I'm building a book shelf using 1.5" angle iron made from hot rolled steel as the frame. It's my understanding that this steel will rust even if left indoors. While I like the raw, industrial look of the mill scale, I would like to prevent the frame from rusting overtime without giving the metal a glossy appearance. Looking at different ways to seal metal, it seems like there are two options; hot methods and cold methods.
Hot methods include rubbing/painting on different oils then heating the metal to the point that the oil polymerizes. Here is a great example, https://www.youtube.com/watch?v=xIG6tQI73_Y. This is beyond my abilities and shop setup, beside it seems a bit egregious to use that much energy for my situation. So, I'm interested in cold methods (i.e. no heating involved).
The best method I've come across so far is apply multiple coats Penetrol (a flow control additive to help paint dry more evenly) which builds up a glossy layer of protection, then go over this with a single layer of oil-based matte polyurethane. I don't mind doing this, but I wonder what other methods could work? Would I be able to just rub the metal down with boiled lineseed oil and give it a few days to cure?