If by oxide you mean ‘white’ (zinc or titanium oxide, for examples), I would begin with either red or orange chalk, or a wash of the same colours of acrylic paint, and give them one or two light coats, with dull-coat in between each. Each application of whatever, Dull-coat or wash, should be allowed to dry. YOU MUST BE PATIENT!
Once you have done, say, two light coats of the colours, try two more of an India Ink and alcohol wa***o get that streaked soot that RVs pick up over the year, 'cuz really everything gets it due to industrial fallout, pollen, automobile traffic, etc.
My first attempt was an unmitigated flop…no two ways about it. I built the Walthers Concrete Coaling Tower, the monster, with the intent of perching it on the side of my rather huge mountain as a coal mine. I used acrylic paints in what I thought was a ‘wash’, but it was way too strong. It is acceptable now but looks like a heavy hand had at it. I then used diluted acrylic on the Walthers Valley Cement complex, and it had promise that is still promise. It is getting better.
I then dove in with my BLI J-1e (yeooowwww!), but didn’t do any harm because I was timid. I can undo some of the not so realistic parts where the acrylic pooled and dried. I won’t use acrylic on that loco again, but the next item, the bridge on my turntable, was a spectacular success…with acrylics!
The last attempt was with chalk, on my P2K 0-6-0, and it took about five light applications of dusted chalk, then Dull-coat, until it began to look quite good, if I may be the judge.
Sorry, I’ve gone on, but I endorse what the others have said. Learn, enjoy, don’t fret when you err, and do by all means expose yourself to this character-building process. It is not for the faint of heart, true, but you will eventually be happy that you went ‘there’.