I found this article which is very similar to how I do mine!!! So here it is and I hope it is of help to you, although there are many other ways that others use.
Painting Sequence
The sequence of paint layers
From above to below in 20 easy steps!!!
Introduction
This article will provide general advice about the order to apply the various layers of paint and other finishing layers that are commonly used. It applies to models that have a realistic non-gloss finish including military vehicles, aircraft, ships and some science fiction subjects. The painting of a glossy finish such as used on automotive models follows a quite different path.
Background
Gone are the days when applying a single layer of monotone paint was considered a good finish in the modelling world. Covering a model with paint in colours that match the original subject may produce a finish that is technically accurate, but it will not produce a realistic looking finish. Furthermore, it will result in a model that is dull, lifeless and probably 'toy like'.
Over the decades, the modelling community has developed a wide range of painting and finishing techniques that have made the best models works of art with a real wow factor. It has been discovered that the best results are achieved by applying many translucent layers of paint and other mediums which build up into a finish that has depth and interest.
Part of this process is attempting to mimic how light and shadows fall on the original subject and part of it is adding the wear, tear and weathering that any real life object receives from day to day. A final element is pure art - adding colour, shading and texture which looks great and emphasises the characteristics of the subject being modelled.
These developments can be confusing, especially to the beginner. Not only are there many different techniques to learn, but which ones should be applied on any particular model and in what order. This article attempts to answer the last question.
Sequence of Painting
The following is a suggested sequence to achieve a good result on most models. Not every step is needed in every case - in fact it would be rare to carry our every step on a model. Furthermore, depending on the results, it may sometimes be necessary to go back and repeat some steps.
It is worth noting that relatively recently a new form of painting known as colour modulation has been developed. Essentially, this consists of deciding where the main light source (the sun) is in relation to the model and shading the model accordingly. Since the sun is generally high up in the sky the upper surfaces of the model would be painted with a lighter shade than the sides which in turn would be a lighter shade than the under surfaces. In fact, the colour on the sides may be gently graduated from light to dark to add interest.
The colour modulation technique can produce spectacular results and bring to life a model painted in a monotone colour such as olive drab. When using this technique some of the stages below such as pre-shading and post-shading would not apply.
1. Wash all parts
Getting a good dust and grease-free surface is essential, particularly when using acrylics.
2. Primer
The reasons for a primer are covered in 'All About Primers' and it would be rare not to prime a model. Then check the surface for defects, seam lines etc.and correct them before proceeding.
3. Pre-Shading
This is optional, but most models will benefit from pre-shading. The primer coat may make this step unnecessary if it is the right colour.
Hint: To simulate paint peeling, put splotches of dry salt or Marmite between two base coats. The top coat will wash away where the salt is placed leaving irregular patches of the undercoat showing through.
4. Base Colour
Apply with airbrush if at all possible. Several thin layers are best, but take care not to totally obscure any pre-shading.
5. Highlight Base Colour and/or Post Shading
Lightly airbrush a lighter shade of the base colour in the centre of panels and upper surfaces. With camouflage paint schemes, spray the base colour and highlighting for each colour before moving to the next.
Alternatively, airbrush a darker shade of paint into recesses and along panel lines to simulate shadows. Post-shading is very easily overdone, so use very thin paint very sparingly and built it up in layers. Too little is much better than too much. You will need a good airbrush and some skill to post shade successfully.
6. Protection layer
Optional, but many modellers find this a good time to apply a thin layer of varnish (often Johnson's Future/Klear). It protects the work so far from the next stages and allows you to come back to this stage if something subsequently does not work well.
7. Filters
Optional, but becoming increasingly popular, particularly now that ready made filters are available. A filter is a very thin tranparent paint layer that subtely alters the colour of the base layer and helps to blend in the colours on camouflage schemes.
8. Detail Painting
This may the best time to brush paint tools on vehicles and similiar tiny items. Some items may be left to the decal stage depending on whether you want them to be affected by the washes or not.
9. Washes
These may be wide area or limited area (pin) washes. It is easy to overdo this and darken a model.
10. Gloss / protective layer
Decals must be applied to a gloss or semi-gloss surface so another protective layer may need to be added now. Some modellers only 'gloss' the areas where decals are to be added and others prefer to put a gloss coat over the whole model and this will depend in part whether a protective layer has already been added to the base coat.
Hint: An alternative to putting a glossy layer under the decals is to soak the decal in gloss varnish e.g. Future/Klear so the decal 'floats' on a layer of varnish.
11. Decals / Markings
As well as applying decals and painting unit markings, this might also be a good time to paint details by brush if it has been done earlier.
12. Dry Brushing
If you plan to do any dry brushing this is probably the best time. Advice on dry-brushing is given in the article 'Dry Brushing'.
13. Protective Layer
Yes, another protective layer. This has two possible purposes. One is to seal in the decals before any serious weathering is done, so you may wish to restrict this to a thin layer over the decals. Secondly, this is a good time to change the surface shine to it's final effect, so you may wish to apply a layer of matt/flat or semi-gloss over the whole model.
Weathering Stages
At this point your model will look like it has just come out of the factory and if that is what you want then you have finished. However, almost all models benefit from some wear and tear to give additional realism. This is often what makes a model stand out and many modeller's find this is the most enjoyable part of the hobby - like Geppetto bringing Pinnochio to life.
14. Chips
Paint chips, rust chips, scratches etc applied with a very tiny brush or even a cocktail stick. This is a stage that can fit in several places and you may wish to add further chipping and scratching later. Remember it is easy to add scratches later, but very difficult to remove them so don't overdo it.
15. Light Dust With Airbrush
The lower parts of vehicles and underside and undercarraige of aircraft may receive a very light 'dust' layer with very thin paint. This layer should be very transparent.
16. Heavy Dirt and Dust
This stage mainly applies to vehicles. Pigments and/or ground pastels applied with a brush and medium such as turps. If you want to slap it on really thick mix with resin or varnish.
17. Light Dirt and Dust
Apply dry pigments or ground pastels in addition to the previous stage or instead of it depending on the degree of weathering and muck you are replicating.
18. Detail Wear and Tear
This includes oil and fuel stains, rain streaks, boot marks and any other signs of usage that the vehicle would have recently suffered and should appear on top of all other weathering.
19. Final Dusting?
You may wish to consider giving the model another very light dust layer with thinned paint from an airbrush to tie everything together. This must be very light at this stage to avoid soaking or removing any of the dry pigments.
20. Final Protective Layer???
Your model is complete and looking great. The temptation is to give it a coat of matt, gloss or semi-gloss varnish to protect it. Unfortunately, doing this is likely to impair the look of the model. If you have dry pigments on the model then any varnish will blend them together and may make them completely invisible. Furthermore, part of the interest in the model is the different sheens on different parts of the model which will all become uniform under a coat of varnish.
If you really do feel the need to protect your model, then keep it to a minimum. Areas that are matt/flatt, or where dry pigments have been added should only receive a very light misting of a varnish that drys to a very matt/flatt surface (matt varnishes vary in just how matt they are) and make sure you mix matt varnish very well indeed. Consider using varnishes with different sheens to add interest to the model. For example, an aircraft model may be mostly an eggshell/semi-gloss finish, but consider making the leading edges of wings and tail a bit more glossy as these are areas that would catch the light.