There are all sorts of acrylics on the market that may be applied with a brush, leaving NO brush marks (use a decent - not fancy -brush). I've done it many times. I painted all
the silver (not the "stainless") on a Lionel Empire State Hudson, drivers and all, to a gray to reflect its later service colors. Excellent finish (I've posted photos here, some time back). I used Model Color paint by Vallejo, from Hobby Lobby. They offer "steel" colors, and the like. Some might do. Check their website, of course.
I applied the paint straight from the small bottle - 1 coat did it. Next day I touched up the bits I missed. Blended in completely. May be thinned, of course.
$3.29/17 ml. bottle; way less than the cost of the loco.