Sanding after applying primer should help keep your surface as smooth and flat as possible, eliminating brush marks, or extra little globs. Also when painting, like others have pointed out, can raise little fibers from the drywall, going over the surface with a sheet of sandpaper can help get rid of those lose fibers.
Once you've completed your last coat of primer, you should go ahead and sand before applying your first coat of base color. However, there is no reason at all to sand between coats of primer. It will only add more work to your plate with no visible benefits.
The quick answer to the question, “Do I need to sand primer before painting my wall?” is YES. Primer should be sanded before you apply the paint. A primer is used to help cover up the existing color of your wall. It is applied to create a blank, neutral surface before you coat your wall with your chosen paint color.
For your finish to be vibrant and not dull sanding after primer will ensure that the finish is not excessively dulled when you apply your paint top coat. You will typically use finer grit sandpaper and aim to achieve a smooth wood finish so that you get a fine appearing wood finish when you apply your topcoat.
Whether it's a wall or woodwork, it should be thoroughly sanded to provide a surface “key”. You may need an undercoat for wood, but there is no need to apply any primer, as the surface is already painted.
Sanding a surface removes the top glossy layer, allowing the substance to receive the primer. Without sanding the surface in advance, the new coat of paint won't stick to the previous layer.
600 to 800 Grit – This sandpaper grit range is perfect for sanding surface imperfections in the primer, prior to paint application. It is recommended to start at the low grit end and work your way up to 800 grit.
Sanding after applying primer should help keep your surface as smooth and flat as possible, eliminating brush marks, or extra little globs. Also when painting, like others have pointed out, can raise little fibers from the drywall, going over the surface with a sheet of sandpaper can help get rid of those lose fibers.
An undercoat is used to create a neutral base for colour to develop. This is because they work to cover surface imperfections, therefore they are often sanded before recoating.
Essentially, wet sanding is the process of sanding a car with water. The water acts as a lubricant and vehicle to remove the paint dust that would normally just clog up the sandpaper and get in the way.
Let the primer dry completely, then sand it down before painting. Use very fine 220-grit sandpaper, and apply light pressure to prevent gouging the primer.
The initial condition of the surface will determine what exactly will happen if you don't sand before painting. A clean, smooth, not glossy surface will take paint well. However paint can feel rough, streak, peel, chip, or even not adhere if not sanded first.
Whether you wet sand primer or not depends entirely on what type of primer you're sanding. For latex paint primer (water, oil or shellac-based,) you don't need to wet sand, since the layer of paint will cover that perfectly smooth surface anyway. Auto-body primer, on the other hand, should be wet sanded.
If the project has a high gloss finish or a clear top coat, lightly sand* the project with a medium grit sanding block. This will enable new paint coats to bond with existing paint coats.
HOW LONG DOES IT TAKE FOR PRIMER TO DRY? Most latex primers dry to the touch in 30 minutes to one hour. But for best results, don't paint until the primer completely dries—which can take up to 3 hours, depending on temperature and humidity. Once the primer is completely dry, it's prime time for painting.
Pro tip: Sanding between coats of paint isn't always necessary, but is highly recommended when applying polyurethane over stain or raw wood.
Use the same technique for applying the undercoat as you used with the primer. Once dry, give the undercoat a rub down with the 220 grade sandpaper before cleaning with the rag again. If you want a nice smooth finish then sand between each coat of paint.
You will want to use 2-3 coats of primer to ensure there is a good bond between the new paint and the wall, and also to cover up any previous colors, especially if they are red, orange, or a strange outdated color. In short, you will typically need 2 coats of primer for most painting projects.
600 grit and 800 grit are used for sanding imperfections in the primer before applying the base coat/paint. Start with 600 grit and work your way up to 800 grit. You can use 1,000 grit or 1,200 grit over the base coat to remove imperfections. It is important to use “wet sanding” technique.
Use fine-grit sandpaper to remove the dips as you don't want to sand the entire primer off, just smooth them out. Can you paint over a primer without sanding? Yes, but you shouldn't. Skimp on the sanding results in a rough finish and paint blemishes.
If you attempt to paint over a primer coat that is not completely dry, it will result in a goopy, sticky mess that doesn't adhere properly to the wall. Even if you can get the paint to stick, there is a good chance that it will peel, chip, or clump, leading to an unattractive finish.
As Phillips mentioned, you want to wait a full minute or more to allow the primer to dry completely before going in with makeup, as this will help to curb pilling or patchiness.
Step #3: Sand Surfaces Before Painting
For previously painted water-based paint, sand with a fine-grit sandpaper. For oil-based paint, a medium-grit sandpaper (100- to 150-grit) should be used.
Could be the surface was too glossy before priming or poor primer for this surface. I would scrape off a few square inches of the new primer, sand the surface with 220 grit sandpaper, and re-prime. After it dries I would check the adhesion. If it adheres well, then you know what you'll have to do.