Soak time and the science of a paste-and-hang liner
Pasted paper expands before it goes on the wall. Understanding that expansion is the difference between a flat liner and a bubbled one.
Cellulose fibres are hygroscopic, meaning they draw in water and swell, and that single fact drives the whole ritual of pasting a liner. When paste soaks into a paper made from wood or cotton fibres, the fibres take up water and grow, and the sheet expands, mostly across its width rather than its length because of how the fibres lie in the sheet. A standard lining paper can gain a noticeable fraction of an inch across a width once it is pasted. All of that movement has to happen before the paper is smoothed onto the wall. If it happens after, the expanding sheet has nowhere to go and pushes itself up into bubbles and blisters.
The way to let it happen is soak time, also called booking. After a length is pasted, it is folded paste-to-paste, left to stand for a set number of minutes, and only then carried to the wall. During that pause the paper drinks the paste, expands to its full width, and relaxes. The right soak time is printed on the roll's wrapper, and it varies by weight: a light grade may need only a couple of minutes, a heavy bridging grade several. The rule that matters more than the exact figure is consistency. Every length in a room must soak for the same time, because a length booked for two minutes and a length booked for six will be two different widths on the wall and their seams will not match.
Soak time is also why ready-pasted and paste-the-wall products behave differently. Paste-the-wall liners are built on a non-woven backing, often reinforced with glass or polyester fibre, that barely expands when wet, so the paste goes onto the wall and the dry paper is hung straight from the roll with no booking at all. That saves time and needs less table space, but the two systems cannot be mixed on one wall without inviting mismatched seams.
If bubbles appear anyway, small ones often disappear as the wall dries and the paper shrinks back down, because a slightly over-soaked sheet tightens as it loses water. Bubbles that are still there when the wall is fully dry mean paste never made contact at that spot, and the cure is to slit the bubble, work a little paste behind it, and roll it flat.