If your pressure washer is a low psi pressure it may not shift the gunk under the base, I have a few I use 1600 psi ,1800 psi ,2400 psi and
the rotating nozzles help a lot supposedly increasing cleaning output by 50 percent (turbo nozzles).
A standard wire brush works well under the base ,use it gently and you won't scratch gouges into the alloy.
I use a scraper to start with sometimes.
