How can I get a textured granite finish for a 3D printed concrete speaker base?

To get that granite effect, you realy need to look at white Portland cement combined with a mix of light and dark aggregates rather than just relying on a grey base. Using a standard grey bag mix with charcoal dye usally ends up looking like a muddy slab. If you go with a white cement base and mix in crushed basalt or black granite sand, the contrast is what creates that high-end stone look. Keep your pigment load under 5% of the total cement weight. If you go higher than that, you'll definatly start compromising the structural integrity, and for heavy speakers, you need that density.

Strength won't be an issue if you use a high-strength countertop mix or add a water reducer. This allows you to use significantly less water, resulting in a much denser, stronger cure that can handle the weight without cracking. If you're really paranoid about it, you can toss in some alkali-resistant glass fibers. They don't show up on the surface if you vibrate the mold properly, but they provide a massive boost to the internal structure.

Don't bother trying to sand the layer lines off the finished concrete. It's much easier to prep the 3D printed mold first. Hit the interior of the mold with a few heavy coats of **high-build automotive primer**, sand that smooth with 400 grit, and then apply a light mold release. If your mold surface is perfectly smooth, the concrete will mirror that finish right out of the pour. If you still see lines, you can wet-sand the concrete starting at 200 grit and working your way up. This also helps expose the aggregate, wich is how you get that speckled granite aesthetic. For the finish, stay away from the cheap acrylic sealers at the hardware store becasue they leave a plastic-looking film. You want a **penetrating sealer**, specifically a lithium silicate or a silane-siloxane. These soak into the pores and protect the surface from oils and dust without adding any shine. It keeps the stone looking raw and matte while making it feel incredibly smooth to the touch.

I've been looking into this for a desk lamp project and tried using regular spray paint on a PLA print, but the concrete basically bonded to the plastic. It was a nightmare to get out. Does that automotive primer act as a barrier, or shold I still use somethig like paste wax? Also, if I add those glass fibers for the weight, does it make the mix way thicker? I'm worried about getting air bubbles stuck in the tight corners of a 3D printed mold if the mix doesn't flow well.

Vibration is key if you’re using fibers. I usally zip-tie an orbital sander—minus the sandpaper—to the side of the mold or the workbench. It rattles those stubborn air bubbles right to the top.

For release, skip the hardware store waxes. Grab some Pol-Ease 2300 or a very light wipe of mineral oil. Paste wax can leave a cloudy residue that messes with how your penetrating sealer bonds. Also, if the mix feels too thick with the fibers, don't add more water. Just use a drop more superplasticizer; it’ll make the concrete flow like a milkshake without ruining the strength.

If you're worried about those bubbles when using glass fibers, try adding a tiny bit more superplasticizer to the mix. It makes the concrete flow like heavy cream into the 3D printed details without needing crazy vibration.

Since these are for bookshelf speakers, definitely glue some 1/8 inch cork or thick felt to the bottom once it's cured. Concrete is super abrasive and will chew up your speaker cabinets or your furniture if they shift at all. I also recommed using diamond hand polishing pads for those rounded edges; they give you way more control than a power sander so you don't accidentally flatten your curves.

If you're going for that basalt speckle, watch your timing on the wet-sanding. If you start grinding too early—especially with a high-strength mix—you can acutally pluck the aggregate pieces right out of the cement paste, leaving tiny pits instead of a smooth finish. Give it at least 72 hours of wet curing under some plastic before you hit it with the 200 grit. Also, check your 3D print settings. Concrete is heavy, and if you're vibrating the mold, a thin PLA shell will bow out. I usually do 4 or 5 wall loops and a dense gyroid infill. It keeps the edges crisp so your speaker sits perfectly flush.