im a newb to clay bars..i used to be just a wash/wax guy...now im experimenting..and liking my results! anyways.....i was reading the box...and it said that you should replace the clay after 3 normal sized cars were done w/ it. do they really only last that long? or is it just a common statement on someones product to get you to buy more? also...are you supposed to knead it everytime you start claying a new panel? or just keep going on the same spot of clay, and just re-knead it after you pull it out for another car? also, can you just buy the clay bar..not the kit? i bought the kit at autozone for 14 bucks.....but id rather just keep on buying the clay bar if i do need new ones.... other then that..i feel pretty confident using clay even though i just did my first car with it....but is there anything else i should know about? tips/tricks?