Printing directly onto acrylic and glass, usually on a UV flatbed, produces striking display pieces: photo panels, signage with depth, splashbacks, awards and backlit features. Customers sometimes assume a protective coat is the natural final step. For these two substrates, it often is not, and it is worth understanding why before offering it.
Acrylic and solvents
Acrylic (PMMA) is well known for being sensitive to many solvents. Contact can cause crazing, a network of fine cracks in the surface, particularly where the sheet is under stress near cut or drilled edges, fixings or bends. Crazing may not appear straight away. Polycarbonate can behave similarly. For this reason we advise real caution about applying a solvent coating such as Colourjet to acrylic, including on unprinted areas and edges the coating might reach.
If you do want to explore it, test on an offcut of the same sheet, cut and fixed the same way as the finished piece, and leave it for a good while before judging. Inspect the edges and around any holes under a strong light.
Glass
Glass is not attacked by solvents, but its hard, non-porous surface makes adhesion difficult, both for ink and for any coating. Direct-to-glass printing often relies on adhesion promoters or primers, and a coating over the top adds another layer that needs to bond. Any weakness can show as peeling at edges, especially where the piece is cleaned regularly or exposed to moisture, as with kitchen splashbacks.
Reverse printing: protection by design
Many acrylic and glass pieces are reverse printed, viewed through the clear sheet with the ink on the back. In that case the viewing face is already protected by the substrate itself, which is one of the attractions of the method. The ink side is then usually sealed by a white backing layer, mounted against a wall or panel, or bonded to another material. A liquid coat on the back may add little and introduces the risks above.
Our practical advice
- Acrylic and polycarbonate: generally avoid solvent coatings unless thorough testing shows no crazing.
- Glass: consider whether reverse printing removes the need for a coat; if coating, test adhesion carefully.
- For first-surface prints that need protection, discuss with your ink and substrate suppliers which overcoats they recommend for that combination.
- Where a protected first-surface finish is essential, a coated print on media, mounted to the panel or framed, can be a simpler and safer route.
Our page on rigid boards covers substrates that usually take a coating more readily, and UV-curable inks explains the ink side. For the general method of trying a coating safely, see sample testing.
We would rather talk you out of a coating than see a panel craze after installation. If you have a specific acrylic or glass job in mind, call 01252 624 411 to discuss it.