Is it better to apply one thick or several thin layers of paint
Science used to be considered an art... and I would bet that most scientists today would agree that it really is an art. I think that the best way to make sure it's nearly even is to use the same amount of paint over the same amount of area. In other words, use one cup of paint for each layer... or three cups of paint for one layer. Now most paint covers 600 sqft per gallon. So I would try spreading a cup of paint onto 36 square feet (that's 6 ft x 6 ft) and let nature run it's course (although it may take years). Alternatively you could try painting swatches of wood, and/or plastic, and or cloth, and/or aluminum, and/or any material.

Tips on painting and science:


Practice once or twice before starting.
If you start with a clean roller the first time, then start with a clean roller every time; or start with a used/precleaned roller... make sure that each painting job is handled "identically".
Purchase and use an accurate scale for weighing the paint and the roller (and maybe the swatch of wood). The idea is to make certain that each piece of wood gets the same amount of paint. But that's probably too tricky to get perfect. So, try to get it close, and record the actual amounts applied to the walls or swatches.
Make absolutely certain that the paint is very well mixed each time.
Record the weather conditions: sun, humidity, wind, time (at the time of painting, between coats, and over time as the wall or swatch is being weathered).
Subjecting different areas to different "accelerated" conditions will help fill out the results. In other words, allow some areas to age naturally, while other areas may be exposed to soil, or milk, or wet rags, and other areas could be solar irradiated, perhaps by adding reflectors. Other stressors might include an acid-wash treatment or a power washing every week. There should also be at least one section from each test that is kept from being exposed to any accelerating elements. Those would be the "control" sections for the experiment.
A time lapse camera (like for hunting) might be a good tool to have... once mounted it could be set to take a picture every day.
Replication is very important. For a good test, you would want at least four of each kind of "identical" test.
Consider the variety of paints and be write about why you chose to test whatever paint you test (any why not x, y, or z paint) and describe the kind of paint(s) with attention to details like the labeled application rate, color, composition, etc. Details might include: Gloss, Flat, Urthane, Oil, Acrylic, Latex, was primer used or recommended?, Aerosol, UV protection, Mildew resistant, Brand differences... it might be best to not mention any particular brand names, but you could mention relative price differences (which is information of value to most people).
Be sure to write the dates and identify/number the test articles on the swatches or wall (or on an attached tag), and keep a notebook with all the details and observations.