I do it all the time from spring to fall. Simple green, a stiff brush, a textured rag, and the garden hose. I use the mist spray on the hose so I can control the spray because I never cover up the electrical parts. I also use simple green on my wheels, diluted down by 50% or more. No problems so far.

I spent about 3 hours just on the under side of the hood because it was so badly caked with black crud (oil/water/dirt sprayed from the belts had accumulated over 14 years). It was back to white and looked great when I finally finished.
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Michael -- 1987 244 GT | Volvo 240 Links