what are the common mistakes people make when detailing their cars?
I agree with everything Chad mentioned, and will just add my own little spin.
I will answer this question in two parts. The first part pertaining to people who only wash and maybe wax their car, but thats it, and the other for enthusiest who get into machine polishing and so on.
As for the harry homeowner that just wants to keep their vehicles clean I would say using improper tools and techniques are the most common mistakes. Believe it or not there are still people out there who will wash and dry cars with beach towels, wash a car with nothing but paper towels and windex, and even use all sorts of brushes on their paint. This leads to nothing but swirls, scratches, and scuffs. It's pretty sad since to just wash a car properly doesn't take much money or time, but a little bit of knowledge. Someone could keep their car relatively swirl free with just a couple grits guards, a couple buckets, a good sponge and microfiber towel for drying.
The second most common mistake I see (my neighbor does this constantly and then wonders why his black Volvo looks almost gray now) is that people with drop their drying towel or wax applicator on the ground and then just go back to using it like nothing happened. If a towel, applicator, or sponge of mine touches the ground it either gets knocked down to cleaning wheels only or tossed in the trash. This is the only way to know for sure that you didn't pick up some sort of debris that is going to mar your paint.
For the enthusiest/pro
I'm with Chad on this one. Impropper polishing techniques are the most common mistakes. I break this down into several categories. First is arm speed. People think that since they have a machine they can just whip it around on the paint as fast as they want. With a DA machine this won't hurt anything, but you won't be remeoving any swirls either. However with a rotary you will be putting in more swirls and holograms, and the vehicle will most likely look worse afterr than it did before.
Second is using the machine on too high of a speed. Lots of people think a higher speed will remove defects fast, and in a way it will, but most times it will also lead to burning through the clearcoat or leaving behind holograms in the paint.