Apply stickers to glass: step by step, without mirror writing

On Friday the lettering for the promotion week goes up in the shop window. By Monday it has bubbles, one corner is lifting, and from the pavement it reads as mirror writing. It did on Friday too, only nobody looked from outside.

Anyone who wants to apply a sticker to glass usually thinks of bubbles first. A squeegee will usually prevent those. The costlier mistakes happen earlier: when deciding which side of the pane it goes on, and when cleaning beforehand.

This guide follows the application guidelines of the vinyl manufacturers. Where the data sheet for your self-adhesive vinyl says otherwise, it takes precedence.

Before you order: inside or outside the pane?

A sticker for the outside is printed normally. If it is to go on the inside and be readable from outside, the design has to be printed in reverse on clear vinyl and backed with white, either with white ink or with white vinyl behind it.

On the inside the sticker is better protected from weather and damage. For large dark designs on sunny panes there is a good case for the outside; more on that below under glass breakage.

Then there is the vinyl itself: depending on the type, it is made for the inside or the outside. The data sheet says which. When you order, tell us where the sticker will be read from. In our free file check we look at your file before printing, so let us know whether it goes on the inside or the outside.

Applying stickers to glass: clean and degrease the pane

Vinyl sticks to whatever is on the pane. Fingerprints, limescale and the film left by the last cleaning product have to go:

• Scrape: remove old deposits with a new blade. Use the stainless steel blade only on ordinary glass, held flat on a wet pane. With coated glass and safety glass, ask the glazier first; on car windows with heating wires or an antenna, never. If in doubt, use only a plastic scraper.
• Wash: clean with soapy water and remove the water with a squeegee.
• Degrease: apply a mix of 70 % isopropyl alcohol and 30 % water, available ready-made or mixed from pure isopropyl alcohol, with a lint-free cloth. Wipe dry before the solution evaporates and let it air off well. Isopropyl alcohol is highly flammable, so ventilate and keep naked flames away.

Leave out glass cleaners with silicone, ammonia or wax. They contaminate the surface and the vinyl adheres less well. The same goes for methylated spirits. On the window frame use only mild products, or the rubber seals will suffer.

The water drop test: spray a little water on the cleaned pane. If it beads up, there is a coating or cleaner residue on the glass. Degrease again and repeat the test. If it still beads, it is probably a coating that may interfere with adhesion. Some coatings are a fixed part of the glass, so do not scrub anything off. Do a test application and check adhesion after 24 hours, or ask the glazier or window manufacturer.

Temperature: do not apply below 10 °C air and glass temperature. According to one manufacturer, wet application should only be done from about 18 °C, ideally at room temperature. The exact limit is in the data sheet. Put the sticker in the room where you will apply it a few hours beforehand.

Dry application step by step

For most applications the vinyl manufacturers recommend dry application. You need a plastic squeegee, ideally with a felt edge, plus masking tape and adhesive tape. Large stickers go onto the pane with the hinge method, so that as few bubbles as possible remain:

1. Position. Line up the sticker, backing paper and all, and fix it with strips of masking tape. Leave a few millimetres of space to seals and silicone joints, otherwise the vinyl will come loose there.

2. Make the hinge. A strip of tape goes along one whole edge and holds the sticker like a door hinge.

3. Remove the backing paper. Fold the sticker back and peel the paper off in sections of 20 to 30 cm. Pull the paper away from the vinyl, not the other way round, and do not crease anything.

4. Squeegee. Work away from the hinge, each stroke from the centre to the sides, firmly and overlapping by about half. That way the air moves out to the edge.

5. Finish. Remove the hinge and go over all the edges again with the squeegee.

Small stickers and labels on glass do not need a hinge. Peel back 2 to 3 cm of backing paper along one edge and fold it over, press this edge down from the middle, then pull the paper out bit by bit and squeegee as you go.

Lettering and logos cut from vinyl on a plotter come on application tape. It holds the letters and spacing in position. That way the lettering goes onto the glass in one piece, not letter by letter. Squeegee before and after removing it, and peel the application tape off slowly and flat, almost parallel to the pane.

Wet application: when it helps and when it does not

On smooth glass wet application is possible, but only if the vinyl's data sheet expressly allows it. The advantage: you can slide and align the vinyl for longer without it distorting.

Perforated window film is never applied wet, because the water stays in the holes and clouds the view through. Vinyl with air channels is designed for bubble-free dry application, so that is out too. For some printed window films the manufacturer specifies dry application. With some clear films the adhesive turns milky with water, so check the data sheet first.

The application fluid is almost entirely water, plus a few drops of mild washing-up liquid without moisturisers, perfume or other additives. The exact amount is in the vinyl's instructions; too much washing-up liquid weakens adhesion. Spray finely from a spray bottle. If it is too wet, the vinyl slides away and water bubbles form.

