The only time it's acceptable to mix water in the paint is when you're using a deep or ultra deep base paint to reduce its stickiness, which is rare with new paint technology. Dark primary colors are composed almost entirely of tint that makes it very hard to work with without adding water. (You never use a sprayer and need to thin paint.?) If i was still in the industry I'd take the time to make a better article than this. Take this for a grain of slat. Use a reputable painter or someone you know you can trust or has been referred to you by someone you trust. I wouldn't hire anyone I had to watch like a hawk to make sure they're not screwing me.
Thorough preparation is essential to ensure a good finish. Contractors might have to fill cracks or holes and remove old paint if it is badly worn, peeling or flaking. They remove all paint and apply a suitable primer to the bare surface. If the existing paint is in reasonable condition, they clean it and rub it with abrasive material to provide a key for the new paint. On new surfaces, such as bare wood or fresh plaster, they apply a suitable seal, followed by a primer coat.