If it's really down to wood (with maybe a smear of old paint/primer) you'd do worse than prime with Universal Clear Primer from International (UCP). It's clear, universal and primes. I've not found anything to match it. It's the very devil to scrape off if ever you need to but a rock solid basis for a new paint job. Perhaps thinned, second coat neat. Moisture helps it cure, but the surface must be dry, preferably very dry. The you can slap on Dulux to your heart's content.