There are various different reasons why a drywall can become uneven. Those horrible valleys and ridges you have found could have been the result of getting wet. The drywall may have been installed badly, or you may find that removing the wallpaper has wrecked things. But fixing it isn’t impossible. The main technique that you will find the professionals swearing by is known as skimming coating. It has the tendency to produce walls with a smoothness level to die for. There are no fancy tools you need to buy, however you will be required to have a level of skill that only practice can achieve. In the case of a beginner taking on this task, they will most likely take longer to do it than someone who does this for a living. Here are the steps.
Get some sandpaper with 120 grit. You will be using this to sand down bumps that are the result of wallpaper removal. In order to get at the areas of wall that are higher up, you will want a sanding pole. You can also use it to give the ceiling a good sanding. If you can see any paper tape, don’t stand in that spot. After that, get the PVA primer. Distribute a layer of that onto the whole wall.
You will need some joint compound powder for this step. Make sure that it is of the all-purpose variety. Mix it up in a bucket with water. If you want, you may like to buy pre-mixed compound. But if mixing compound powder is what you live for, you will be wanting to get it so that its consistency roughly matches that of peanut butter. If you are able to pour it, you have done it wrong. If you put some on your mixing tool and it doesn’t fall off, you have done it properly. Now get your mudding trough and put some compound in it.
Be sure you have a drywall knife for this step. You will need to use it after applying the compound to any valleys or depressions you can find to scrape the area so it is flat. You may need to apply a few coats if the flaws are particularly big.
After the compound has dried, give the area a light sand. You will need to clean any dust that results from the sanding process before dispersing a further coat of compound. Make the area that this coat covers wider than the last. Wait until this has dried and then do it all again. You want to have the wall’s surface gradually get to the same level as the ridges and seams so they are non-existent.
Make sure that the whole time you are doing this, you are keeping the drywall knife free of debris. If you don’t, it will show up on the wall and make more work for you to do.
Give the wall a final light sanding before priming the wall with more of that PVA. Then you are free to paint the wall to your heart’s desire.