> I agree, they should have changed the verb "aufwenden" to "aufwänden", to make it completely consistent But then you also have to change "wenden" -> "wänden"

> I agree, they should have changed the verb "aufwenden" to "aufwänden", to make it completely consistent But then you also have to change "wenden" -> "wänden" and "abwenden" zu "abwänden". If we conjugate this, we get "ich wände mich ab" (1st person singular). But unluckily "ich wände mich ab" is already the Conjunctive II of "abwenden" (also 1st person singular). So the consequences of changing the the spelling of "aufwenden" are much deeper than it looks on the first view. My answer is rather to find an abstract, but completely general rule from which words adjectives are to be derived: the noun or the verb.