as little as two or three hours just sitting in the sun. Others need days.
The parts will discolor again in time. How long it takes is controversial, partly because it depends on too many unpredictable factors. I find items stored in my basement, we’re at the temperature is always relatively low and I have LED lights, I don’t have a problem with them yellowing again. Fluorescent lights definitely did cause some of my old computers to yellow, but now that I’ve replaced those bulbs, I haven’t had a problem. To a certain extent, exposure to LED light seems to have helped. And spraying 303 Protectant on it also slows down re-yellowing by protecting the plastic from UV.
I’m generally willing to tolerate a bit of yellowing, so I don’t worry too much about it. If a piece is very discolored, I set it out in the sun to brighten it up a bit. Keeping them in a room with LED lighting and out of extreme temperatures seems to help. At least it’s working for me.