There are a number of considerations to ensure a surface is ready for a durable vinyl graphics application, that should under ideal circumstances (such as good surface and normal European weather conditions) last 3-5 years or longer outdoors, depending on the grade of vinyl used, and even longer indoors.
It is important to know what the surface being applied to is made up of, and how it has been treated, e.g. with paint, varnish, potentially residual cleaning fluids or other more unusual treatments such as French polish or wax. Common "bare" materials used for sign making include plastics like acrylic (Perspex), Correx, aluminium and many more. Such surfaces do not need to be treated provided they are clean, and so are a popular choice. Wood, if used, is usually treated with a varnish or combined primer/undercoat with a satin or glossier finish for best adhesion of the vinyl graphics to this layer. Matte finishes can still work, but it depends highly on the quality of the paint's composition. For outdoor applications, it's especially important that the paint is rated for outdoor conditions such as wet, damp and sun exposure.
Self-adhesive vinyl graphics, that are cut out from larger rolls of vinyl (PVC) film, are backed with either an acrylic or solvent-based adhesive. These adhesives vary in strength from a tacky but removable temporary stickiness, common for short-term exhibitions, to extremely high tack adhesives that are suitable for use on sports cars, boats and even planes.
Whatever the type or grade of vinyl or how long it is rated to last in different conditions, there are two surface aspects that are key to a successful application that will withstand time and the elements:
1. Total surface area (surface smoothness or roughness)
Generally, the smoother the surface to which vinyl graphics will be applied, the more surface area is available for the adhesive to form a bond. Therefore, a very smooth surface such as glossy plastic, glass, bare metal or a well varnished, satin or gloss-painted wood provides a strong bond. Conversely, a relatively rough surface, such as a painted surface with large brush strokes, rough wallpaper, rough plasterwork or even brick will offer OK to poor adhesion. This is because the vinyl is generally less able to conform to the rough shapes at a microscopic level, and so the adhesive bond is less strong. This can be handled using very expensive cast vinyl or vinyl especially formulated for this purpose, but usually it is easier to make sure the surface itself is good.
2. Surface solidity (strength or soundness of the topmost layer of the surface)
Sometimes surfaces have a poor integrity to which many pressure-sensitive adhesives (such as tape) will struggle to "stick". For example, a surface might be spotty with oil, grease or waxy substances, or it might be powdery or flaky. This can be cleaned using household cleaning products or chemicals such a white spirit, isopropyl (rubbing) alcohol or acetone. On the other hand, a powdery, dusty, or otherwise not solid and sound surface is problematic because this can mean the surface is not good to begin with. For example, paint may not (and may never) fully cure, if it was not well mixed in the tin due to separation, it is constantly exposed to damp, or it has not been given adequate time to dry fully, which can sometimes take weeks. Some formulations of matt emulsion paint are inherently powdery in nature. In this case the vinyl's adhesive layer may not be able to form a bond at all or application will prove very difficult.
A simple test for surface solidity can be performed with just a piece of sticky tape (masking tape works well). Rub a piece of tape down firmly on a small, inconspicuous area (this might leave a mark), wait a few moments for the adhesive to bond, and then tug it away. You can gauge how solid the surface is by seeing how much is removed on the sticky side of the tape. Ideally, you want to see little to nothing here, as this roughly emulates what the much stickier adhesive of the vinyl graphics needs to "grip" onto. Durability is hampered if the vinyl graphics can only adhere to a layer that is just "sitting" on top of the surface.
What happens if we try the application anyway (i.e. time limitations)?
Even if an application is possible with enough skill (carefully and greatly applied pressure in the right places, conforming the vinyl film into the "grain" of a rough surface, etc), the durability of the finished graphic is unpredictable. It is better, in a pinch, to seal and/or smooth the surface completely with a reliable agent such as watered-down PVA glue or primer/blockout paint (if it is outdoors, an exterior-rated product must be used). Ideally this should be done when the original surface has been allowed to cure as much as possible (for example moving it to a dry space for a while or perhaps treating it with heat). Otherwise, "onion-skinning" (cracks and bubbles between the layers) may occur over time. If there is not time to prepare the surface fully, for example because of an upcoming event or opening, this may be a risk to take, nonetheless the long term durability implications are there.
If the current treatment of a surface is not curing or is just not good (for example it is powdery, soft or sticky to the touch or flaking away), even after being given time to dry, then it should be stripped away with a scraper, perhaps with the aid of heat/chemicals, or sanded off (synthetic wool AKA Scotch-Brite pads are great for this). Then replace this layer with a product that will perform well. This will need to be done if interior grade paint is used outdoors, or a powdery matt emulsion has been applied on top of a glossy substance such as varnish or oil paint (enamel), or an existing graphic, where it cannot bond. Applying successive layers of any material to this, whether vinyl graphics or something else, is going to create problems, so wherever possible, pare back to a sound layer.
Unfortunately, not all paints, varnishes and other treatments are made equal, and some bare or treated surfaces are challenging to impossible to apply vinyl graphics to, and so generally it is important to make sure a substrate destined for vinyl graphics is good for it. Otherwise, the surface is likely to compromise the end result, even if it takes a long time for problems to show. If in doubt, enquire about pre-preparing the surface you want to use, covering it with a suitable plate or "block-out" layer, or fitting the sign to something different that will work best.