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What do you use instead of a sponge ?
I use a sponge, but a seperate one for the paintwork, and make sure there is no debris in it.
The best thing to use is a lambswool washmit as it works the same wasy as microfibre cloths do
i.e. the dirt and debris from the paintwork is drawn into the fibres of the washmit which keeps it away from the paintwork so the possibilty of scratching the surface is greatly reduced.
A sponge however will always trap the dirt between itself and your expensive paintwork so that as you draw the sponge across the paintwork you will always drag the dirt and grit across the surface,
end result = lots of scratches which stand out like a sore thumb in the sunlight and therefore spoiling the whole look of the car
Two Bucket Method (for those that do not know it) Use one bucket for your wash solution (use a good quality shampoo)
Then use a second bucket filled with clean water (preferbly with a gritguard in the bottom) for rinsing the washmit in between dips in the bucket containing the wash solution, that way you decrease/virtually eliminate contaminating the wash solution with the debris you have just taken off the paintwork.
There is a little bit more to it than this but it'll give you the general idea.
HTH