Safest way to ensure that everything is fair is to get it ALL in writing , signed by both parties. Specify each item that needs repair. Also, BUY the paint YOURSELF. That way, there is no incentive to water it down, and you KNOW that you are getting the grade/quality you actually purchased. Don't be penny wise and pound foolish; if you are paying to hire a painter, buy the best paint that you can afford, to ensure maximum life of this home improvement.
I do not mean to come off with an edge, however, this is an article for people hiring low bid, uninsured, one man band painters out to "hustle" a buck. Hire reputable, established companies or hire someone you know or have a strong referral from. If you are arguing over watered down paint, ticky, tacky deposits, buying your own supplies, you have hired the wrong guy.
To win work, contractors generally have to bid against other suppliers. To make an accurate bid, they estimate the time and materials required to complete the job. They measure the area to be painted and discuss the type of finish the customer requires. They assess the quality of the surfaces to work out how much remedial work they must carry out before painting. They might have to remove old paint or wall coverings, or repair cracks and other damage. They also calculate the cost of any essential equipment, such as scaffolding for exterior walls.