Paper Stretch Calculator — Watercolour Paper Stretching

Paper absorbs water and expands unevenly, causing buckling and cockling. Stretching wets the paper fully, then holds it flat as it dries. When you paint on stretched paper, it stays flat because it cannot expand further.

Paper of 300 gsm and above is less prone to buckling and many artists skip stretching. However, heavy washes on any weight can cause cockling. If in doubt, stretch it. 190 gsm paper should always be stretched.

Gummed tape (brown gumstrip) is the traditional method and works reliably. Stapling is faster but leaves holes. Clips and stretcher frames are reusable but may not hold heavy papers. For beginners, gummed tape is recommended.

St Cuthberts Mill (maker of Saunders Waterford and Bockingford) advises about 3 minutes for 190 gsm and about 8 minutes for 300 gsm in clean, cold water. Heavy 425-640 gsm needs a little longer (roughly 10-14 minutes). The clock is only a guide — the real sign it is ready is the sheet going floppy and limp, like wet cloth. Do not over-soak: excessive soaking washes the gelatine size out of modern mould-made paper and weakens the surface.

St Cuthberts Mill simply advises leaving the board to dry overnight before painting (about 8 hours for 300 gsm). Drying time is approximate and depends on humidity and temperature, ranging from a few hours for light paper to most of a day for very heavy sheets. Do not use a hairdryer — uneven drying causes warping. Let it dry naturally, flat and horizontal.