How do I seal air-dry clay, and can it get wet?

Air-dry clay is porous all the way through. A sealant coats the surface, and any chip, crack or unsealed area — most commonly the underside — lets water into the body, where it softens the clay from the inside. That is the whole reason for the honest limits here, and it is why sealed air-dry mugs fail.

Safety

What you'll need

• Fine abrasive paper for smoothing
• An acrylic varnish, or PVA and varnish
• A soft brush, or an aerosol varnish
• Somewhere dust-free to dry

Step by step

1. Let the piece dry completely first.Fully dry means it is the same colour throughout, no longer cool to the touch, and no longer losing weight. Days, not hours, and longer for thick pieces. Sealing damp clay traps moisture inside where it stays.
2. Smooth the surface before sealing.Fine abrasive paper, worked dry, takes off the fuzz and the tool marks. Air-dry clay sands very easily. Do this outdoors or over a bin, and wipe the dust off with a barely damp cloth.
3. Fill any cracks before sealing.A slurry of the same clay mixed with water, worked into the crack and smoothed, then dried and sanded. Sealing over a crack locks water in and the crack will still be the failure point.
4. Paint first, seal second.Acrylic paint goes onto bare clay well, and the sealant goes over the paint to protect it. Sealing first and painting after gives a slick surface the paint struggles to key into.
5. Apply thin coats, and seal every surface.Two or three thin coats of acrylic varnish rather than one thick one, and — critically — the underside and the inside as well. An unsealed base is where water gets in on almost every failed piece.
6. Use a PVA sealer first on very porous clay.Dilute PVA soaks in and seals the pores, giving the varnish something to sit on. It goes on milky and dries clear. It is a primer, not a finish, and it needs a varnish over it.
7. Choose the finish deliberately.Matt varnish keeps a natural ceramic look. Gloss looks more like glaze but shows every imperfection. A satin is a reasonable middle. Aerosol gives the most even coat on a textured piece.
8. Accept the limits and design around them.For a plant pot, put a plastic pot inside it rather than planting into it. For a dish, treat it as decorative. For anything outdoors, expect it to fail eventually — frost is what finishes air-dry clay off.

Tips

• Sealing the inside of a hollow piece is difficult and is exactly where water gets in. Pour dilute PVA in, swill it round, tip the excess out, and repeat.
• Epoxy resin coating gives a genuinely tough waterproof-feeling surface, and it is still not food safe and still fails wherever it is thin. It is a better finish, not a different answer.
• If you want a piece that genuinely holds water, the answer is fired and glazed clay. A studio firing service is not expensive and it is the only route that works.

Common mistakes

• Making a mug or a planter from air-dry clay — Water gets through any unsealed point, softens the body from inside, and the piece fails — usually at the handle or the base, usually while it is full.
• Sealing a piece that is not fully dry — The trapped moisture cannot leave, so the clay stays soft inside and the sealant eventually lifts or the piece grows mould.
• Leaving the base unsealed — It is the surface in contact with any water on a shelf or a windowsill, and it is the one people forget. Water enters there and the piece deteriorates from the bottom up.
• Believing a waterproof claim on the packet — Some air-dry clays are more water resistant than others and none of them is watertight. Testing an offcut in a bowl of water overnight settles it definitively.

If it doesn't work

Questions people ask