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.
If you decide to hire help, have several contractors inspect the job and provide proposals. You'll probably find huge price differences for the same job. A Checkbook undercover shopper got quotes from nine Washington-area contractors to repaint the walls, ceiling and trim for a living room, dining room, family room, bathroom and kitchen. Including paint and supplies, prices ranged from $2,650 or less to more than $6,500.
Back to the article. You can add water to all latex based paints / thinner to oil based paint. The tinting base has absolutely nothing to do with it. Say you are working outside and throughout the day you have to add a little water to keep the same consistency. If somebody really tried to add 20% to 50% water they no longer would be painting they'd be performing a whitewash or pickle finish.
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