The secret to a finish that's free of lap and brush marks is mixing a paint extender (also called a paint conditioner), such as Floetrol, into the paint. This does two things. First, it slows down the paint drying time, giving you a longer window to overlap just-painted areas without getting ugly lap marks that happen when you paint over dried paint and darken the color. Second, paint extender levels out the paint so brush strokes are virtually eliminated (or at least much less obvious). Pros use extenders when painting drywall, woodwork, cabinets and doors. Manufacturer's directions tell you how much extender to add per gallon of paint.
Whether you buy cheap or expensive roller covers, washing them before their first use gets rid of the fuzz that inevitably comes off once you start painting. Wash them with water and a little bit of liquid soap, and run your hands up and down the covers to pull off any loose fibers (a practice called "preconditioning covers"). You can start using the roller covers right away—you don't need to let them dry.

Use a roller to paint the rest of the wall. A good method to use is the 'W method'. You start by painting a large 3 foot (0.91 m) square W on the wall. Then, without lifting the roller, you fill in the W. You can paint a wall section-by-section, and do the walls one at a time for best results. It's generally a good idea to use an extension pole for your roller instead of standing on a ladder. Make sure that neither the extension pole nor the roller has plastic handles, as plastic handles are flexible and this makes it difficult to control the painting.
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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