I've just clayed my bonnet yesterday and boy do my arms hurt today it was the only bit left to do and hadn't been touched in 10 years so it was really bad with contaminates. Still after a wash, clay, Meg's ultimate compound, polish and wax oh and 6 hours later it's looking very shinny (a bit ott maybe lol)
Be carefully which clay bars you buy and how often you clay your car as some clay bars can be very aggressive, personally I use Sonus SFX ultra fine which is very mild.