Cutting In Before Rolling Out
Most ceilings and walls can be painted with a roller, but the roller's shape makes it hard to get into the corners and tight angles along edges.
Painters will usually start a job by cutting in along the edges with a brush, spreading a band of paint a few inches wide to cover areas the roller can't reach.
TIP: It's tempting to cut in a whole room first and then use the roller. But it's better to cut in a manageable area, then roll it. This way, you keep a wet edge (see below) and never paint over dried paint.
Keeping a Wet Edge
Painters always try to stroke or roll the paint from dry areas back into wet areas. That way, a coat of paint always blends back into itself to create a smooth, continuous layer.
That's what's called "keeping a wet edge," and it's most critical with latex paint in a warm area when the paint seems to dry seconds after you put it on. So you have to move fast under these conditions.
If you paint one area and it dries, it will often produce a lap mark at the edge of the dry paint if painted over again. It may not show when wet, but the lap mark is visible when dry.
When you're rolling out a ceiling or a wall, it's nice to have an extension handle which screws into the handle of your roller and allows you to reach down without much strain. It also allows you to reach up to the ceiling without a ladder (although you'll still need a ladder for cutting in with a brush).
Besides keeping a wet edge, the only other trick with rolling out paint is to get an even coat across the whole wall or ceiling.
Work in small sections, about 4' x 2'. First, load up the roller with paint so it's full all the way around, but not dripping wet. Start high and roll up and down in the shapes of Ns and Ws. Then roll over the Ns and Ws to cover the entire surface with paint.
Finally, smooth out the section by rolling down from the top to the bottom of the section, lifting the roller off the wall after each downward stroke.
This works equally well on walls or ceilings, but if you're painting a ceiling wear a hat because there's bound to be some paint coming down off the roller.
The main consideration with painting trim (and walls) is keeping a wet edge and avoiding any lap marks.
Always paint from a dry area back to a wet area. Of course, this is less critical if you're using an alkyd-based paint which dries more slowly than latex. Then your main concern is applying a smooth, even coat.
TIP: Occasionally, one or two bristles will spring out of the pack making it hard to paint neatly. Just cut those bristles off back at the handle or yank them out.
Paint sprayers are often the choice of professionals who want to lay down the smoothest finish possible. And they may be appropriate for your job, but beware of the wind direction, overspray, and cover everything that would be harmed by it, including yourself.
Many pros thin down their paint a bit so it won't clog the sprayer's nozzle. Latex paints which can't be thinned are too thick for some sprayers. Alkyd-based paints can be thinned with mineral spirits or paint thinner so those usually work best in sprayers.