Updated on 26 February 2017 (Harvard referencing).
2 May 2016. So, here comes disaster diary for the last step in this exercise.
I think that everything that follows can be pinpointed to my introduction of gloss medium as a diluting agent instead of water. In my previous exercise I had noticed that I was unable to create uniform layers of paint, i.e. such that were not totally opaque to the sides of the brush and totally transparent in the brush track. After having got the advice to use gloss medium to dilute instead of water the first experiments looked quite promising except for the weird bubbles created by mixing more than the tiniest amount of paint and medium. While I thought that the background created in this way looked fine and the surface was smooth and shiny, I will have to work some more on my composition skills (Fig. 1).

When next I added another layer of paint indicating the position of my objects and their future colours, I noticed straight away that painting on top of that smooth surface would not be as straightforward as it had sounded on the internet. I found that I was unable to go over areas already covered in wet paint a second time, because this would remove all the paint in an instant and I would not be able to close the gaps produced in this way until the layer had dried completely. This allowed no spontaneity im my use of the paintbrush whatsoever and prohibited the correction of mistakes. Also, and what I did not expect to see to such an extent was the extreme darkening effect. Acrylics become darker on drying, student qualities such as I have been using so far more so than professional quality paint (which I will buy from now on!), but the gloss medium made this far worse. The application of mixed hues became totally unpredictable, since I did not know what tonal value to mix for it to dry up to create the tonal value I wanted. And furthermore, the medium and paint reacted together in a way which produced a fluffy, creamy stuff difficult to apply to the smooth surface, which a second later was dry enough to allow no correcting. I spent two days trying to do my best to finish my painting, but I gave up when I saw no way I could improve what I had (Fig. 2):

While there is no point in wanting to see anything nice in the overall work, I do think that some parts may be worth remembering for later for their effects. If they had been on a separate canvas each I might have been quite pleased with the outcome. So here are the details I liked. The “coral” (or whatever it may be) was believable overall despite the lack of detail provided at closer look. I also want to remember the effect of having outlines contrasting in colour to both the object itself and the adjacent negative space, see top right of following photo (sorry for the poor quality, neither camera nor scanner nor the later removal of highlights on the computer provided a realistic image) (Fig. 3):

In the following two details I liked the texture of the background surrounding the petrified branch (Fig. 4a) and the contrast between the pointed bit of rock and the background, making it look really 3-dimensional (Fig. 4b). These I was also pleased with, because I noticed that for the first time my artist research seemed to have an effect on my style of painting (if one may call that so):
What was impossible to improve in the painting was my piece of pumice. Whatever I tried became so dark in the end that I could only guess at the final colour.
I will resist, however, the urge to apply a pair of scissors to this piece of work and stick the nice bits in my sketchbook. By having it in front of me in my workshop I may learn more from it that I am am aware of at this point.