The Farr vertical grilles made by Plano for EMD E and F unit locomotives pretty much push the limit for chemical etching as far as the width of the slots is concerned - undercutting of the mask prevents even HO vertical grilles from achieving ultra-realistic spacing of the vertical slots, since there is so very little material separating the slots.
Perhaps the best method of producing one's own Farr vertical grilles for any locomotive model would be to affix a styrene strip to an advancing table which moves in thousandths of an inch increments and lightly scribe vertical lines with a #11 x-acto blade as close as possible to each other into the styrene. Then, paint the scribed styrene black and then remove the paint from the raised surfaces, leaving the black paint in the scribed furrows to represent the open slots of the grille. It would be maddening and tedious work, but you will achieve better results than with any pre-scribed materials that are readily available.
