I used titanium white for the fading, since that was what I had at hand, although Jeremy says that zinc white is better since the titanium tends to give the car a blueish hue. And as you can see below he was perfectly correct (I have since bought some zinc white to use on the next car).
I then used some acrylics and weathering powders to add grime and rust effects. I am not all happy with the result, even if it looks better IRL than on the photos. Judge for yourself.
I added ACI labels, since that would be appropriate for my era (late 1970s). For that purpose I had bought myself a decal set (Microscales 48-650). I thought it would be cool if at least part of the identification number coded on the label would match the actual car number, so I did some digging into ACI labels.
The first part of the bar sequence is an owner identification. I managed to find a list of such identification codes (http://eaneubauer.ipower.com/aci.pdf), prepared by Eric A. Neubauer. The list showed that a BN owned car should be coded starting with 0076. The list also shows how numbers are translated into bars. Prepared with this information I managed to actually find some plates on the decal set that started with 0076 for BN. But what I also found at that point was that the ACI plates on the decal set are all wrong! An ACI plate shall have 13 bars, including start, stop and check bars, but the Microscales plates have only 12. No big deal really (who would notice or even care?), but when having managed to gather all this info I thought it would be even cooler to make my own labels, actually matching both the owner road and the car number in question. The result is what you see in the picture below. Believe it or not, but the bars really translate to BN 450661.