Cedarwoodron
I have had fairly good results painting the grills of BB F7's in Santa Fe freight colors. I first paint the grill and background stripe yellow to get the yellow background color. I then mask around the grill and carefully airbrush aluminum paint across the grills at a shallow angle (top-to-bottom and bottom-to-top). This colors the raised portions of the molded on grills aluminum while leaving most of the yellow color in the recessed portions of the grill. While not the same as a separate etched metal grill, it does at least imply that you are looking through a grill at the paint behind the grill.
Hornblower
Here's Detail Associates' etched stainless steel Farr intake grills on a Model Power E-unit...
They also offer similar ones to fit F-units, and versions with horizontal openings, too.
Wayne
Are you by any chance describing the Farr grilles? These are stampings in stainless steel, and the weathering effects you would use should be ones appropriate to that metal.
In my opinion, this is one of the best diesel details to provide as an etched or laser-cut piece in appropriate metal, with the carbody carved or cut away underneath with correct details and framing. It can be difficult to get the right metallic finish on the grills while keeping any trace of silver out of the paint or ink wash in the 'interstices' even if you don't care to detail what is behind there.
I tend to weather everything with powdered charcoal after the decals are sealed in with flat finish, so I'd probably go with aluminum paint and then fairly heavy weathering.
BTW the Erie Mining units were F9s not F7s, so there will be some differences...plus the Athearn F7s have the boiler vents on the rear that was only really used for passenger units.
That being said, a friend of mine did create a set of Erie Mining F's using Athearn bodies that turned out quite nicely.
On Athearn BB A &B units, there is a molded-on vent feature on each A and B model. I'm repurposing an AB set for Erie Mining Company and I notice that the real ones had vents that appeared to be exterior screens fitted to the upper body, as some body color (yellow) is visible underneath in photos I obtained online. As this detail is molded on, I would rather just paint the vent screens than try to find a detail part and increase my cost basis. I've already ordered the Walthers F7 detail kit with windows and grabs as well as the Microscale decal sheet for Erie Mining. In looking at the photos it appears that the vent screens are a "dirty aluminum" color, so...any suggestions on painting this detail? Aluminum with an ink wash?