Cheers for the comments
Clay is used to get rid of all grime road tar tree sap all sorts. to tell if your car needs doing once washed get a clear type sandwich bag and rub the paint work if it not smoth then clay will make feel like glass afterwards, great prep for a polish & wax
I would only recomend doing clay in the summer months or it can damage your paint work if the clay is hard.
this is tken from megs desc
Quik Clay Detailing System
Tree sap, overspray and industrial fallout are just a few of the contaminants that bond to your paint’s finish making it rough and lacklustre. Meguiar’s Quik Clay Detailing System will quickly and safely restore a smooth-as-glass finish. Polishes and waxes then apply and wipe off in half the time with dramatically improved results.
have a look on http://www.meguiars.co.uk/ under surface prep