Sand down the bad areas until you can feel a smooth and flatter surface, wipe clean with a terry cloth, and repaint with a 3/8'' NAP paint roller. You'll want enough paint in the roller that it's ``loaded'' but won't cause the paint to drip.
Either skim coat the entire wall with drywall compound, or have a plasterer come in and skim coat a thin layer of plaster. The drywall compound you could do yourself. It is messy, but not brain surgery. A plasterer will put a bonding agent over the paint and then put a thin coat of plaster over the entire wall.
Lightly sand the troublesome area with 150-grit sandpaper, then tack the walls. That's painter lingo for ``Wipe it with a cloth to get rid of dust.'' (Easier than the lingo makes it sound!) Then re-paint the area, maintaining a wet edge--meaning, don't let the paint dry until you finish the wall.
Step 1: Wet Sanding Rough and Sagging Paint
When I wet sand, I like to add a few drops of soap to the water in order to allow the sandpaper to glide more easily over the surface and not grab as easy, biting into the paint. I used 3M 600 grit wet sanding paper to make small circles and knock the bumps down.
Buffing it Up
Now it's time to use the rotary buffer, want to set it on the lowest speed, this process comes in three stages: A heavy polish for taking out scratches. A medium polish that will remove fine scratches and imperfections. Glazings polish to give you a mirror like finish.
If you want to paint an uneven wall, you'll have to use a special type of very thick paint designed to fill in hairline cracks and surface holes. As long as your wall only has superficial faults, a renovation paint will be able to give you a good result without you having to fill in and sand down the wall first.
Glossy and satin sheens are more likely to show imperfections, but some matte sheens can be challenging to clean despite being better at hiding blemishes. Choosing a more middle-ground sheen, like eggshell, could be the best solution when your goal is to hide imperfections.
If the wall is in good condition and the paints are chemically the same (both latex, for instance), you have a few options when the new paint is the opposite shade of the old paint. You can use a primer to thoroughly cover the old color, then apply 1 or 2 coats of the new paint.
Wall condition: If your walls are in good condition, you may only need one or two coats of paint to achieve a smooth, even finish. However, if your walls are rough or damaged, you may need to apply additional coats to cover up imperfections.
Sand over the drip with medium (100-grit) sandpaper on a sanding block in a circular motion until you can no longer feel the paint ridge or any surface unevenness. Switch to fine (220-grit) sandpaper and sand over the same area to remove any scratches from the previous step.
As a general rule, if the surface you just painted doesn't feel or look smooth, sand it!
Old paint might have debris or clumps (even if it's an unopened can!) and new paint might need to be stirred.
If you want your paint to have a smooth finish, the technique you use is very important. Keep your brush at a 90-degree angle while applying the paint and feather as you go. Between coats, sand the wall lightly using a very high grit sand sponge of at least 220 grit.
Wet flatting stops the sandpaper becoming clogged with old paint residue. To wet flat, have a bucket of warm water to hand (adding washing up liquid helps ensure the sandpaper does not clog and further ensures a clean surface onto which you can apply your primer).
Patchy paint can result if paint has been applied unevenly. Patchy paint can be down to a variation in surface absorption. Walls might appear patchy if they have variation in texture. Using paint that is not fully mixed can cause a patchy appearance.
Sand Down the High Spots
For slight unevenness, you'll need a very fine grit sandpaper of around 320 to 400. To sand down lumps of paint, choose a coarser grit of around 80. With a block and paper or an electric sander, apply gentle pressure to sand off the top layers of paint, leaving the paint underneath.
If the paint has dried, use some warm soapy water and scrub at the paint. This should soften it and help it come up easily, then use a flat-edged scraper to get rid of the paint.
Use 400 grit (or higher) piece of sandpaper all over your piece and I PROMISE you that you WILL feel a difference. I use this one, but any soft sandpaper with a high grit will work. I know many paints say “no final sand required” but trust me, you don't want to skip this pro step.
Use light and steady pressure when wet-sanding. Keep the surface wet by spraying water on it if it begins to dry out. Repeat the sanding process using the same sanding technique but with ultra-fine 2,000-grit sandpaper. This removes the grit lines and makes the paint smoother.