Why does my watercolour paper buckle, and how do I stretch it?

A buckled sheet is not just ugly. The paint pools in the low points and dries darker there, so a wash that would have been flat comes out mottled, and the whole surface becomes unpredictable. Stretching takes ten minutes of work and an overnight wait, and it removes the problem entirely.

Step by step

1. Choose a rigid board bigger than the paper.Marine or exterior plywood, MDF sealed with a coat of varnish, or a purpose-made stretching board. It must not warp when one face is wet. A drawing board with a laminated face works well.
2. Cut four lengths of gummed paper tape first.Brown gummed tape, the kind you wet to activate. Cut them before the paper is soaked, each a little longer than the edge it will cover, because once your hands are wet everything becomes difficult.
3. Soak the sheet evenly.In a bath, a tray or a clean sink of cold water. A few minutes for lighter papers, longer for heavy ones. The sheet must be uniformly wet — soaking one side only causes it to curl violently.
4. Lift it out and let it drain.Hold it by one corner for a few seconds. You want it wet, not dripping. Handle the surface as little as possible — wet paper is fragile and fingernail marks are permanent.
5. Lay it on the board and smooth it from the centre out.Use the flat of your hand or a clean cloth to push out air and excess water towards the edges. Do not scrub the surface.
6. Wet the gummed tape and stick down all four edges.Draw each strip through water or over a damp sponge, then lay it so roughly half its width is on the paper and half on the board. Press firmly along its whole length, particularly at the corners.
7. Dry it flat and slowly.Lay the board horizontally at room temperature and leave it overnight. Do not use a hairdryer or a radiator — fast uneven drying tears the paper away from the tape, which is the most common failure.
8. Paint on it while it is still taped down, and cut it off at the end.Trim just inside the tape with a scalpel and a straight edge when the finished painting is completely dry. The paper will stay flat.

Tips

• Watercolour blocks — pads glued on all four edges — are stretched for you and are excellent for working away from home. Slide a palette knife round to release the sheet only when it is dry.
• Heavy paper of 300gsm and above resists cockling well for anything short of very wet work, and many painters never stretch it. Lighter papers of 190gsm or less will buckle under almost any wash.
• Staples round the edge into a softwood board are an alternative to tape and hold very reliably, at the cost of a border of holes.
• If a finished unstretched painting has dried cockled, you can sometimes flatten it by dampening the back lightly and pressing it under a board and weights for a day.

Common mistakes

• Using masking tape or parcel tape — Neither has the adhesive strength to resist a sheet of paper shrinking as it dries. Gummed brown tape bonds to the board and to the paper as its adhesive dries, which is why it is the only thing that works.
• Drying it quickly with heat — It shrinks faster than the tape can hold and rips off, usually in the middle of the night. Slow, flat, room temperature.
• Over-soaking heavy paper — Very long soaking can soften the internal sizing, which changes how the paper takes paint. A few minutes is enough for most papers; follow the maker's guidance for heavy ones.
• Stretching on an unsealed softwood board — The board absorbs water on one face and warps, and the paper goes with it.

If it doesn't work

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