Paper defects refer to any flaws in paper not covered by the technical specifications for paper quality. Paper defects include certain performance‑related flaws that are invisible and intangible and can only be detected through testing or printing, such as brittleness, strike‑through, linting and dusting. They also cover various blemishes and flaws on the paper surface that can be directly seen or felt, for example dirt specks and stains. The latter category is herein defined as paper surface defects.

There is a wide variety of paper surface defects, including dirt specks, spots, wire marks, felt marks, cloud formation, seersucker, wrinkles, creases, stains, shives, transparent spots, moisture spots, holes, dents, scars, pulp lumps, hard chunks, as well as basis weight variation and poor formation. Some of these surface defects originate from stock prior to papermaking; some are caused by improper or inadequate technical operations during papermaking; others stem from poor plant sanitation. Certain surface defects, such as felt marks and cloud formation, tend to occur continuously. They will persist unless corrective measures are taken once they emerge. Other surface defects appear sporadically or at long‑time intervals, for instance stains and holes. Some defects degrade paper quality, such as dirt specks and felt marks. Others render the paper completely unusable. For example, offset book paper containing hard chunks or coated paper with large stains must be discarded as waste.

Common paper defects are classified by type as follows:

Fluctuation in paper basis weight and cross‑machine basis weight variation

The first type of fluctuation is generally caused by periodic vibration in the stock delivery system feeding the headbox, shaking of the rotor screen ahead of the headbox, or vibration of the pulp pump. It may also result from a deformed roll agitator or poorly designed perforations inside the headbox. The second type of fluctuation is usually attributable to unreasonable production processes or improper operation and management.

Poor formation. Its manifestations include:

Flocculent structure: fibres agglomerate into clumps.

Cloud formation: also known as cloud‑like structure, describing fibre distribution resembling clouds within the paper sheet.

Discontinuous fibre structure: poor interweaving of fibres along the machine direction of the paper machine.

Wavy fibre structure: wavy thickened layers running transversely across the paper web width.

Pulp streaks: longitudinal pulp marks visible against light on the paper web.

Dirt specks, spots, sand and hard chunks

Dirt specks: divided into three categories: fibrous dirt specks, non‑metallic dirt specks and metallic dirt specks.

Spots: classified as wet spots and steam spots, dryer spots, calender dark spots, pulp spots, bubble spots, filler and coating spots, and bright and white spots.

Sand: hard mineral particles such as silt, lime residues and carbon slag present on the paper surface.

Hard chunks: occasional hard, raised lumps or coarse particles on paper, for example wood knots, straw knots and pulp lumps, which pose the greatest hazard to printing.