Steps required prior to painting exterior wood

patchyX2

I've got a couple of exterior doors that I need to do paint and I'm not sure what I need to do to the wood before the final coat. The final coat will be something like Cuprinol Garden Shades matt wood treatment or Sandtex 10 year exterior satin paint.

The first door will be brand new unfinished pine. The second door is already painted. I'm not sure what's underneath, but it's starting to peel off and the wood is starting to split. I'm planning on sanding this back and filling the small cracks with some Ronseal wood filler.

What do I need to do to the doors before painting? Do I need some kind of sealer first, then a primer, then my finishing coat, or is the sealer only required if I'm trying to keep the wood grain visible (I'm not). Any recommendations on suitable products?

Grenage

The paint you're using will probably give the best advice.  For example the Dulux Weathershield has three stages; wood preservative, undercoat, and top coat.

The Sandex seems to suggest just sanding painted wood for a good key and a couple of coats.

Chickereeeee

I can recommend Zinsser Allcoat Exterior. Seems to stick to anything, 16 years guarantee (FWIW). Two coats and your done. (Zinsser 1-2-3 or BIN primer if its particularly difficult surface).

I tried Dulux Weathershield (waterbased) and it was poor, even using all three (expensive) coats and proper surface prep. Lasted 12 months on a window ledge. I put on two coats of Allcoat with no prep other than a wipe down - still very good after 2 years.