Wood is made mostly from cellulose, however this is not what makes wood 'woody'. Cellulose is bound together by lignin, a resin-like substance. The harder the lignin the harder the wood.

Lignin decays more rapidly than cellulose (especially in oxygen and UV light). Lignin gradually vapourises, called 'off-gassing'. This vapour creates a natural seal for the wood. However, when the vapour is pushed away by wind or motion, the lignin disperses, leaving more space for new lignin to decay into gas.

When we seal a piece of wood we are not keeping the elements out, we are keeping the lignin in. So we want to seal the wood front, back, and sides, not one hole open.
Once we have completely sealed the woof the lignin vapour remains locked within the structure and decay time reduces dramatically.
Some seals are better than others. A gesso prime is good, but still lets lignin gas through. A good oil-based structure is much better.

You want your primer to have some grip, or 'tooth'. That's why matte is better than gloss.

Modern method
Preparing a Masonite panel
A very good sealer is spirit Urethane, or you may know it in the shops as Matte Polyurethane Varnish. However the Urethane must be oil-based, not water based or the alkyd 'marine' type.
Cheap and easily available, masonite is a great way to start with serious painting.
A. Wear clean gloves.
1. Wash the panel with a spirit, as in Artist's Spirit, not Casper :ob
2. Do not touch the wood with bare fingers from this point on
3. Lightly sand the surface to create better grip for your primer.
4. Seal the wood all over with Urethane
5. Prime with Gesso. Or, use an oil-paint of your choice (mixed 1 part oil to 5 parts artist spirit). Never use raw turpentine.
6. Paint a mid-tone undercoat using Raw Umber and Titanium white.

Classical method
Preparing an oak panel
A. Wear clean gloves
1. Wash the panel with spirit
2. Lightly sand the surface
3. Apply a coating of 1 part linseed oil (or a nut oil if you can find it) mixed with 5 parts spirit to all surfaces (front back and sides)
4. Leave to dry for at least 30 days
5. Prime the painting surface with gesso or a coat of raw umber and lead white (mid-tone grey)