Is Jesmonite® Waterproof? (And How to Make It)

The honest answer

Raw, unsealed Jesmonite® is porous. Because it’s water-based, it draws water in rather than beading it off – a quick wipe is fine, but any water left sitting on the surface soaks in and leaves a water stain. Sealing changes that completely: the water stays on top, so the piece wipes clean and stays unmarked. So:

• Unsealed: keep it to dry, decorative use – away from standing water, damp and spills.
• Sealed: water stays on the surface, so it’s safe for vases, soap dishes, bathroom and kitchen pieces, and outdoor use with the right variant.

Jesmonite® is not designed for constant submersion – it’s a decorative casting material, not a swimming-pool liner.

How to make Jesmonite® water-resistant

Sealing is quick and makes a big difference. For AC100 (our indoor variant), use the acrylic sealer, available in satin or gloss:

• Use it neat, or thin it 50% sealer / 50% water – the diluted mix often leaves fewer streaks.
• Apply by dunking the piece, brushing it on, or wiping with a sponge.
• Always seal after the piece is fully cured (24 hours) – and after any sanding.

Or reach for a wax paste. It seals the surface too – and because it goes on top of the cast, it’s a lovely way to give a piece a new life: cover a colour you’re not keen on, disguise a water stain, or add a gold, silver or bronze metallic sheen. Rub it onto the dry, cured piece with a gloved finger, sponge or brush, and let it dry.

Outdoor & heavy-water use

Going outside, or somewhere that gets properly wet? Reach for a weather-resistant variant:

• AC730 / AC84 (cement-based) for outdoor pieces – sealed with a penetrating sealer and Flexiguard for full weather protection.
• AC100 is indoor only – keep it out of the rain and frost.

See our guide on plant pots for indoors and out for the full outdoor breakdown.

Vases & anything that holds water

For a vase, you’ve got two easy routes:

• Fresh flowers – seal the inside well (a couple of coats), so it holds water without soaking in.
• Dried flowers – no sealing needed; use the piece as-is.

Quick FAQ

Will water ruin an unsealed piece? Not instantly – but water left sitting on it soaks in and leaves a stain. Seal anything that will meet water.

How many coats of sealer? One is enough for light use; two or three for vases and wet areas.

Can I make it fully waterproof? You can make it very water-resistant with sealing. For permanent water contact, seal thoroughly and avoid leaving it standing full for long periods.

Get the right sealer for your piece

Not sure which sealer suits your variant? We stock the full range.

👉 Shop Jesmonite® sealers