Here's some step by step photos of a painting called 'Liquid Crystal' (180cm x 100cm) using a collage technique I developed some years ago. For full details of the technique see my thread 'Collage Technique WIP' in the acrylic forum. I was asked if this could be applied to oil painting so I thought I'd post this.
The image is not based on any source photos but rather constructed from simple patterns.
The image is an impression of sunlight dancing on rippled sand underwater, a favourite theme of mine and one that attempts to conjure up the sensation of diving into the bay where I live on a hot summer's day. Firstly the sand ripples are underpainted in thin acrylic. They are deliberately dark in tone, subsequent layers will cover a lot of this and it needs to have a strong enough contrast to show through and act as a guide.
The same underpainting treatment is given to the surface of the water, again in thin acrylic. I almost invariably underpaint oil paintings in acrylic and have never had any problems with this method, it's fast and easy to change.
A thin wash is applied to stain the remaining white canvas. The surface is washed with ultramarine blue and the sand with a graduation from pthalo blue to a sandy colour ... the colour is not all that critical at this point, I usually am in a hurry to get rid of any white surface remaing.
The shell and sand collage is applied. For details of how I do this, please refer to my thread in the acrylic forum "Collage Technique WIP". You can see now why the underpainted sand ripples needed to be quite dark, at this point there is just enough remaining to act as a guide for the oil layers.
The surface is painted in one session in oil, I premix about 5 or 6 tones and work with a fairly 'fat' medium, usually a mix of stand linseed oil, liquin (to accelerate drying a bit) and turps. I paint with medium stiffness acrylic brushes and smooth with a fan brush.
The sand is painted in oil with two sets of premixed graduated colours, one for the light and one for the shade of the ripples, The paint is almost opaque and again smoothed with a fan brush. This is also done in one session to keep the surface consistent and even. Once the oil layer is applied I carefully wipe some of it off to reveal a small amount of the actual shell colour. This is then left to fully dry, or at least have a tough skin (if I'm feeling impatient).
Finally the pattern of light is superimposed over the sand ripples. This is done using the same medium as the previous oil layers and is painted on thinly and the edges diffused with a fan brush.
I have revisited this image a few times now, each time with variations of pattern and colour. It is a simple image but one that I really enjoy painting.