When considering wallpapering over existing wallpaper in a purchased or rented home, it's crucial to investigate the wall's history. Existing layers could harbor mold and bacteria, jeopardizing both health and the durability of your DIY project. 

Before proceeding, carefully inspect inconspicuous areas for multiple layers. Use a utility knife to make a small incision, revealing potential hidden complexities. By systematically checking and addressing any existing layers, you ensure a safer and more robust foundation for your wallpapering endeavors, promoting a longer-lasting and healthier living space

Wallpapering over different wallpaper surfaces depends on material types. Breathable wallpaper, such as paper and fabric woven types, offer a smoother base, facilitating easier application of new wallpaper over top. Their porous nature allows for better adhesion. 

In contrast, shiny non-woven wallpapers such as vinyl, with a more rigid and less permeable texture, pose challenges. These types resists moisture and can prevent proper adhesion. To check if the existing wallpaper is woven or non-woven, dampen a small section - if it darkens then it is most likely woven.

When wallpapering over old wallpaper, a smooth surface, ree from bumps and cracks, is always recommended. Uneven textures can compromise the adhesion and appearance of the new wallpaper. Ideally, choose a thicker wallpaper, as it helps conceal minor imperfections on the underlying surface, creating a smoother finish. This approach not only enhances the aesthetic outcome but also ensures a more forgiving application process, and better adhesion of the new wallpaper.

Thoroughly examining the wall under existing wallpaper before starting a new project is essential. The existing wallpaper might have been used to mask underlying imperfections or damage. Investigating ensures you address any issues like cracks, moisture damage, or uneven surfaces, preventing these hidden flaws from surfacing later. 

Uncovering any issues and fixing them beforehand guarantees a more stable foundation for your new wallpaper, enhancing its longevity and maintaining a pristine appearance over time.

When applying new wallpaper over existing wallpaper, be clever with color choices. Bold or dark colors beneath lighter wallpaper may not stack well, potentially showing through. 

For instance, a vibrant red or deep blue under a light, pastel hue might cause visibility issues. Opting for a thicker new wallpaper can mitigate this concern, providing better coverage and minimizing the risk of underlying colors affecting the final aesthetic.