That last question brings up an excellent point. I've found that granular
consistency is very important in a rub. Why? It's all about gravity and
suspension.
You don't want a situation where your various grain sizes (weights, really)
are all over the place. Gravity will cause the larger grains (typically salt
and/or sugar) to settle to the bottom, while the smaller grains (the
seasonings) tend to settle on top. When this happens, guess what comes
out of the shaker first. Yup, it's the potent (and expensive) seasonings.
The salt and sugar stay at the bottom.
What you want are the most similar grain sizes that you can get. This will
ensure that everything stays more equally in suspension when gravity
takes over.
In short, when mixing a rub you want to avoid a gravel and sand dilemma.
Imagine a bucket of sand and gravel that is shaken. The gravel goes
straight to the bottom and the sand sits on top.
John