painting treated pine exterior porch stairs

I have a tough paint problem and am looking for success stories.
The house has a large wrap-around porch which was painted by the previous owner. Most of the painted area is fine.
BUT, the porch stairs are exposed to sun, wind, rain, snow, and heavy foot traffic, and so far I haven't had any luck keeping a coat of paint on them.
Last year we removed all the treads and risers, scraped and wire-brushed as best as possible, applied an oil-base primer all around (top, bottom, edges, and ends), and put on a top-coat of latex rated for exterior horizontal surfaces.
Within a year the paint (and primer) was blistering and flaking off.
SO... I just finished gutting the stairs (removed all treads and risers) again, and will be replacing with new treated pine (purchased 3 years ago and has been drying in our basement).
Since I've gone to all this effort, I want to use the absolute BEST primer and topcoat for this particular application (exterior, horizontal, treated pine with heavy foot traffic).
Are there some pros reading this who would be willing to weigh in? How about some DIY homeowners who have battled this problem successfully?
BTW, simply waterproofing the stairs is not an option. They need to match the rest of the porch.
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