As a beginner, you might like to bring your painting to a certain stage and leave it to dry before proceeding to the next stage. Check out part 2 on the advantages of painting in stages as opposed to finishing a painting in a single session (alla prima).
I use and would recommend ‘Liquin’ as a painting medium. Its made by Windsor & Newton and their website gives a lot of information on this medium and others they produce. On the info. page on ‘Liquin’ they say “Not suitable as a varnish or final coat”. Unfortunately, they don’t say why. Alla prima painters paint a single coat, and therefore a final coat only. So you are left guessing, should you use ‘Liquin’ or not. I can only guess that as synthetic material it remains soluble after it dries. This is a characteristic of varnish, to be removed at some time in the future as the painting is cleaned. If the layer of paint, under the varnish, had a soluble constituent (like Liquin), it would also be removed with the varnish.*
* Update 19/11/2013. W&N express concern about the speed at which Liquin dries. A painting produced in layers, each drying before the next is applied, can be prone to cracking if the final layers contain Liquin and this will seal off non-Liquin layers underneath – retarding drying. As you can imagine, a painting with a final layer of hard, dry Liquin sitting on a wet layer can only end badly when removing varnish – both varnish and Liquin will lift off the wet layer.
To return to the beginner (leaving the painting to dry between painting sessions), for various reasons the painting when dry will have dull patches and glossy patches. The matt version and the glossy version of the same paint layer will be different in colour. The matt will be lighter in tone than the gloss. There are a multitude of reasons why this would happen, some colours dry matt, paint applied over an already dry layer will tend to be glossy, etc. The reason I mentioned the ‘Liquin’ conundrum is because applying fresh paint on a dry layer from a previous session (which has dull patches) will be different from the the same paint applied previously. My solution is to ‘wet’ the area with a layer of ‘Liquin’ before I apply the fresh paint. After the ‘Liquin’ is applied the painting will look the same as it did at the end of the previous session with the advantage that the under layer won’t mix with the later applications of paint.
Even after several layers of paint there can be dull patches and some areas will have a layer of pure ‘Liquin’ (the bits you ‘wet’ and did not paint over). Varnishing is not recommended as a method of removing dull patches and Windsor & Newton recommend using an oil based medium for the final ‘oiling out’, probably because of the solubility of ‘Liquin’ and similar mediums. They recommend using a clean cloth to apply the oil checking that the colour is not ‘lifting’ by the application. This will leave a layer of oil which will bond to the dull patches and you wipe off the excess where its sits on the non absorbent glossy bits. This film of oil will harden by oxidation and in time will not be dissolved by a later layer of solvent containing varnish. At least 6 months is recommended before varnishing.
All this is about having a uniform surface on a painting. The finish can be matt, gloss or an in between satin which is dependant on the varnish used.