So how much paint do I really need? How far does it go? Is that with or without thinning? Why did I end up with a litre leftover? Just what is a spreading rate? Is spreading rate simply how far paint will spread or is there some magical number?
Let's have a look at the current situation. A very common spreading rate quoted by the industry is 16 sq metres per litre - but where did this come from and how relevant is it? Looking for the source of 16 sq metres per litre will take you into the pre-metric days when the standard spreading recommendation was 800 sq feet per gallon. This was in the days of mainly enamel paints that had a volume solids of 50-60%.
Volume solids? This is the volume of goodies left in a paint can after all of the solvents have evaporated off. Oil-based paints have invariably had higher volume solids than their waterborne equivalents; which typically pan out into the 35-45% range. An enamel paint of volume solids 55% spread at 16 sq metres per litre will give a film that is about 35 microns thick (1,000 microns equals one millimetre) and a three coat system will give a total film thickness of about 100 microns. A waterborne system spread at the same rate will only give about 25 microns a coat and 75 microns for a three coat system (50 microns if a two coat, direct to substrate, system is used).
Hundreds and hundreds of exposure panels have confirmed, beyond argument, that thicker films offer more protection and the Resene philosophy has been to quote a spreading rate that will result in a film thickness of 35 microns per coat; 100 microns for a three coat system. This requires cognisance to be taken of the volume solids of each paint.
This sort of regime is the absolute norm for steel protective coatings where the film thickness of each part of the system is critically specified.
There is a property of a paint called the 'natural spreading rate'. This rate is tied up with the application properties of the paint and reflects how far a paint would be spread by someone simply painting by 'feel'. The 'easier' a paint is to apply, the further it will be spread and the thinner the resulting film will be. A very easily applied paint may be able to be spread to 25 sq metres per litre but the end result will not be satisfactory and result in further coats having to be applied.
The paint formulator endeavours to build a viscosity profile into the paint such that the specified spreading rate results no matter how facile the painting process. However, changes from brush to roller, or the addition of thinners or even tinters, can negate these efforts to precisely control spreading rate.
So how about hiding power as a mechanism for film build control? Unfortunately, different pigments have such wildly different hiding powers that it is not a useful tool. A cheap fence paint, based on an equally cheap natural iron oxide, may achieve fantastic obliteration at films so thin that they do not provide sufficient protection from water ingress.
The best idea when painting is to estimate the amount of paint needed based on the area being painted and the recommended spreading rate. Then as the painting progresses keep an eye on what is being used. When the midpoint of the job is reached, around half the paint should generally have been used.
But what about interior, where appearance, rather than protection, is the key? In these circumstances, it is correct that hiding is the key rather than film thickness. The predictive issues here are: What colour is to be used in the new coating and what colour is it going over?
An 'off-white' going over an 'off-white' may well produce an acceptable finish in one, carefully, applied coat. A bright yellow going over a dark purple feature wall may take four coats! So basically, while we can bring technology to bear on exterior situations, where protection is paramount, for interior, you may end up with a litre or so over. But what the hell; you will not get a better colour match for 'touch ups' and paint is not going to get any cheaper. It could be the best investment you make!
Getting the spreading rate right on the wall is the key to getting a quality finish. Put on too little and you'll waste time and money having to put on an extra coat just to get on the paint that should have been applied in previous coats. Get it right and you can get the optimum finish in the prescribed number of coats