I feel that the following sentence is not correct:
He wants to paint the walls of his flat, for what he needs the best tools.
So 'what' refers to the whole first clause in the sentence.
Okay, I could say something like:
He wants to paint the walls of his flat, so he needs the best tools.
or I could use therefore, thus, etc. But is there a solution which is somehow nearer to the first sentence in meaning? Can I use a preposition before 'what' in a relative clause, similarly to the case of 'which'?