How to Sand Jesmonite®: Reveal Terrazzo, GlassRoxx & StoneRoxx

What sanding actually does

It’s a common misunderstanding: sanding doesn’t polish Jesmonite® to a smoother finish. It removes the thin top layer of the cast – which is exactly what you want when there’s something underneath to show. On a plain piece there’s nothing to reveal, so there’s no reason to sand.

Do you need to sand?

• Solid colours & marbled pieces – no. The colour and swirl run all the way through, and the surface is already smooth from the mould. At most, give the bottom a quick sand so it sits flat and even.
• Terrazzo, GlassRoxx & StoneRoxx – yes. The chips and speckle sit just below the surface. Sanding the top layer back is the only way to bring them out – without it they stay hidden and the piece looks plain and cloudy.
• Cement-based AC730 & AC84 – optional, for looks. Sanding these back gives a robust, etched, stone-like finish – raw and concrete in character rather than polished.

Revealing terrazzo, GlassRoxx & StoneRoxx

This is where sanding earns its place. Work the surface back until the chips and speckle sit flush and show clearly and evenly across the piece. Keep going until the pattern is as bold as you want it – the more you take off, the more comes through. (New to it? See our terrazzo effect guide.)

A robust, etched look on AC730 & AC84

On the cement-based variants, sanding is a finish choice rather than a reveal. Take the surface back for a raw, stone-like texture – the more you sand, the more etched and rugged it looks. (You can get a similar effect by etching with acid or, more gently, household vinegar.)

How to sand: wet vs dry

Whenever you do sand, do it as soon as possible after demoulding, while the cast is still relatively soft – fresh Jesmonite® sands far more easily than a piece that has fully cured (after about 24 hours).

• Wet sanding (recommended) – dip the sandpaper (or the piece) in water as you work. The water keeps the Jesmonite® soft, so it sands more easily and evenly – and it keeps the dust down. It’s the easiest way to reveal terrazzo and Roxx.
• Dry sanding – you can sand dry too, but it’s harder work: without the water the surface is tougher to cut, and it throws up fine dust. Wear a dust mask and work in a ventilated space.

Which sanding sponge to use

Always work in small circular motions, and go from coarse to fine. Sanding sponges are ideal – they flex around curves and edges, and come in three grits:

1. P100 (coarse) – start here to really scrub away the top layer and reveal the terrazzo, GlassRoxx or StoneRoxx underneath.
2. P150 (medium) – smooth out the coarse scratches and even the surface.
3. P240 (fine) – finish here for a clean, refined result that isn’t too rough.

For the etched look on AC730 or AC84, you can simply stop after the coarse P100 sponge.

Seal to bring the shine and colour back

Sanding a terrazzo, GlassRoxx or StoneRoxx piece leaves the surface looking a little dull – that’s normal. Once it’s fully cured (24 hours), add a coat of sealer: it brings the shine back to GlassRoxx and StoneRoxx and the colour and depth back into terrazzo, so the pieces really pop again.

Common questions

Does sanding make Jesmonite® smoother? No – the mould already gives you a smooth surface. Sanding removes the top layer to reveal terrazzo, GlassRoxx or StoneRoxx underneath, or to give the cement-based variants an etched look.

Do I need to sand a plain piece? Usually not – just a quick pass on the bottom so it sits level. Save your sanding for pieces with something to reveal.

Can I sand out an air bubble? Small surface bubbles, yes. For deeper holes, fill them with a little fresh Jesmonite®, let it set, then sand back.

Everything you need to cast and finish

From base and liquid to GlassRoxx, StoneRoxx, sanding pads and sealers, it’s all on GigaCrea.

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