Before painting bare wood, the first thing that you need to do is seal any knots with knotting solution. This is because these areas have a tendency to bleed resin and often a yellow stain will bleed through the paint, even oil-based paint.
A stain produced from a knot.
Knotting solution is a shellac based varnish (interestingly shellac is made from crushed insects' shells!) and this will block any stains or resin from bleeding through. I'd recommend that you use at least 2 coats. After the knotting solution is dry you can then apply your primer. Some primer is designed for exterior use and some for interior use but many can be used in both scenarios. I tend to use Zinsser Bullseye 123 primer as it performs well indoors and particularly well outdoors, it dries quickly and it's easy to clean brushes afterwards.
The other mistake that a lot of homeowners make is that they waste time sanding the wood before priming it. Priming the wood will raise the grain and cause the wood fibres to swell, so it's better to wait until after you have primed it before sanding as the surface will feel rough after the paint has dried even if you do sand it beforehand. You'll want to use a fine grade sandpaper (I find that the foam sanding pads are good for this). Sandpaper comes in different grades and I'd recommend P120 grade.
After you have sanded the wood, dust the surface with a dusting brush and it may also be worth wiping the surface down with a damp cloth. After this, you can use you undercoat and top coat/coats. Note that if you have used an oil-based primer you must use an oil based undercoat and top coat, but if you have used a water-based primer then you can use either oil or water-based paint on top of this.
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