I want to preface this by saying that I have absolutely no experience in painting or anything of the sort, so please do keep that in mind.
Recently I decided that I would like a Pull to Open sign to have around, kind of like a poster. After some debating with myself, I decided on the 1966 prop's main style, around Season 9 or 10.

I got a rectangular piece of wood (surprisingly close to the measurements, only half an inch short) and went at it with 3 colors of spraypaint.

Much too bright grey for the dark, chalkboard-like color. So today I went back over it with several types of liquid paints, blacks, blues and greys, using several layers and methods of application, rags, paintbrushes and rollers. I came out satisfied with the result, but it's in my mind not dirtied down enough.

I want to give it the scuffs and marks that characterized the real prop, but I've not got much of an idea as to how to do this. The methods specified that I've read through seem to be for brighter blue or white coloring and not black, where I feel that they would be non-visible. Would anyone be able to give some advice on this? It'd be heavily appreciated.
(The text will be applied last, once the background is done)

Do the text now and weather it all together, like a real one is.

I appreciate the suggestion and will definitely be using it, though my main worry is how to weather it in the first place aha.

If you have built up various layers of different shades of blacks/greys, then get some fine sand paper, 300 to 400 grit(?)  And gently work over it, concentrating on some random areas rather then others.

There also - getting a little bit of lighter greys/whites on a rag and rubbing here and there?

I would start with the fully assembled sign, using acrylic paint heavily watered down I would apply very thin washes building up dirt and grime with each layer which will take several days as the watered down paint dries. You could also experiment with weathering powders or dirt, and dry brushing. There are several YouTube videos on model weathering which might be useful. Adam Savage from Mythbusters fame in the USA has a YouTube channel Tested, which has some tutorials on paint application and weathering.