One of the most important considerations before any decorating job is how much paint you actually need. As a rough rule of thumb and assuming you'll be doing a couple of coats, it's usually the case that 2.5 litres of emulsion is enough for each wall, but this, of course, can depend on the size of the walls and whether any contain lots of windows, for instance.

For a more accurate calculation, simply work out the area of each wall (plus the ceiling, if applicable), which is the length times the height (or width), and then add the areas together to get the total for the room (preferably do your calculations in square metres).

Any paint you buy should specify how many square metres it covers, so you can work it out from there.

You may like to make deductions for the doors and windows - unless these are an unusual size, allow around 1.5 square metres per window and two square metres per door. Alternatively, you may prefer to ignore them for these calculations, so you have some extra paint to play with.

The surface you're going to paint is another consideration. For example, if the walls are smooth new or as-new plaster, you'll need less paint than if they're lumpy and bumpy (problem surfaces often require around 25 to 50 per cent more paint).

Incidentally, in the case of the latter, you'll get a far better finish if you use a flat matt emulsion, as one with a sheen will only accentuate the imperfections.

In the case of new plaster, it's essential to seal it once it's dried out thoroughly and before you paint it. You can use watered-down emulsion to do this, but it's best to use a dedicated plaster-sealing liquid.

This should both stop the salts in the plaster from bleeding through and prevent the wall from being ultra absorbent, so you use less paint.

If you're painting over dark-coloured walls with a light colour, it's a good idea to apply a basecoat emulsion first. These aren't particularly cheap but they should save you time and money because you won't need to do endless layers of topcoat to cover the original colour.

Estimating the paint required for the exterior of your home is trickier, but don't let this put you off.

To make it manageable, divide the exterior into rectangles and work out the area for each, as before, then add them up for your total.

When it comes to triangular gables, multiply the width (base) by the height and divide by two to get the area. And this time, don't make deductions for doors and windows, unless they are more than 10 square metres, because you often need the extra paint when doing exteriors.

When painting the doors themselves, allow around two square metres of paint per door or four square metres if you're doing both sides.

For a window, expect to use around one square metre of paint, though, of course, windows vary considerably in size, especially period ones, which can range from tiny cottage ones to vast floor-to-ceiling sash windows.

Quick-drying, water-based satinwoods and glosses are recommended but they do require a lot of coats, so you may use more paint than you expect. The advantages are that they dry in a fraction of the time, making them ideal for exterior use, and go on a lot more easily than the oil-based equivalents.

If the paint has a batch number on it, make sure all the tins you buy are from the same batch. This minimises colour differences and thus DIY disasters.




Q&A
What's the best type of paint roller to buy?

Look for one with bevelled edges, as these are less likely to leave tracks of paint that spoil your finish. It's also wise to steer clear of cheap roller sleeves because they're likely to shed fibres when you use them, which you then have to keep picking out, again potentially spoiling your paintwork. A good way to rid a new roller of loose fibres before you start using it is to use masking tape to "defluff" it.




If you have stucco or render on the exterior of your home, don't be tempted to clean it with a power washer because this can remove the finish and leave you with a major repair job on your hands.

• In two weeks' time we will look at another part of preparation that isn't necessarily as straightforward as you might think: how to clean and clear a room ready for painting.

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