Warren Block wrote:
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? ...even a latex primer will have
a better chance of a long life.

How is latex primer chemically different from latex topcoat paint?
Does it have a different binder system than latex paint?


Oil-based primer should sink in better,

Why not oil primer for this job? Here's why:

Many claim that oil-based primer penetrates better, but the caveat is
that the wood must be BONE DRY. The least bit of moisture appears to
greatly interfere with absorption and adherence. By contrast, latex
seems to be fairly forgiving: to apply latex, the wood can't be wet or
damp, but it doesn't have to be baked dry for a week. This is a
significant issue for this outdoor project, especially at this time of
year, when finding a whole week where there is no rain and the humidity
is low and there is no dew in the morning is next to impossible.

The area to be painted is a high-use area; it is a great inconvenience
to take it out of service. If I power-wash it, I have to keep traffic
off it until it dries or it will get dirty again. For oil primer, that
means a full week of dry weather after washing. If it rains, I have to
wait another week.

Once the wood is dry and the oil primer is applied, it takes several
days to dry properly so that it can be painted with latex. If it rains
during this period it seriously compromises the primer. All during
this period, the area must be off-limits to traffic.

Once the latex is applied, I need another 24 hours of dry weather or
the latex will be compromised.


On the other hand, if I use latex:

After power-washing, the wood is ready to accept a coat of latex within
24 to 48 hours. Within 4 hours I can apply a second coat. Within 24
hours it is rain-proof and ready for foot traffic. Done.


Besides the significant inconveniences of oil (as detailed above), my
experience will oil-based primer has been highly disappointing. See my
other posts in this same thread for more details, but here is the gist
of it. I tried using latex topcoat over oil primer previously and the
system failed within one year. The latex topcoat bonded to the oil
primer, but the oil primer started blistering and peeling away from the
wood in large chunks. I tried to follow all the rules. I do not know
if I somehow did something wrong, or if oil primer just isn't the right
solution for this particular application (exterior, wood, horizontal,
foot traffic, sun, rain, snow). Some boards were coated all six
sides, and some were coated on the top and ends only. All of them
failed prematurely.