Smoothing rough textured walls before painting them is best achieved by using a sander. You will firstly need to make sure the wall is totally clean by wiping down with a damp cloth. This could be quite difficult, as the textured wall will naturally attract dust and dirt, which will show up if the wall is a light colour.
The next stage is to make sure that the area under the wall is covered, as the job itself is a messy one. You can use dustsheets for this, which will enable the cleaning up process at the end to be a lot quicker and easier. You will need to start with 60-grit sandpaper, which is available from most hardware stores. This sandpaper is a medium sandpaper, 50-grit is too coarse for the job and will actually damage the wall. Attach the sandpaper to the sander and sand the entire wall, making sure to apply even pressure throughout, the easiest way to achieve this to move the sander in a circular motion.
Wipe the wall down again to remove any new dirt from the first sanding. Replace the 60-grit sandpaper with 100-grit sandpaper and repeat the process, again ensuring that even pressure is applied. 100-grit sandpaper is fine and is normally used for sanding bare wood, although it is suitable to use in this process, as the 60-grit sandpaper will have removed the majority of the texture. Wipe the wall clean again.
Next, you will need to replace the 100-grit sandpaper with 150-grit 180-grit sandpaper and repeat the sanding. The 150-grit and 180-grit sandpaper is very fine and will smooth out the remaining bumps on the wall. Wipe the wall clean again and your wall should be ready for the fresh paint. Replace the dustsheets and paint the smooth wall.
Skim coat with plaster, prime, then paint.
I'm trying to find out myself. You don't just sand, as SOME would suggest! I wore out my orbital sander in the first week sanding textured walls to smooth them! you can smooth out the serious peaks and then I think we'd use either drywall filler (spackling compound) or perhaps ready mixed plaster. So, check out DOB products on Home Depot under Drywall repair. They seem to be what most are suggesting I use. also check out www.diychatroom.com
You can use a palm sander with an 80 to 100 grit sandpaper to smooth the wall, but if you have a high gloss enamel paint over top of the texture it will be difficult sanding through the paint in order to get to the texture.
You don't mention anything in your question as to whether the texture is painted. If it has a flat or satin paint application over it, no problem.
A latex enamel will most likely start peeling when you sand it. Still, it could be done, but would be a whole lot of work. If so, would recommend you start with a 60 grit sandpaper to get the paint off. Then use an 80 up to 120 grit for smoothing the wall.
If it is an application in which the color was mixed in with the texture, it shouldn't be much of a problem to sand.
As Aicha said, be sure to apply a primer before painting because the surface will most likely be quite porous once you have sanded and you don't want an unevenly colored wall.
Is there any product that can be rolled on dry wall to smooth rough drywall