Mike has it pretty much right on spot.
One thing that I've learned working with my century old home is that removing as much of the greyish weathered wood as possible is critical to proper, long-term adhesion of the paint. In fact, I was surprised to learn from researching the issue several years back that at both paint manufacturer and lumber supplier sites is that wood ages very rapidly, and. the most durable paint adhesion is made by priming freshly cut wood, then applying finish coat within a few days of priming.
- As soon as the wood is exposed to air, it begins to oxidize, (very slowly burning...) -
so even with store bought wood, it's best practice to at least lightly sand, then dust-off the wood surface right before the first coat of paint is applied.
Also, it's often not said or ignored by people who just grab a bucket and slop it on, but primer is not the same as regular paint. It has a much better ability to soak into the paint (adhesion) and a greater adhesive properties on a chemical level. A simple test to prove disprove that claim is this. thoroughly wash a piece of glass. Apply a single coat of primer to one side and a single coat of finish paint to the other. Allow to dry for a few days and then use a single-sided razor to remove the paint...I've done it several times just to prove myself wrong...I think you'll find that the primer is a bit more difficult to remove. It's by design
Fort best results, always prime with an actual Primer. It's inexpensive (a 2 gallon tub of moderate quality primer cost me about $23 a few weeks ago) and imo, well worth the trouble, both for mundane tasks like siding or balustrades, or for more sentimental (and potentially much more valuable) work like art on wood...
Latex primers are the easiest to use for wood, as their cleanup is water and soap, instead of spirit based products that are both toxic to humans and the environment. They can be tinted to reduce the amount of top coat (finish paint) that needs to be applied or in the case of art work, to provide a backdrop like water or sky...
here are two pictures showing finish coat (dark green on left) and a tinted primer coat (light green on right)....(and yeah yeah, I know, I haven't finished the job... bigger fish to fry right now)
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No matter the type of paint you choose to use you should prepare the surface by sanding to to remove any old paint and create a good surface for your paint to bond to.
To prime the wood you can use an acrylic primer (typically a runny white paint) or you can simply find some plain white acrylic paint and thin it with some water (as acrylic paints can be diluted with water). The idea behind priming is that you are giving your top coat a solid uniform base to build on.
The method of application is simple:
-Sand surface (and brush clean so there is no debris)
-Apply one or two coats of primer (dried fairly fast)
-Apply your paint in successive coats (waiting for the previous coat to dry) until the desired colour and finish is achieved (could be 2-3 coats)
Another trick to consider is to ask your paint supplier to tint your primer the same colour as your finish coat. This will allow less coats to achieve the same results!