When you are on a tight budget and are purchasing an inexpensive framed canvas, remember these usually have a somewhat porous fabric. If you wish to avoid using a large amount of paint/thinning agents for the finish you require, buy a large carton of Gesso (a type of chalk based bonding agent) and prime the surface with several coats of this quick-drying material. The results will give you a smooth and relatively non-absorbant surface which is ideal to work on and which gives very good results to your artwork. I have to stress, however, that in order for your portrait work to maximise your art skills, only the very best quality canvas will suffice. And even they, may require a coat of this primer to give you the very smooth surface you may require.
Perhaps you work in watercolours? In this instance, a small bottle of artist’s gum arabic or drawing glue is an excellent investment. This permits (a) colours to flow freely and easily under your brush and (b) it does not allow the paper to become saturated and thus unworkable. However, if you are painting with acrylics or gouache, then conventional water is probably still the best material for use as the spreading agent.
Now I come to a somewhat contentious piece of advice. Artist who work in oils, know that they can take a very long time to dry, even when used in conjunction with a drying agent. Water-based oils, though, dry quite quickly, but aren’t all that popular as to date their finish and spreadability and colour range is frankly not the same as their oil counterparts. I have found, that adding a little bit of acrylic to an oil colour (obviously exactly the same colour, hue, tone, etc.,) enables the oil paint to dry very quickly. A word of warning though. This mix has to be applied with a soft brush and brushed very well onto the canvas, so as to avoid brush marks when the colour dries out.
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Thank you for reading this, and I hope it will be useful to you.