I will only work for Time and Materials.
The truth is that we do much more than just spray paint on vehicles. Actually, spraying the paint is the part I enjoy doing.
The restoration shops in my area charge on average about $90 per hour. That is how I got my 1968 Plymouth GTX for such a low price. The owner had taken it to a restoration shop nearby, they disassembled the entire car, then hit him with their estimate. He was shocked at the cost and decided to take the car home - he got all the parts in boxes, realized he didn't have the ability to restore it, so put it up for sale.
Too often, you don't know what you are getting into when someone brings a car to you, that from 20 feet looks pretty decent. I have found cracked frames, rusted out floors, hack repairs, etc. Hence, Time and Materials is the best method for both you and the car owner. He gets an itemized bill listing the materials, labor and work performed each month. He pays the bill, you keep working or he pays the bill and takes his car to someone else.
Personally, I only work about 40-60 hours per month on people's cars since it was never my intention to be in business doing this. So the customers are told this up front and sign a contract before I start.
The first job I did, I quoted it at $4800 and charging $10 an hour for labor because I didn't feel that I had enough experience to charge more. On that first job I began keeping track of everything on a spreadsheet and that was when my eyes were opened. I had my estimated time to do a certain repair and then a column for my actual time. Nearly everything was in the red, sometimes under estimated by 6 - 10 hours.
After that job, I said no more estimates. I currently charge $35 per hour for my labor.