Most ready made paints are created for certain average conditions which generally do not exist on the day you choose to do your painting! The result of that is the paint does not flow out well to leave a smooth finish and you end up with ugly roller marks or brush marks or streaks or lap marks.
Paint in general terms is made for what would be an average room temperature or outdoor temperature for the market where those paints are sold. The problem is, whenever there is a deviation in temperature either up or down that paint will be more difficult to brush out or roll on to the wall. Indoor water based paints, Emulsions or Latex tend to dry too fast when the temperature is warmer than average, and the result of this is that each new section painted can have streaks because the piece before has dried out too quickly.
Painters / Decorators describe this phenomenon as ‘losing the wet edge’. Painting ceilings can be especially troublesome because all the room’s warm air rises up to the ceiling compounding the drying too fast problem. If you are applying water based paint / masonry paint on to outside plastered or similar walls and the weather is dry or dry with a breeze or worse still, if the sun is shining straight on to that wall while you are painting… you will have a really tough job because those weather conditions mean that you will lose that ‘wet edge’ almost as soon as you apply a roller full of paint.
It is best to choose a day that is neither too hot nor too cold for exterior wall painting etc. If you are applying oil based paints or varnish, especially outside on a good day, you will notice that your paint becomes very heavy and the brush will tend to drag making the painting process a lot more tedious. The reason for this is because the solvent in the paint is evaporating quickly in the warmer conditions and it will be necessary to thin the paint a bit to ease application.
To get your paint or varnish to flow, one historically adds some water to water based paints and some while spirit to oil based paints. That usually makes the paint flow better. There are a few negative consequences when thinning paint in this way though, for example… the paint loses some of its ‘hiding power’ which can be a nuisance when using ‘weak hiding’ colour paints (yellows, reds etc) because you will probably need to apply extra coats to get a finish and block out previous colours etc.
With gloss & other oil based paints etc, thinning in this way will make it flow better but it will kill the hiding power and lose a lot of the shine after a time. Another alternative would be to utilise a paint additive which is made to make your paint flow out and cover better in less than ideal conditions etc. You can obtain paint additives (sometimes called paint conditioners) for emulsion and oil based paints in most of the better paint stores. The water based additive (conditioner) is ideal for any colour paint but will not be suitable in water based varnish because of its milky appearance. However it does not change the colour or finish etc.
Water based paint additives are designed to keep the wet edge open and usually enable you get a finish with no brush marks or streaks. In fact some water based paint additives actually help hiding power in a dramatic way especially in weak colors like reds, yellows etc where you can save a few coats and a lot of hard work. Oil based paint additives will work well in any oil based paint and varnish. Check your paint to see if cleanup is with white spirit etc and if so it means it is oil based and suitable for that paint.