This is an extract from an article I had published in Model Boat magazine a year or so ago. If you want to read the entire article it was published in two parts which can be obtained from the Back issues section of the magazine.
This is the section on washes:
Like so many things the term “Washes” covers quite a wide area in itself and a wash can be of any one of a number of mediums. The way to think of it though is that a wash will enhance areas of shadow. A wash consists of a solvent such as water, enamel thinners, turpentine etc. containing a small amount of pigment such as enamel paint, water colour paint, pastels, powdered pigments or even oil paint. The idea is to make the wash very thin or watery and will consist of only a few drops of pigment in a large quantity of solvent. Some people, including myself, have actually used dirty thinners from the brush cleaning jar as a wash, which is about the level of pigment you want and it actually works very well. The medium you decide to use for the wash is generally dictated by the finish of the surface you want to apply it to as basically you do not want the solvent to affect the base. If you are applying a wash over enamel paint you could use a water colour wash or a turpentine and oil paint wash but the important thing is that you do not use the same solvent as the base. One thing to remember is that if you are using a water based wash over a gloss surface it is always best to add just a spot of washing up liquid. This will break the surface tension of the water and stop it from forming beads that sit on the surface. If you see bubbles coming from the area of your model you may have used too much!
The colour of the wash will also very much depend on the model you are building and where the wash is to be applied. Areas around a coal bunker will almost certainly benefit from a dark colour, almost black, whereas a light coloured vertical bulkhead plate joints would look odd in such a stark colour so a much toned down brown would be more appropriate. Always good advice is to experiment with scraps until you are happy with the overall effect and try to imagine what the entire model will look like when completed.
There are two basic ways of applying a wash, one is by applying it all over a model and allowing it to flow into the shadow areas and the other is to apply it more carefully to areas of recessed detail, called a pin wash, only then remove the excess afterwards leaving only the detail with wash in it. This is the normal technique used on model aircraft panel lines but can just as easily be applied to plate joints on a boat.
Considering the first method, this may be for a model that requires heavier weathering such as a coaster, collier or barge where you want to simulate heavy use. The wash is best applied over a matt surface where it will also shade the surface as well as collecting in the recessed detail. If you are applying a wash such as this then you should apply an area in one session as successive coats will overlay the first and could cause patches and uneven cover. You should also bear in mind how gravity is going to affect the wash and such items as a box shape could well be done one side at a time to better control the coverage. A number of light coats could help you to build up the effect but if you are using a water based wash this will almost certainly dissolve the previous coats and you will loose control over the effect. The challenge with this type of wash is that it is a lot more difficult to remove if you are not happy as it is over a matt surface so possibly you need more experimentation beforehand before you commit to your model.
The second method should only be applied over a gloss surface so if your model is to be a matt finish then it will require a clear gloss coat first to apply the wash to. This can consist of a clear gloss enamel or even clear gloss varnish but one of the most widely used on the plastic model world is Johnson’s Kleer floor polish. This is basically no more than a clear gloss acrylic covering that dries thin and evenly without hiding detail or colour. It may well require two coats of the Kleer to give a reasonably glossy surface for the wash. Once the gloss surface has dried you can then apply the wash but not all over with a large brush but a bit more controlled over the recessed detail only. The wash will flow into the detail and dry and, because it is on a gloss surface, the excess can now easily be removed. If it is a water based wash this could even be done with a damp cloth simply very lightly wiping over the surface of the finish, being careful not to remove the wash in the recessed detail but only the wash on the surface. If you have used an oil based wash then you could be removing it with turpentine with either a rag or cotton buds but the idea remains the same, you are removing the excess and leaving the wash in the recesses only. One beauty of this technique is that if you are not happy then the wash can relatively easily be removed and you can start all over again.
That is then two very basic methods of applying washes for different applications and which give quite different results. As with all aspects of modeling but especially finishes I would always strongly recommend practicing on scraps with different paints and washes to give to the best chance of achieving the results you are after when you actually decide to commit to the model.. When the wash process is completed and you are happy with it you are usually best applying a top clear coat in the finish you want to seal it all in. This is obviously considerably more important with a water based wash on a model boat! Clear coats of varnish, enamel or Kleer can be either sprayed or brush painted to enhance the surface texture if that is what you are trying to achieve.
