Wall Prep - F-A-Q - expansion and contraction, testing, priming, common problems

Sep 28, 2026

Uploaded by Marglet

Wall Prep addresses why preparing walls for wallpaper is challenging and increasingly necessary. It explains causes of failure, including expansion and contraction after paste, porous or moisture-sensitive paint films, and weak paint layers from prior coatings. The guide provides practical wall testing methods, criteria for sealing porous surfaces versus other painted surfaces, and recommended primer types. It also outlines the most common prep mistakes, such as insufficient dust removal, low-quality products, and loose paint layers.

LanguageEnglish

CategoryExam

FormatPDF

Pages2

Reading Time5 min

AccessFree to download

F-A-Q: Wall Prep

Why wall prep is important?

Why it is more difficult

Why prep differs for painted vs papered walls

How to test a wall

If you need to prime, what should you use?

Most common wall prep problems

F-A-Q: Wall Prep 1. Why is wall prep important? It was John Cox, founder of the American School of Paperhanging Arts, who said “you never do exactly the right amount of prep. It’s either too much or too little.”More than ever, professional paperhangers need to do TOO MUCH prep and cannot afford TOO LITTLE. Wall prep for wallpaper has never been easy. But recently, it’s become more difﬁcult. 2. Why is it more difﬁcult? The most recent problem is that born-yesterday wallpaper companies on the internet release new products without adequate testing or guidelines. Therefore more caution for wall prep and installation is needed. Other problems seem to be economics-based: highly skilled, well-paid paint crews are less evident than they once were, and formulations for prep products have changed, while other problems are due to wallpaper having a lower proﬁle in the culture. Several generations have grown up without wallpaper. These last few problems have a common result: more and more walls are being prepared for wallpaper by people who don’t understand how a papered wall differs from a painted wall. Meanwhile, old problems have not gone away. Previous layers of paint are only as good as their weakest link. Skim coatings of hot mud or joint compound and walls painted with so-called builder’s ﬂat are extremely porous. These must be sealed. 3. You’re depressing me. Why don’t we take these one by one? Why should prep for a painted wall be different than prep for a papered wall? Let’s start with expansion and contraction. Paper expands from 1 to 2% after being pasted and contracts while drying. Our testing shows that most pasted papers, if they were simply left to dry without installing them on a wall, would contract as much as a half-inch. Let me say that again. Our testing has shown us that most pasted papers, were they not anchored to the wall, would contract as much as a half-inch. With pulling power like that, it’s no wonder that many paint ﬁlms can’t take the strain. The result: popped or compromised seams. So, while a cheap paint might give a nice white ﬁnish to a painted wall, the same paint might cause a disaster if wallpaper is put over it. 4. How can you test a wall? First, run a sponge over the wall. If it darkens, it’s porous and should be sealed. Some builder’s ﬂat paints have such low amounts of binder that you can wash them off the wall without much trouble. If the paint ﬁlm doesn’t darken but instead lightens, it’s probably an acrylic topcoat that’s sensitive to moisture. Usually, these dry out OK and are reasonably stable. Another test is to cut a good-sized “X” into the wall with a razor blade. Use some pressure to penetrate the paint ﬁlm with the blade without cutting into the sheetrock or plaster underneath. Put a piece of blue tape over the "X," press down, and pull. A weak paint ﬁlm will often come off with the tape. 5. If I need to prime, what should I use? Let’s draw a distinction here: a.) very porous surfaces, and b.) all other painted surfaces. a.) very porous surfaces: For many years, an alkyd (oil) based primer was speciﬁed for sealing very porous wall surfaces. This is still the gold standard. Many substitutes for oil-based primers are sold, but, in our opinion, water-based primers do not penetrate as much, dry as hard, and seal as well as oil-based primers. A popular solution among professional paperhangers for these concerns is to use a masonry- type sealer for very porous surfaces. These are water-based, readily available, and do a reasonably good job of sealing raw sheetrock, drywall damage, skim coats, and suspect paint. Some brand names are"Drawtite" from Scotch, "Gardz" from Zinsser, and "RX35" from Romans. A good-quality acrylic primer such as Ben. Moore "Fresh Start" (046, NO23) is another popular choice. 046 and NO23 can seal many porous surfaces such as raw sheetrock effectively, but neither of these primers are speciﬁed for wallcoverings, so caution is advised. They should

FAQs

Why is wall prep important for wallpaper installations?

Because the right amount of preparation prevents failures caused by too much or too little prep. It is especially important now since wallpaper products may be released online without adequate testing or guidelines.

How can you test whether a wall needs sealing?

Run a sponge over the wall: if it darkens, the surface is porous and should be sealed. If it lightens instead, it may be an acrylic topcoat sensitive to moisture.

What should you use when priming very porous surfaces versus other painted surfaces?

For very porous surfaces, oil-based alkyd primer is the gold standard, with water-based masonry-type sealers as an alternative. For other painted surfaces, use a prepcoat (acrylic wallpaper primer); some strong latex/acrylic paints may already hold wallpaper, but adding a primer is recommended as insurance.

Sep 28, 2026

Uploaded by Marglet