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Here are a few of the rules of paint rolling:
1) Be sure to never use a roller that is too dry. You should have enough paint on the roller so that it is fully covered but does not drip. Trying to "stretch" the paint on the roller will only result in an uneven paint thickness and uneven appearance, forcing you to apply unnecessary additional coats. With a standard 9" roller, you can cover about two to three feet square max. If you roll out farther than that, you will have less even coverage and the too-thin coat of paint will not blend into subsequent roller work.
2) Don't overload the roller with paint! Though it is a temptation to try to speed the job along, all this poor technique will produce is unsightly paint lines and excessive dripping. The extra time it can take to smooth out overly heavy paint application and wipe drips off moldings will eliminate any savings from a few less dips into the roller tray.
3) Roll in multiple directions. Don't just roll up and down. Go over each area a few times at many different angles. This technique evens out the thickness of the paint and also spreads out any lumps or lines of paint.
4) Look back over your work frequently for missed spots and errors. After every two or three rollers-full, go back and check your work with a critic's eye. This is especially crucial with latex wall paints. Unlike oil or alkyd-based paints that can be sanded smooth (which is why they are still the professional choice for shelving and trim work), latex paints do not sand well but instead tend to tear and scratch. To make a long story short, it is extremely difficult to eliminate a rough, amateurish paint finish with latex paints… so do it right the first time!
5) Don't paint in the dark! In painting, "dark" means not having enough light to catch your mistakes! It is especially helpful to have a light at an angle to the wall so you can see the "gloss" of the paint. This helps you to see missed spots and unspread drops, lumps and streaks. This "angled" lighting is even more important when you are repainting a wall in the same color or for second coats.
6) Though it's hard to control the weather, don't paint in extremely hot conditions! And, please, don't use a fan in the room you are painting! If you are dealing with a solvent-based product and are concerned about the fumes, wear a respiration. Then turn on the fan when you are done. Hot and/or moving air will cause the paint to dry more quickly and produce more roller marks and brush marks!