Ether Jones wrote:

I took a representative sample board and power-washed it to remove the
old blistering paint, then I let it dry and gave it a single coat of
aggressively thinned latex (I would guess 50/50 although I didn't
measure it). After letting it dry thoroughly I blasted it with the
power washer (1700 psi) and it didn't budge. So I figured, on that
basis, that using the thinned latex as a first coat, to be overcoated
with 2 coats of unthinned latex, was a reasonable thing to do.

It sounds like you've invented primer. The unknown is how long it'll
last.

Why not just use a real primer? Oil-based primer should sink in better,
but even a latex primer will have a better chance of a long life. Both
can be topcoated with latex.

--
Warren Block * Rapid City, South Dakota * USA