The main reason I painted the one so far, was because I could see that there WAS some drying and cracking of the sidewalls on them, and I wanted to prevent any rust. The choice of an interior/exterior latex paint with water clean-up was to avoid environmental issues.
As you look around, you see thousands on thousands of homes painted with it, and in the old days LEAD was a big issue with house paint. Children were eating paint chips (because they don't know any better) so a huge pogrom was mounted at a point to make these kinds of paints safe.
While I have no real idea what the chemical makeup is of them, the notion that they should be safe around foolish kids is a leading factor.
I could also have used metal paint - a sort of 'Rustoleum' clone that I have a gallon of - but I know from experience that the stuff I have takes a decade to completely dry. I have trellis grids that I painted five or ten years ago that still leave green marks on my fingers when I handle them, so that wouldn't be good. In theory Rust preventive paints use fish oil to penetrate and protect metal, but looking at the labels there is a complete witches brew of petro chem stuff in those...
Another thing I like is that the drainage of the containers is right through the center, but yet the sidewall on the bottom forms a kind of a circular well, or reservoir, that can help keep some of the water inside during a dry spell.
They are robust enough that using a weed-trimmer around them doesn't damage them. The string just bounces off
Elsewhere on the web I have seen images of tires whoes sides have been artfully cut to resemble flower petals when turned inside out. They are stackable - as witness the one next to my "MALABAR TOWER" that I have set a Cumin plant in the middle of.
I also sometimes use uncut tires as supporting bases to center narrow planters that could be knocked over. Again - they form a buffer that will protect against trimmer damage when policing up the yard.
Because the potted banana on the right is in a container with such a narrow base, I used a tire I had no other plans for to straighten it and keep it from falling over
Another view of how the Red Stemmed malabar is doing
In the forground I have a recent addition that I found I had cut just about right for the tire to grip the trellis/cage I planned to set in it. It will be painted sometime soon, I hope.
Beyond it however - you see another narrow based planter with a tobacco plant in it standing on very uneven ground. IT TOO is set in a rubber tire to hold it straight up.
~The tire also allows me to turn the planter easily, and aim the trunk of the plant straight up if it should grow towards the sun which here comes from somewhat to the south
I may eventually have all these planters either standing on gravel or concrete depending on how my plans for the driveway go. But the soil in them can be moved right along with them, and the planters themselves turned on their sides and 'Rolled down the road' so to speak...
By clicking on these pictures, you can see them better. There is no "X" that appears to close them, you just click on the image a second time and they go back to normal.