Regular is our firmest clay and will be most useful on heavily contaminated vehicles, medium for moderately contaminated vehicles, and soft for less contaminated paintwork. Additionally, cold weather will cause the clay to be firmer, while warm weather will make it softer, so it is recommended that the firmness of the clay is selected with weather temperature in mind. (E.g. If the weather is warm and your car is only moderately contaminated, you should choose a Regular clay bar, as the warm weather will soften the bar.)