UV printing on acrylic and glass: white underbase, mirroring and file prep

Why these substrates are different

On a white substrate the material does the work: CMYK inks are translucent, and the white behind them makes the colours look right. Acrylic and glass take that away.

• Black or coloured acrylic absorbs the light that would have bounced back through the ink. Colour printed directly on it looks dark and muddy.
• Clear acrylic and glass let the light straight through. Colour printed directly on it looks like stained glass: pretty, but not opaque, and it changes with whatever is behind the piece.

In both cases the answer is a layer of opaque white ink, and the whole craft of printing on these materials comes down to where that white goes relative to the colour.

The three layer orders

1. Dark or opaque substrate: white under colour

The standard underbase. The printer lays white first, then the colour on top. You look at the printed side. Use it for black acrylic, coloured acrylic, frosted or painted glass. The white should sit slightly inside the colour (choke) so none of it shows at the edge; this guide explains how much.

2. Clear substrate, viewed from the printed side (first surface)

Same order as above: white, then colour. The print sits on top of the clear material and you look at it directly. Simple, but the ink is exposed to handling and scratching, and the edges of the white layer are visible from the back as a plain white silhouette.

3. Clear substrate, viewed through the material (second surface)

This is the premium look for photo blocks, keychains and signage: the image is printed on the back and you view it through the acrylic or glass, which gives it depth and gloss and protects the ink. Two things change:

• The order flips: colour first, then white on top of it. The white becomes a backing instead of an underbase. From the front you see colour with white behind it, which is what you want.
• The artwork must be mirrored, because you're looking at it from the other side. Text printed unmirrored reads backwards through the glass.

Some RIPs also offer a colour–white–colour sandwich for pieces meant to be seen from both sides, such as hanging signs. The second colour layer is the mirrored copy of the first. It's the same idea with one more pass.

Where to mirror

Mirror in the RIP, not in the design file. Nearly every RIP has a mirror or flip setting intended for second-surface work, and keeping the file unmirrored means you can print the same artwork first-surface tomorrow without a second file. If your RIP genuinely has no mirror option, flip the file in your design software, but flip every plate (colour and white) together so they still register.

One more check for second-surface jobs: the white plate must cover the colour completely. Any colour pixel without white behind it is a translucent spot you can see straight through. That's the opposite of the first-surface rule, where the white has to stay inside the colour. The file prep below handles both, because the white plate is built as the colour shape shrunk by a small, known amount; for second surface, set the choke to its minimum.

Adhesion, briefly

This guide is about the file, but a perfect file still fails if the ink doesn't stick:

• Acrylic usually bonds well with UV ink. Wipe it with isopropyl alcohol and a lint-free cloth to remove the mould-release film and fingerprints, and peel the protective masking right before printing.
• Glass is non-porous and chemically inert; most UV inks need an adhesion promoter (primer) wiped on and allowed to flash off before printing. Without it the print may look fine and then peel under a fingernail. Test on an offcut.
• Keep the print bed and the piece at room temperature; a cold piece straight from storage can cause adhesion and curing problems.

Why edge defects are worse here

On a dark shirt or a white tile, a slightly messy edge gets lost in the texture. On acrylic and glass it doesn't:

• Black acrylic is the classic white-halo showcase. Every semi-transparent edge pixel in the PNG gets white ink under it, and a thin bright rim traces the whole design. The surface is glossy and flat, so the eye goes straight to it.
• Clear acrylic and glass show the edge from both sides. A white rim is visible from the front and as a halo around the white backing from the back. Stray semi-transparent pixels away from the design print as tiny white dots floating in clear material.
• Second surface adds a third failure: any edge pixel with colour but no white behind it is a see-through speck at the border of an otherwise opaque image.

All three come from the same thing: semi-transparent pixels in the PNG, left behind by background removal and anti-aliasing. The printer is doing what the file asks. What ghost pixels are explains the mechanism.

File prep checklist

1. Final size at 300 ppi or more. Small pieces (keychains, tags) are inspected up close; 600 ppi is not excessive for anything under 5 cm.
2. PNG with transparency, no background layer. Any opaque background prints as a rectangle of white and colour.
3. Hard alpha edge. Run the PNG through Ghost Pixel Cleaner: every semi-transparent pixel is shown in green, then removed or promoted to opaque according to the threshold you choose. The edge colour is corrected so promoted pixels don't carry a tint from the old background.
4. Separate colour and white plates. The export ZIP contains _cmyk.png and _white.png. The white plate is always a subset of the colour plate, shrunk by the choke amount: use 1–2 px for first surface, the minimum for second surface.
5. Preview on the substrate colour. The editor composites the result on black, so you see the edge the way the acrylic will show it before you spend a sheet.
6. Mirror in the RIP for second-surface jobs, and set the RIP's own white choke to 0 since the plate is already choked.

Desktop UV printers: eufyMake E1 and similar

Desktop machines such as the eufyMake E1 have brought acrylic and glass printing to small shops and home businesses, and the same rules apply at that scale: white under colour for dark pieces, colour then white (mirrored) for second-surface clear pieces, and a clean file before any of it. Import the colour plate and the white plate as separate layers in the printer's software, assign the white plate to the white channel, and leave any extra white choke at zero. If the E1 community thread you're reading says "the halo is a printer problem", it almost never is; this guide walks through the E1 case specifically.

Frequently asked questions