Freshly painted walls often look blotchy. The color is uniform, but the sheen isn’t consistent. This usually occurs over the holes and cracks you patched with a filler or drywall compound. The porous fillers absorb the paint, dulling the surface (a problem called “flashing”). When light hits these dull spots, they stick out like a sore thumb. The smooth patch also stands out in contrast to the slightly bumpy texture of the rest of the wall. A quick coat of primer is all it takes to eliminate ﬂashing and texture differences.
Once you have the approximate square footage that you’ll need to paint, remember that a gallon of paint generally covers between 250 and 400 square feet. You’ll need more if you’re painting a rough, porous material, or if you want to paint a dark house a much lighter color. Depending on your color choice, you may need a more expensive tinted primer to help with the color transition. Thornton House Painters