Clean valve cover contact surfaces?

Hi everyone, I had to replace the valve cover (because of KGE) and the intake manifold gasket on my car (1.9 TDI AVF).

I've tried a few things online, but how do I properly clean the contact surfaces for the valve cover? I've tried throttle body cleaner, cleaning gasoline, brake cleaner, etc., but there's still residue. I tried using a CSD/grinding disc on the intake manifold. It definitely gets clean, but of course, that leaves very fine grooves. Is this a problem with the intake manifold, or is it an option for the contact surface of the valve cover? I've attached a few pictures of the current condition.

I would be very happy to receive some tips.

Thanks!