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.
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.
Sand the Primer
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. Remove the primer dust with a quick brush from the shop vacuum, followed by a light swipe of the tack cloth.
After the primer dries, lightly sand away bumps and ridges using very fine grit sandpaper folded into quarters. When the grit of one section of sandpaper becomes covered with dust, switch to an unused section and continue. Wipe the wall clean with a damp towel or sponge and allow it to dry before painting.
Now you may be thinking "SAND AGAIN?!?" after just finishing the messy drywall stage, but a light sand with 150 or 180 grit sandpaper is very important. Whether you use a separate primer or self priming paint, both will raise the nap of the board making a slightly rough surface.
Most drywall projects should be finished with sandpaper in the 150-220 grit range, depending on the methods you use. Higher numbers mean a finer grit sandpaper and lower numbers are coarser.
A drywall pole sander is perfect for sanding the drywall primer coat before you paint–a step that requires little pressure and control. Using a drywall sander: sand with minimal pressure around screws and the edge of seams to avoid the drywall paper "fuzzing".
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.
A minimum of two coats of PVA primer is necessary when painting new drywall in order to prevent the paint finish from looking splotchy and uneven after drying. New drywall is like a sponge, and it's better to let it soak up inexpensive primer than expensive paint.
While sanding is not required for every paint project, rough spots on walls, whether they have been previously painted or not, need to be sanded before they are painted to ensure the paint goes on smoothly.
New paint will not easily stick to a smooth surface. Thus, if you don't sand the paint job (at least a little bit), your paint will begin to flake and peel in no time at all. A little bit of surface roughness will provide the traction that is needed for the paint to stay in place.
If you don't have a properly sanded surface, paint may not adhere to certain items properly. You can oversand, undersand and often people underestimate its importance. But, you don't have to be a pro to sand a surface. You just need patience, some protective gear, and some elbow grease.
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.
Professional Painters Usually Don't Wash Walls
Washing cuts into painting time, which cuts into their income, so don't expect your hired painter to wash down your walls unless you agree to pay them more for the service. Better yet, hire a cleaner to do this work, or do the cleaning yourself before the painter arrives.
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.
For best results allow Primer to dry for 1 hour before dry sanding. Test in an inconspicuous area to be sure Primer is dry enough for sanding. Block sand the area using 320-400 grit sandpaper.
Since primer's function is mainly to bond and cover porous surfaces, it is not made to withstand elements like paint. Because of this, if you leave primer without a top coat (or paint) it will deteriorate and break down, probably in a chalk like form.
Most water-based primers dry relatively fast, in 3 or 4 hours. After that, they will be good to use for the next 30 days. As long as you paint your surfaces in that time period, your paint will stick with no problems.
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.
Wet-sanding drywall compound is a slower process than traditional sanding. If you're interested in speed, you'll want to dry-sand. Nor does wet-sanding produce a perfectly smooth surface. Because you are using a sponge―which is flexible―your finished wall may exhibit gentle waves.
Hand sanding – Power sanders are crucial for getting the majority a job done quickly and efficiently as well as giving you a great finish. However, if you want to finish off little bits that you may have missed with the Power Sander, hand sanding works a treat, even on Drywall.
Yes, you need to sand between coats of mud. However, perfection isn't needed on those first few coats. Knock down ridges and obvious bumps so that your next coat can be smooth.
Generally, 180-220 grit is a good choice for sanding between coats. These extra fine grits do a great job on most interior walls. Grits of 240 and higher are best suited for projects where polishing is involved.