Start at the top in the middle so that no water gets trapped, and squeegee outwards. Adhesion only builds up once the water is out or has evaporated. So squeegee thoroughly, wipe the edges and pane dry and press the vinyl down again after a few hours. For lettering, application paper is used here and peeled off carefully after a short drying time.

Fixing bubbles under the vinyl

Applied dry: prick the bubble with a fine pin and smooth the air from the edge towards the hole. No knife, no cutter, or the vinyl may tear.

Applied wet: do not prick or press out water bubbles under window film. According to one manufacturer they disappear by themselves, usually within a few weeks, faster in warm weather. The same manufacturer advises not to touch wet-applied window film for 24 hours and not to clean the window for about a month. According to this manufacturer, streaks and slight haze are normal in wet-applied window film during this time.

Later, clean the pane with mild soapy water and soft cloths, without ammonia. Alcohol only goes on the glass before application, never on the printed vinyl.

Special cases on glass

Glass doors and large areas

On a glass door the hinge is worth it: the vinyl hangs firmly and you work section by section. If the area is covered in several panels, printed window film is butted edge to edge, not overlapped, according to one manufacturer.

Vehicle windows

Rear and side windows of vehicles are subject to their own rules, which this article does not cover. If you want graphics on a vehicle, you will find our service under vehicle graphics.

Coated glass, insulating glass and glass breakage

Large dark or highly opaque designs on sunlit glass heat the pane unevenly. That can cause stress cracks and even lead to the glass breaking. Insulating glass that is fully or partly covered can also be damaged by temperature changes.

Whether a pane cracks depends on many factors, such as shading, heating, pane size and type of glass.

The vinyl manufacturers' application guidelines generally exclude liability for glass breakage. With insulating glass, heat-insulating glass or coated glass, ask the glazier or window manufacturer first. The manufacturers also advise keeping large designs on sunny panes light, applying them on the outside and leaving a narrow margin free all round. This guide does not replace an on-site assessment.

The promotion lettering: stuck inside, read from outside

A typical case from retail: a shop is planning a promotion in its window, with logo and lettering, easy to read from the street. It is to go on the inside, because rain and slush reach the outside of the pane.

The print file arrives the right way round. Applied on the inside, the lettering would read as mirror writing from outside. For the inside, the design is printed in reverse and backed with white. You supply the file the right way round and state "inside"; we do the mirroring.

Then there is the design: a large black bar across the whole lower half, on a window that gets full afternoon sun. The first thing to clarify is whether it is insulating glass. A lighter area, or the outside of the pane, is the safer choice either way, but then with normally readable print.

A bubble can be rescued, a mirror image cannot

We will commit to this: on glass, the side decides, not the adhesive. A bubble can usually be fixed afterwards. A design printed the wrong way round cannot. Clarify reading direction and sun beforehand and you avoid that detour.

We still have a view on the adhesive. One vinyl manufacturer recommends removable adhesives for glass, because they leave less residue when taken off later. We agree: on glass they are often the better choice. Removable does not mean nothing is ever left behind, though. If lettering is meant to stay up for many years, we discuss the material case by case.

You will find an example of a shop window promotion in Stickers you can't overlook. Privacy films and removable promotional films are under Foil technology.

Before you order: four questions about the pane

If the pane is properly degreased, the temperature is right and you squeegee from the centre outwards, the sticker will usually sit neatly. Before you order, check:

• Inside or outside: where will the sticker be read from, and does it need reverse printing with white?
• Wet or dry: what does the vinyl's data sheet allow?
• Which glass: does water bead up? Is it insulating glass?
• Which position: is the pane in the sun?

Rather not apply it yourself? Then our sign-making team will do the installation. Write to us via Contact and tell us where the sticker is going. Custom stickers can be ordered online at koeck-print.at.

Frequently asked questions

Can I apply stickers with soapy water?

Yes, if the vinyl's data sheet allows wet application. Use water with a few drops of mild washing-up liquid without additives and spray finely. Too much washing-up liquid weakens adhesion. Perforated window film and vinyl with air channels are only applied dry, and with some clear films the adhesive turns milky when applied wet.

Should the sticker go on the inside or the outside of the pane?

On the inside the sticker is better protected from weather and damage. To be readable from outside, the design is printed in reverse on clear vinyl and backed with white. Large dark designs heat a sunny pane unevenly, and in the worst case it cracks. Keep such designs light and preferably apply them on the outside; with insulating glass, ask the glazier first.

How do I get bubbles out from under the vinyl?

With dry-applied stickers, prick the bubble with a fine pin and smooth the air towards the hole. Do not use a cutter, or the vinyl may tear. Leave water bubbles under wet-applied window film alone; they usually disappear by themselves within a few weeks.