There are two types of paint, trade and retail, in general retial paint doesnt need thinning...trade formulations sometimes do.
There are various reasons for thinning paint, some paints are 'sticky' when your trying to appy them, especially oil based glosses...so you need to add a drop or white spirit to them (not turps substitute, which can have an effect on the sheen of the paint)
Sometimes the paint is two thick...this is often common with some undercoats, if its too thick it will leave brushmarks..so it need thinning
It is also hard work using a paint thats too thick...again a little thinners makes it easier for the person putting it on
Another reason is to allow a paint to semi penetrate a surface, I have just done an outside job, new rendering, the first coat which was an oil based masonry paint needed to penetrate the surface, this enables it to 'lock' on the surface and provides a good based for subsequent coats...so I used 15-20% white spirit
Some surfaces like rendering are porous, which causes the paint to suck into them...thus making it hard work to apply and youl end up using to much paint (which doesn't penetrate or prime and just sits on the surface)
The other and maybe most obvious reason it to help make the paint go further...(hence white spirits nickname...'doomore') it will help to a certain extent but the type of surface will have a bareing on this, imagine a gallon of water and a gallon of slightly thinned paint on a plaster surface...the paint allthough thicker will go further becasue the water being thinner will be absorbed more into the surface....so thinning paint paint purely to make it go further in order to save money doesnt always work plus, by thinning it you are reducing its covering power.
Most painters beleive that it far better to give something two decent coats than having to give it three because the second one has failed to cover.
When you thin an oil based paint you are breaking down the carrier oil and resins..remember that the highest percentage of gloss paint is this oil....think of it as a varnish with with powder in....if you break the oil down in gloss paint to much it will go flat
Too much thinners can also have an effect on the binder in a paint...the binder holds the pgments together...it this happens the surfac of the dried film will become chalky and powdery
Water based paints acrylics, masonry etc...thin with clean water
Oil based, gloss, undercoat etc...thin with white spirit
There are also some speciality paint that have their own types of thinners...hammerite for example
Hope that helped.
