Our white car was at a local shop that shall go nameless--- KARL'S CUSTOM VW --
and was oversprayed black, badly.
It was 'clay barred' by a detail shop, -- it came out tip-top.
A fella from Moe Allen's body shop on 16th Street demonstrated for me how to do it.
He had a spray bottle in one hand and the clay bar in the other.
He would spray, what he said was tap water, from the spray bottle and then lightly buff with the clay bar.
He would, after a few strokes, take his thumb and 'roll' the surface of the clay bar to expose fresh clay.
He said the trick was to always 'wet sand' with the clay and to 'roll' the clay regularly as to not abraid the paint with particulate that the clay will pick up, probably this was more of an issue due to the black overspray.
John
P.S. Any paint shop in town will have clay bars, you already have water, don't you?