On Monday, June 6, 2016 at 12:45:11 PM UTC-4, dpb wrote:
On 06/06/2016 10:34 AM, HerHusband wrote:
...

There's obviously an oil or something on the old siding that is preventing
the new paint from sticking. But I'm not sure how to treat the old siding
so the new latex paint won't just flake off again.

...

I've come to the conclusion there is nothing that can restore an old,
weathered surface to hold paint other than to completely remove
sufficient material from the surface so as to expose, essentially, "new"
wood.

We did all you described to the old barn where it _looked_ as clean and
fresh as the new wood and it, too, has failed while the replacement
pieces are still holding.

If you can find anybody who can provide any other solution as well, I'd
surely like to know, too.

--

I've had very good results using PeelBond Primer. It's a very thick
latex primer that's made to help level old rough wood. It's good for
areas that you've scraped to avoid having to sand the surface perfectly
smooth. It will help hide the transition areas. I've used it on weathered
wood and it's held up fine. Kilz is good at stain blocking, not sure
how it ranks as a primer when adhesion is the main goal.