Second: all the tricks of the trade in regard to "cheating" customers is for hustlers and cheaters and NOT established businesses. At the end of the project the job should come out looking professionally painted as specified in the contract. A selected color that takes multiple coats that was not calculated by the contractor should cost more money. It's not the fault of the painter.
My husband has been a professional painter over 30 years. He prides himself in his high level of work ethic and customer satisfaction. He stays up to date on techniques and finishes. He gives Very detail and accurate appraisals with contracts. At an alarming rate, as he starts to finish the last day or the day before, the client starts nit picking and being disrespectful towards his work when every day prior to that, they were very pleased, as he request ongoing satisfaction throughout the job. Then they don't to want pay remaining balance, bicker about final cost, or stop payment. He has a crew he has to pay whether the customer does or doesn't honor the contract as well as our own household expenses. Wasted time ,labor, money and effort lost. Now how do we fix this? Remind yourself and clients that a contract is based on honor.