We had a bad experience with an interior painter years ago, with the crux of the problem being him overcharging us at the end for "extra work" he didn't anticipate. One thing I'd strongly recommend is making sure it's in the contract that any additional work or growth work is estimated and communicated to the owner as soon as it is identified, otherwise the owner is not liable to pay it at the end.
Inspect the surface area for holes, cracks or other problems and apply acrylic caulk to fill them in. Use a putty knife to smooth out the caulk and remove any excess to prevent bumps. For particularly thin or narrow crevices, you may have to use your finger to smooth out the caulk. Caulk contracts and shrinks, so apply a second layer once it's dried.
I turn away any job when the client refuses to pay anything up front. It sends a red flag. I also charge a scheduling fee which is non-refundable. I get 33 percent when I show up and begin work. Another percentage halfway through, and the balance upon completion after client is satisfied. There needs to be skin in the game for both parties as a measure of good faith. If you are dealing with a reputable company (did your due diligence, right?) why wouldn't you want to pay something as work progresses? We do this not only because we love to paint but we require cash flow to stay in business. There is not always 'money in the bank' as you suggest. It's tough these days. The suggestion buy 'Kim' 'Never pay a contractor a deposit' is nonsensical.
I agree with JHs. If there is no new color on it at all, just talk to the painter about it, if it was an honest mistake (which can happen very easily while painting) your painter should have no problem fixing it. However, in my years of experience, it is not unusual for 2 coats of door paint (good quality) not to cover very well at all. I once painted a red door 7 times, plus a tinted prime coat before I found the door to be a solid color.
Generally, it will take a minimum of two gallons of paint to cover a room. At the highest end, paint will cost anywhere between $30 and $60 per gallon and come in three different finishes: flat, semi-gloss or high-gloss. Flat finishes are the least shiny and are best suited for areas requiring frequent cleaning. Semi-gloss finishes are a bit shiny but can also be easily cleaned. A high-gloss finish is stain-resistant and easy to clean. Traditionally, living rooms should be painted with a flat finish to allow the paint to stand out. Glossy finishes should be reserved for hallways, and a semi-gloss is best suited for trim. Invest in pre-primed paint whenever possible. This cuts down on time by combining the layering process. Almost any paint job will require a primer, which will cost anywhere from $7 to $15 per can. Primer helps the paint to stand out against the underlayer of paint it's covering, especially if the new paint is lighter than the old coat. You will need at least two cans of primer, if not more, to cover one wall. Here are some paint brands and their average price per gallon: