Sealing your wall is advised by everyone but is it necessary? Its a really technical questions. We have wallpaper hundreds of times not using a sealer and its been 100% fine. But every time we did it - we knew the history of the wall and what we were wallpapering over.
So the question that might be more appropriate is what does sealer do? And why do I need it?
Wallpaper size (or sealer or primer), is a liquid acrylic–based barrier that is applied under wallpaper to assist with installing the wallpaper and with removing the wallpaper at any later time.
When you use a wallpaper primer/size/sealer it helps fills pores in absorbent surfaces, creating a smoother surface for sliding the wallpaper into position.
Flat, matte, or uncoated walls—and even some glossy paints —can create a drag that makes it harder to slide wallpaper and position it. When you size the wall you are actually allowing the wallpaper glue to do its job, rather than being immediately absorbed by the wall's pores.
It can also help with painted surfaces. If you don't have a new house or brand new gib, chances are you don't know what paint or what additives have been added to the paint. If the paint is high gloss the wallpaper paste may not stick as well to it as you might hope - sealer can really work here by preparing your walls for paste over the paint.
Wallpaper seal and size can also work when you want to remove wallpaper easily. It helps provide a shield between the wallpaper and the wall making it easier to remove later on down the track.
How do you know if you need a wallpaper sealer?
Water absorption is one of the key factors in determining whether you should use wallpaper primer. So, it helps to test out the wall with a small amount of water to gauge the rate of absorption.
With clean, clear water in a bowl, flick a few drops of water onto the wall and rub in some of the drops. If the wall turns dark, that means that the surface is quickly absorbing water and likely requires wallpaper primer.
Other factors (as already mentioned) influence the decision, including paint gloss, age and condition of the paint, and the presence of primer or dust.
So really the answer of sealing or not sealing depends on the condition of the wall, previous use/history and if you think in a few years you may want to change the wallpaper and be able to easily (or more easily) remove it.