Typically, the "painted" grates aren't paint, they are a baked on ceramic coating, no way to really replicate this with a rattle can or brush at home. That fire under them could be 800 - 1000F, maybe something exists to recoat at home, but probably more expensive and time consuming than just replacing the grates.
I would defiantly skip the steel wool, could end up in your food. In fact, if the rust is bad, or you want to bring it back to square zero, get a cheap plastic bin that is just a bit larger than the gate and a grab a quart or gallon of muriatic acid. Knock the big burnt bits off the grates and then stack them in the tub and cover with acid so they are submerged by about 1/2". If it doesn't happen right in front of your eyes, come in back in an hour and they should be sparkly clean and rust free.
The moment they come out of the acid, they will start to rust, fairly quickly I might add. So, do this right by your grill, and have a can of cooking spray oil handy. Light the grill on a low-medium fire, pull a grate out, wash it off with a garden hose, dry quickly with a towel, then spray both sides and put it on the grill to fully dry with heat. As others have said, repeat the oil a few times like it was a cast iron pan
If you are going to replace the grates, as an option to stainless which actually has a horrible heat transfer coefficient, and unless you get decent non-ferritic SS grates where the quality of the crap recycled chinesium SS is actually monitored, so basically virgin 308 or 316, they are going to rust on you too. Check out https://www.grillgrate.com/ It's a unique design grate made from extruded aluminum, it prevents flare ups, evens the heat out, leaves the most incredible grill marks I think I've ever seen (not that grill marks mean shit 95% of the time, but they do look cool) and, they never rust (they may corrode if you are very near the ocean). When you look at them on the website, it'll make more sense.
