Painting exterior trim is difficult. With preparation, ensuring the conditions are suitable, and the right technique, you will carry out the job just fine.
Painting exterior trim is among the more difficult exterior paint jobs. It requires more preparation and greater care during execution. But it can also be the most satisfying, since good trim really makes a house stand out.
Proper preparation is key, as it is with any painting job. Trim will need to be masked exactly. If wall painting is done first, it can be very difficult to achieve the proper color if wall paint has touched the trim first. If trim is being done alone, it's necessary to mask around the trim to avoid getting paint on the walls. Even if the trim and walls are the same color, new paint will show as a different shade.
Windows represent a special challenge. Mutins (the wooden or metal dividers between panes), make necessary precision masking to avoid getting paint on the glass. It's usually easier to mask the glass carefully than to scrape off paint afterward. Also, it's safer, since a blade used to clean off the glass can slip and scrape the newly painted trim.
As with any exterior painting job, it's critical to ensure that surfaces are dry before beginning. That goes double for trim. Large wall surfaces exposed to the sun will dry off more quickly than sections of trim that contain angles that trap moisture.
Wait 2-3 days after it has rained before beginning an exterior job. Also, start the job a little later in the day, to allow morning dew to evaporate. By the same token, it's a good idea to finish a couple of hours before dusk, to let new paint dry a bit before the evening moisture arrives.
More than just moisture, temperature is important, too. Apart from the fact that moisture gathers more readily in cooler temperatures, cold weather causes paint to dry too slowly. In extremes, cold temperatures can cause wrinkling. About 50F/10C is the minimum for most climates.
The opposite extreme can be just as big a problem. High temperatures cause paint to dry too quickly, which can introduce surface irregularities. Blistering is common when oil-based or alkyd paints get too hot. The compounds separate and a bubble forms with solvent inside.
As with any painting job, always start from the top down. Dip the brush a couple of inches into the paint, but remove any excess by wiping the brush against the rim. Don't try to rush the job by using too much paint at once. Then pull the brush up and hold it parallel to the ground with the paint filled surface up.
Work the brush perpendicular to the grain for the first few strokes. That gets paint into all the miniature grooves and crevices. Then brush with the grain to smooth everything out.
On smaller surfaces such as window mutins, use a smaller brush or at least turn a large brush on its side. Even when the areas are masked be prepared to take the time needed to do trim right. A fast brush will flick paint onto surfaces where it's not desired. This is not a part of the paint job where you can expect to do large sections quickly, as with wall surfaces painted by a roller.
Smaller rollers can be used, though, on thick trim. A four inch wide roller can do a four to six inch piece of trim just fine. Go slowly though. A fast roll will inevitably fling paint onto the walls.