You scuff up existing paint for two reasons. First, you want to get rid of any oil, grease or dirt trapped in the top layer of paint. This is the stuff that cleaners and degreasers can't get off. The second reason to scuff is to set up a physical bond for the new primer and paint to adhere to. You want to give the old paint some tooth so the new sticks better. Water-thinned interior paints are calcimine, casein, resin-emulsion, and gloss water paints. Calcimine consists of powdered whiting and clay mixed with an animal-glue binder and a preservative. It cannot be recoated, but can be easily washed off before redecorating. Once the drywall compound/spackle has had sufficient time to dry, you will need to lightly sand the surface of the repair to smooth out any ridges that may have been caused while "coating" the patch. Once you have the surface sanded smooth, take a wet sponge or cloth and, by lightly washing/rubbing, blend the outside edge of the drywall compound/spackle back into the wall. For a smaller repair, you can wash away the excess drywall compound, leaving only the compound/spackle only in the area where the indent was filled. For a larger hole, after filling, sanding and using a wet sponge to blend the outside edge of the compound back into the wall, it may be necessary to texture the area in order to give the smooth patch the appearance of the existing wall.
Step three: go online and study the basics of shadow and light. It's a visual thing, and too complicated to talk about here. But once you see it, it'll come to you quickly. Simply Google "shadow and light in painting" and the basics will come up. Once you know these rules, you can apply it to any shape, any form, any painting. And it'll make you keenly aware of shadow and light on virtually any object on earth. It's what makes a painting three- dimensional and expensive looking. You can also buy art books on shadow and light at any bookstore. But make sure you sit down for a couple of hours and study it. It will come quickly to you, I promise. Even if you don't need to apply a coat of primer before your new coats of paint, doing so will always save you money. Whether you use white or tinted primer, a coat of primer is always more cost effective for one simple reason: It is cheaper! In fact, primer may cost as little as half as much as standard paint. If you get it tinted the same as your paint, then it is cheaper by the coat. If you use white, as in the first scenario above, it will also minimize the number of coats of paint you have to apply. Either way it reduces how much paint you have to buy. Do you want to learn more about maximizing your paint color coverage, minimizing your paint usage, optimizing coats, and perfecting your primer? Masonry paints come in a wide variety of finishes, from textured to ultra-smooth. Opt for a texture if you need to disguise fine surface cracks. If you favour traditional finishes, then consider limewash, which is available from specialists such as Francesca's Lime Wash. The beauty of this paint is that it will mellow and weather with time. However, do check with the supplier first to ensure that the surface is suitable for this finish. You're not looking for perfection here, but you do want to eliminate every shiny spot of paint. When properly scuffed the paint should have a smooth, but dull matte finish.