is a widely used phrase to define any type of media, that can be used to attach a text or a picture onto a subject.
In this paragraph we will only deal with the water soluble type of decals, not the self adeshive.
"Water slide decals", as they often are referred to; can be sub divided in three main categories, depending on the way they are printed.
1). Screen printed.
2). Off-set printed.
3). Ink-Jet printed.
All three types have advantages and disadvantages.

Ad. 1). :
1a. Opacity is very high. i.e. You can pose a decal in any colour on a dark object, and the colour of the object will not shine through the decal.
2a. High flexibilty.  Almost without any help from decal softener, you can pose a complex decal to a curved surface without any wrinkles.
3a. Well suited for any quantity. With decent starting costs, and decent production costs. Screen printing is suited for small as well as large quanitites.
4a. Long lived. Modern screen printed decals can, when stored properly, last up to 20 years or more, without loosing any qualities.
5a. Resolution is not as high as 2) and 3), depending on which sort of repro You use.
6a. Slow production time, the more opaque the colour is the longer is the drying time. Up to 12 hours for the slowest drying  colours.
Ad.2). :
1b. Very high resolution.
2b. Quick drying time.
3b. Extremely high starting costs. Off-set is not well suited for
runs below 1.000 pieces.
4b. Only fair opacity.  You must underprint Your decals with white to uptain a decent opacity.
5b. Flexibility can be poor. This means You must use a lot of decal softener to be able to mount an off-set printed decal to a complex surface.
1c. Fairly high resolution.
2c. Well suited for runs below 25 pieces.
3c. Extremely poor opacity. Ink-jet printed decals can only be used on white (or yellow) surfaces. 
4c. Far too expensive for large runs. 


We will here deal with problems, that can occur to decals, and how to solve them.

1. Yellowing.
2. Stiffness.
3. Cracked decals.
4. Decals out of size.
5. Decals with poor registration.

Today yellowing of decals is only occurring when a wrong type of carrier warnish is used.
It is often seen on old types of decals, as yellowing is a result of oxidation, or the influence of thalates (the softener found in plastic bags (or old tires, that "sweat")).  To prevent yellowing allways take Your decals out of the plastic bag at once, and remove them from "sweating" tires, (and do not forget to put them in a flat and dry place); for instance a photobook serves as a brilliant decal saver.
If You have a decal sheet, that has turned yellow, You can either partly or totally remove the yellowing of the warnish, by hanging Your decals in the window, facing out, with a piece of cellotape, and expose them to the sun for several weeks. This will bleach the warnish on most types of decals.

Stiffness of decals is also a result of using the wrong type of carrier warnish.
In most cases, the use of decal softener is the answer.
Following are the three most widely used:
2. MICROSOL (red or blue).
The use of these three types normally solves the problem.
You can also, in addition, put the decal in very hot water, but be carefull, and try with a sample first, as this operation may soften the decal too much.
It is commonly known, the warmer the water is, the more soft will the decal be when mounting, but the flexibility disappears again when the decal cools off, so you must be fast when applying a decal with this method.
If You are applying for instance a long stribe around a model, it is advisory to use a brush to apply the softener, as you work Your way around ths model.

Cracked decals is the result of stiff decals stored wrong in the box; bended or folded.
You can with a trained eye see if the decal is cracked before You dissolve them in water, being put into water it is too late to repair.
Use the following tip to repair cracked decals:
Fold out the decal sheet with 4 pins on a piece of wood or cardboard; one in each corner, and spray the sheet with a clear automotive warnish, and let dry well. Before applying, cut along the motif and dip into water as usual, but note that it takes a little longer for the water to penetrate the decal paper. And for gods sake do NOT apply warnish to the back side of the decal sheet, as the water will not be able to penetrate the decal paper.

Decals out of size, is very hard to repair, especially if they are too large. What concerns stribes, they can in some cases be cut up, and applied as a patchwork, but it is time consuming and stressing.
Stribes too small, might also be cut up, an filled between the edges with a carefully mixed tin can colour.

Poorly registred decals is hard to repair too, they migth be cut apart and replaced with each other, normally they only can be replaced by aftermaket decals, if the motif exists.



Not only have the decals turned yellow (and the car must be white!!), the artwork is far below, from what You might expect today.


Well laid-out decals for an allmost identical car. Unfortunately decals from this manufacturer have shown a tendency to crack. so overspraying with clear warnish is mandatory.