How to Seal Jesmonite®: Waterproof Your Casts Guide

Why you should seal Jesmonite®

Jesmonite® is a water-based mineral material. That is exactly what makes it lovely to work with, but it also means the cured surface is open and absorbent. An unsealed piece will darken when it gets wet, pick up rings from cups, and stain from anything oily or coloured.

Sealing solves three problems at once. It protects the surface from water and stains, it makes the piece far easier to wipe clean, and it can deepen the colour or add a subtle sheen depending on the product you choose. In short, sealing is what turns a raw cast into a finished object you can actually use.

One important point for beginners: Jesmonite® is not waterproof on its own. Do not put an unsealed piece in regular contact with water, and never assume a decorative cast is dishwasher-safe. Seal it before it meets water, every time.

Prefer to skip sealing altogether? Reslin®, a plant-based casting resin, is naturally waterproof because it is oil-based, so finished Reslin® pieces need no sealer at all.

Which sealer for which Jesmonite®

Different Jesmonite® variants call for different sealers, so start by matching the product to your cast. You can browse the full range on the accessories page, but here is the short version.

• Jesmonite® AC100: use an acrylic sealer, available in satin and gloss. It sits on the surface, gives a soft satin or glossy finish, and adds everyday protection against water and stains for indoor homeware.
• Jesmonite® AC730: use a penetrating sealer or a Flexiguard sealer. These soak into this tough, cement-based composite and provide invisible water protection without changing the natural stone look.
• Jesmonite® AC84: a versatile, weather-resistant cement-based variant with a smooth or acid-etched finish. It works with all four options, satin, gloss, penetrating and Flexiguard, so choose your sealer based on the effect you want and where the piece will live.

If you are ever unsure, the rule of thumb is simple: an acrylic sealer (satin or gloss) for AC100, and a penetrating or Flexiguard sealer for the cement-based variants, with AC84 happy to take any of them.

Sealing indoor pieces

Most makers start with indoor homeware such as trays, coasters, vases and candle holders cast in AC100. For these, the acrylic sealer is your friend. It gives a gentle satin or glossy sheen, lifts the colour, and protects against splashes and the odd spill.

One thing to keep in mind for candles: Jesmonite® base products (Jesmonite® base plus Jesmonite® liquid) are made to withstand the heat of a candle and will not melt, so a base cast is safe as a candle vessel. Sealers, however, are not fire-tested, so do not apply a sealer to a piece that will hold a burning candle. Using one on a candle piece is at your own risk.

Indoor sealing is forgiving, but it still has limits. A sealed AC100 coaster will shrug off a sweating glass, yet it is not built to be submerged or scrubbed under a running tap. Treat sealed indoor pieces as splash-resistant rather than fully waterproof, and they will look good for years.

Working with AC100 for the first time? Our Jesmonite® starter kits are an easy way to get everything you need to cast and finish your first pieces.

Making pieces suitable for outdoors

Outdoor use is a bigger ask. A piece left outside faces rain, frost, sun and constant temperature swings, so both the material and the sealer have to be up to the job.

For genuine outdoor durability, choose a variant built for it. Jesmonite® AC730 is a tough, cement-based composite with a stone-like finish that is suitable indoors and out. Jesmonite® AC84 is another versatile, weather-capable option. Seal AC730 with a penetrating or Flexiguard sealer so the protection soaks in and stays invisible against the textured surface.

Even the best sealer is not permanent. Outdoor pieces take a beating from the weather, so plan to re-seal them periodically to keep the protection fresh.

Step-by-step: sealing your cast

Once your piece is fully cured and dry, sealing is straightforward. Always follow the specific instructions on your sealer, but the general process looks like this.

1. Cure fully. Let the cast finish curing before you seal. Demould times vary by variant, around 30 minutes for AC100, 2 hours for AC84 and 4 hours for AC730, and all three keep gaining strength until they are fully cured after 24 hours, so give your piece a full day before sealing.
2. Check it is sanded and smooth. Sanding is easiest right after demoulding, while the cast is still soft, so by now your piece should already be smooth. Touch up any remaining rough edges or seam lines with a medium grit such as P150 sandpaper on a sanding block.
3. Clean and dry. Wipe away all dust and let the surface dry completely. Any dust or damp trapped under the sealer will spoil the finish.
4. Apply thin coats. With the acrylic sealer (AC100) you can use it neat, or mix it 50% water and 50% sealer. Apply it by dunking the piece, brushing it on, or using a sponge, many makers find the water-diluted mix leaves fewer streaks. Either way, keep the coats thin and even, as heavy coats can pool, streak or stay tacky.
5. Let it dry, then recoat. Allow each coat to dry as directed, then add a second (and, for water-exposed pieces, a third) coat for extra protection.
6. Cure before use. Give the sealer time to cure fully before the piece meets water or heavy handling.

Test on the base or an offcut first if you are trying a new sealer. That way you can check the finish before committing to the whole piece.

Care and maintenance

Sealed Jesmonite® is easy to live with. Wipe pieces clean with a soft, damp cloth and a little mild soap, then dry them off. Avoid abrasive scourers, harsh chemicals and soaking, all of which wear the sealer down over time.

Keep sealed indoor homeware away from prolonged standing water, and never treat decorative casts as dishwasher-safe. For outdoor pieces, check them each season and re-seal when the surface starts to look thirsty or begins absorbing water again. A few minutes of upkeep keeps your work looking its best for a long time.

Ready to protect your next project? Grab the right sealer for your cast on the accessories page, or start from scratch with a Jesmonite® starter kit and finish your pieces the way they deserve.