Lap marks, roller marks, brush marks in your paint finish is usually caused by variations in temperature, solvent evaporation or other conditions and can be extremely annoying. Paint is generally made for what would be described as an average room temperature or outdoor temperature for the geographical location where those paints are sold. The problem is, whenever there is a difference in temperature either down or up, that paint will be harder to brush out or roll on to the wall etc. Indoor water based paints, Latex or Emulsions tend to dry too fast when the temperature is warmer than average, resulting in each new section painted may contain streaks because the piece before has dried too quickly. Painters / Decorators describe this as 'losing the wet edge'. Because warm air rises in a room, it can painting a ceiling all the more difficult because it makes that paint dry faster, usually ending up in roller marks or lap marks etc. If you are applying water based paint / masonry paint on to outside rendered or stucco or similar walls and the weather is dry or dry and a light breeze or worse still, if the sun is shining directly on to that wall while you are painting… you will have a hard job because this weather condition means that you will lose that 'wet edge' almost immediately as soon as you apply a roller full of paint. It is wise to choose a day that is neither too cold nor too hot for exterior wall painting etc. If you are applying oil based paints or varnish, especially outside on a warm dry day, you will notice that your paint becomes very heavy and the brush will drag making the painting job a lot more tiring. This is because the solvent in the paint evaporates quicker in the warmer conditions and it will be necessary to thin the paint a bit to make it flow easier. To get your paint to flow, one historically adds a little water to water based paints and some mineral (white) spirit to oil based paints. This generally gets your paint to flow out better. There are however some negative consequences when thinning paint in this way, for example… the paint loses hiding power, particularly with yellows, reds etc and you will more than likely need additional coats of paint to get a proper finish or hide previous colours etc. With gloss & other oil based paints etc, thinning in this way will make your paint flow better but it will weaken the hiding power and dulls a lot of the shine after a time. Another alternative would be to utilise a paint additive or also called a paint conditioner which is designed to make your paint flow out and hide & cover better in less than ideal conditions etc. Additives or conditioners for oil based paints or varnish can be bought in some stores or probably best online. Water based paint additives are ok in paints but not in clear water based varnish because of their milky appearance. However the color of the paint will not change. Water based (Latex) paint additives are designed to keep the wet edge open and usually enable you get a finish with no brush marks or streaks. Some of the better known water based paint additives help paint hiding power significantly especially in weaker colours like reds, yellows and so on and usually you save a lot of time and hard work because of the fewer coats required. 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 (mineral spirit) etc and if so, it means it is oil based and suitable for that paint.
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