Half the joints in a modern shopfront or fit-out are glued: flat-cut letters on a fascia, a trim on a reception desk, a composite tray on a subframe, a mirror on a wall. No visible fixing, no penetration of the finish, and nothing to look at when it goes wrong. Adhesive is a legitimate engineering joint, but only when it is sized like one. The word "glued" on a drawing is not a specification, and getting it wrong costs a scaffold and a reinstall.
Glue is a fixing with an area
Double-sided acrylic foam tape is the default in signage because it is instant, invisible and seals the joint at the same time. It also has a published load rating, which most people never look up. In shear, which is how a sign or a panel hangs on a wall, the working figure is about 55 square centimetres of tape per kilogram of dead weight: roughly 290 mm of 19 mm tape, or 430 mm of 12.7 mm tape, per kilogram.
Do that arithmetic once and the illusion goes away. A two-metre run of 19 mm tape carries about 7 kg in shear, which a 3 mm aluminium letter 400 mm high never troubles. A 2.5 m by 1.2 m composite tray with a folded return and a frame behind it is tens of kilograms, and the tape length needed comes out longer than the panel perimeter. That is not a product problem, it is the answer: the panel wants mechanical fixings, with tape doing the flatness and the sealing. Anything cyclical gets designed against dynamic load instead, where the working figure is about 85 kPa across the bond area: wind on an exposed elevation, a door leaf, a hinged lid.
The gap and the movement decide the thickness
Thickness is not a comfort choice. Foam tape swallows roughly half its own thickness in mismatch between the two surfaces, so it has to be at least twice the gap it bridges. On a wall out of plane by 2 mm across a panel, 0.6 mm tape makes contact in strips and the rest of the length is decoration. A second rule catches panel work: the bonded material should not be thicker than about twice the tape, which is why 1.1 mm tape is the normal panel-bonding grade for sheets up to around 2 mm.
The third rule is the one that breaks jobs. Movement in the joint must not exceed about three times the tape thickness. Aluminium moves roughly 1.6 mm per metre over a 70 degree surface swing, which is what a dark panel sees between a frosty morning and July sun. Bond a 3 m aluminium panel to a steel frame and the differential movement is a little over 2 mm, because the steel behind it moves about half as much. Against the three-times rule that means 0.8 mm of tape as an absolute floor, so the 1.1 mm grade is the honest choice and the thin roll on the shelf is not. Acrylic is worse again, close to 5 mm per metre, so bonded plexi wants a thick bond line or a joint that lets it slide.
Surfaces lie
Adhesive sticks to the top layer, not to the material underneath, and the top layer is usually paint. That is where most tape failures are born.
- Clean with isopropyl alcohol and water, roughly half and half, and let it flash off. Not thinners, not a panel wipe with silicone in it, not a dry rag.
- Scuff anything oxidised. A lightly abraded surface exposes substantially more area for the adhesive to flow into, and it is free.
- Anodised aluminium gets cleaned and nothing else. Abrading it removes the finish you were paid for.
- Powder coat is a lottery. General-purpose grades stick to most powder coatings, but "most" is not a specification, and a fluoropolymer coated composite skin behaves nothing like a polyester powder. Painted metal and low-surface-energy plastics, polypropylene and polyethylene among them, need a primer.
- Press it properly. The bond exists only where the adhesive wetted out, and that takes about a kilogram per square centimetre, applied with a roller and not with a thumb.
One bonus and one limit: the foam core keeps dissimilar metals apart and takes galvanic corrosion off the table, and none of these tapes belong anywhere permanently wet or submerged.
Cold metal and the three days nobody waits
Tape has an application temperature and it refers to the surface, not the weather app. Below roughly 10 to 15 degrees, depending on grade, the adhesive is too firm to wet the surface at all and the bond that feels fine on site never formed. A few grades go down to freezing, one of them only on clean metal. A proper bond then holds in deep cold without complaint, which is the cruel part: the joint made on a cold morning is the one that lets go in the first gale.
Strength also arrives on a timetable: roughly half at twenty minutes, ninety per cent at a day, full strength at three days. A part can be handled after ten minutes but should not be loaded for seventy-two hours. On a fascia that means the props or the temporary screws stay up overnight; in a workshop it means gluing up the day before dispatch, not on the van.
Wet adhesive is not fast tape
Cartridge adhesives solve what tape cannot: uneven substrates, thick bond lines, porous materials, long gaps. They bring their own rules.
| Product | Cures by | Paintable | Watch out for |
|---|---|---|---|
| Neutral-cure silicone | Air moisture | No | Contaminates surfaces for later painting |
| Acidic silicone | Air moisture, releases acetic acid | No | Mirror backing, bare and reactive metals |
| MS polymer hybrid | Air moisture | Yes | Will not bond to old cured silicone |
| Polyurethane | Air moisture | Yes | Isocyanate handling, weaker in sunlight |
The cure mechanism is the practical constraint. Moisture-cure products set from the outside in at something like 2 to 2.5 mm a day indoors. A 6 mm bead sandwiched between two sheets of metal has no air to work with and cures inwards from its exposed edges over weeks, which is why a thick continuous bead behind a sealed panel is a bad detail, and why the usual approach pairs beads of adhesive with strips of tape: the tape holds the panel and sets the bond line while the adhesive takes its time.
One hazard is missing from that table. Acrylic and polycarbonate craze under solvent attack, so a sealant touching either has to be confirmed compatible rather than assumed. Low-modulus neutral cure is the usual answer, but some products warn explicitly against both plastics.
Where adhesive stops being the fixing
The sign trade's own guidance is blunter than most specifications: tape is for non-loadbearing work, meaning lightweight panels, trims and small signs. Above head height, over an entrance, over a footpath, adhesive is not a fixing. It is a flatness and sealing system sitting alongside fixings that are sized, installed and where appropriate pull-tested, with a safety factor around four to one for anything overhead.
That is a design decision, not an installation one, and it belongs in the model. A bonded letter gets a drawing showing the hidden stud or the return flange that catches it when the bond is gone. A bonded tray gets the low-torque fixings into the frame, the oversized washers that spread the load and the expansion allowance the panel maker asks for. Composite panels delaminate around a careless hole, so that detail gets drawn with the bracket and the wall rather than improvised on a ladder, alongside the rest of the production pack.
What goes on the drawing
A bonded joint takes one line per joint, and the line contains numbers:
- Adhesive type and thickness, not "VHB" and not "glued": foam tape at a stated thickness, or a named chemistry with a bond line dimension.
- Bond area or tape layout, derived from the weight of the part, with the gaps left for the cure if anything is wet-applied.
- Surface preparation for both faces, including the primer where the substrate needs one.
- Minimum surface temperature at application, and the hold time before load.
- The mechanical fixing that shares or secures the load, sized, wherever the part is above head height or heavier than lightweight.
None of this slows a job down. It avoids the only conversation worse than a price argument, the one that starts with a panel on the pavement.
