Often I see "ing" endings on nouns, such as cowling instead of cowl, tubing instead of tube, and casing instead of case. While verbing nouns is not unusual as in "housing," it seems wrong to call a tube tubing, or a cowl a cowling, etc. What am I missing WRT grammar rules?

Less to do with grammaticality, and more to do with semantics. For me, tube and tubing are different things. As the Wiktionary entry puts it: "Used to form nouns denoting materials or systems of objects which are used or employed in an action, or considered collectively."

Or put another way... a collection (or system) of tubes is tubing.