Scrape, sand and strip until you feel your arms are going to fall off. Now keep on scraping, sanding, stripping until you get to a good/solid substrate.
Then use the best long oil primer money can buy. Ask the paint store to tint the primer to half the color formula of the topcoat to give you a head start on coverage. (or as much tinting as the primer's manufacturer recommends)
Good prep. will maximize the time when you have to repeat this miserable process.