If it isn’t raw, new wood, it may be enough to fill any cracks or holes with wood filler and smooth the entire surface with several grades of sandpaper – from medium-coarse to fine – before applying the new paint. The exception to this is if you are changing the colour dramatically, particularly from dark to light, although light to dark can have problems too. The darker colour tends to ‘ghost’ through the new paint no matter how many coats you apply, so you will need to use a primer.
If you are painting bare, untreated wood, the answer is definitely yes, you will need to prime the surface first.
Why do I need primer on bare wood?
A priming paint has a high solids content that fills the grain in the wood, making for a smooth finish. A primer also seals the absorbent new wood, preventing the paint coats from sinking in immediately and going patchy. Applying a coat of primer can make such a difference that only one top coat may be needed, instead of two or even three.
If you decide to forego the primer, the results might look fine for a while, but a couple of years down the road when the paint starts to flake, you’ll wish you had been more thorough.
What sort of primer should I buy?
There are two main types:
- Traditional oil-based primer
- Newer latex-based version
The oil primer can give off nasty fumes so apply it in a well-ventilated area. It takes about 24 hours before it can be re-coated, although there are some new oil primers that dry faster. Clean-up will usually be with white spirit or similar.
The latex-based primers are not as smelly, dry faster and the clean-up is usually with water, but they are not suitable for every type of wood. Cedar, for instance, actually needs oil-based primer to prevent tannin stains from the wood coming through to the surface.
There are some paints that combine primer and top coat in one, but using separate products achieves better results.
Get a primer that is close in colour to your paint, especially if you choose a dark shade, or go to a shop that mixes paint and ask for a suitable colour to be added to your pale primer.
How do I prepare for and apply a primer coat?
Rub down the wood with two or three grades of sandpaper, as mentioned above, always following the direction of the wood grain. Sanding across the grain will break and fluff up the wood fibres, making the job more difficult.
If there are knots in the wood, as often found in pine, seal them with a knot sealer. If this isn’t done, over time the knots will leak sap, which will penetrate the paint looking like a surface stain.
Apply the primer as directed on the tin, by brush, roller or even by spraying if it’s a large area. Allow it to dry for the stated time, but don’t leave it more than a few days beyond that time before you apply the top coat. If the primer dries out too thoroughly, it won’t bond well to the top coat.
Sand the primer gently to remove any imperfections before applying the top coats.