Completely off-topic: Paint for an outdoor plywood ceiling

[Delta]

I've just done an emergency repair of a ceiling, in what could best be described as a porch, using 12mm outdoor ply.  Now they need to paint it.  All the stuff I've found online assumes that the ply is going to be exposed to the elements (ie it is a floor or wall), and thus suggests a lot of priming and sealing.

As this is a ceiling it will only be exposed to humidity, so I don't think they need to go to such lengths.

Is there a paint (white) that will cover the grain of the plywood with a few coats?

Erm, erm, I used a cordless screwdriver during the job, is that sufficient to legitimise such a post on an electronics forum?

[tggzzz]

What grade of plywood is it?

If it is modern cheapo-grotto stuff from a local "shed" (think B&Q!) then it will be sensitive to moisture and will probably warp and/or delaminate over the years.

If, OTOH, it old ply or marine ply, that will be far less of an issue.

[Delta]

It's hardwood BB/CC ply, it is "outdoor" specced (whatever that means), rather than the cheapest cheapo stuff.  The customer wouldn't stretch to marine ply.

How susceptible is it to general airborne moisture as opposed to rain?

[tggzzz]

It's hardwood BB/CC ply, it is "outdoor" specced (whatever that means), rather than the cheapest cheapo stuff.  The customer wouldn't stretch to marine ply.

How susceptible is it to general airborne moisture as opposed to rain?

I've just replaced a 25 year old very exposed door panel with some marine ply. IIRC that was from an "internal" door, not an "external" one. But the standard of door construction has fallen through the floor in the last 25 years; I've had to replace a door after only 4 years

Summary: I'll tell you in >5 years.

Personally, unless the ply panel will be maintained by someone else, I'd do everything I could to protect it: the correct primer, undercoat (at least one coat) and several top coats. I used Sikkens Rubbol Primer and Sikkens Rubbol XD Gloos, but it is too early for me to recommend them. They have to be ordered at a dulux paint centre.

[Delta]

Well I know the manager there is a bit of a penny pincher, and certainly not one for maintenance, and left to his own devices would have lads out with an old tin of gloss...

I'll recommend that they at least prime it, then over to them...

[tggzzz]

Well I know the manager there is a bit of a penny pincher, and certainly not one for maintenance, and left to his own devices would have lads out with an old tin of gloss...

I'll recommend that they at least prime it, then over to them...

... in writing, so there's no chance they can come back and blame you in 2 years time.

[babysitter]

Make them aware that you either put loads of expensive liquids on it or that they have to expect the need to apply cheaper ones regularly.

[GreyWoolfe]

Well I know the manager there is a bit of a penny pincher, and certainly not one for maintenance, and left to his own devices would have lads out with an old tin of gloss...

I'll recommend that they at least prime it, then over to them...

I would say to use several coats of oil based outdoor rated primer.

[CatalinaWOW]

Too many variables to give you a solid answer, but I'll give you a few data points. 

1.  T1 siding on ceiling of north facing patio with full walls on three sides.  Isolated from weather roof by insulation.  Power sprayed economy grade outdoor white with no preparation.  Southern Arizona location.  Started showing need for repaint after 5 years, but no evidence of degradation of the T1.

2.  OSB roof of south facing patio roof.  Weather roof directly applied to OSB.  Three sides open.   Again Southern Arizona.  Power sprayed with primer and economy grade outdoor paint.  Paint and OSB failing after little more than a year.

3.  East facing patio, closed on two sides.  Ceiling of drywall, mudded, primed and painted.  Separated from weather roof by attic space and insulation above attic space.  Southern Arizona.  Lasted 20+ years (still going strong when I left that house).

Seems to me that environment has a lot to do with the longevity.  If you are in a wet environment, or one that has large temperature variations I would not skimp on the preparations.  Your material should be comparable to the T1 siding in terms of resistance to moisture.  Both are layered wood products fabricated using outdoor rated adhesives.

[Delta]

Cheers for all the detailed replies!

Are acrylic primers any good?

[tggzzz]

Cheers for all the detailed replies!

Are acrylic primers any good?

That entirely depends on just two things: what is put on top of them, and what is underneath them.

RTFM

Treat it like selecting electronic components: if the data sheet doesn't give you the necessary info, would you use it?

Primer should be extremely thin; its purpose is to fill nooks and crannies, and to sink into the wood so subsequent layers adher properly. Some types, e.g. aluminium primer, can also provide a barrier against stuff in the wood affecting the paint. Is the acrylic primer that thin, or does it mean primer-for-acrylic?

[Delta]

Well I can't find a FM to fucking, but I've RTFR, and the reviews are very positive, and they do mention it being very thin, so it should soak into the timber well, with many people saying they've used it on exterior plywood with good results.

Cheers! 😀

[tggzzz]

Well I can't find a FM to fucking, but I've RTFR, and the reviews are very positive, and they do mention it being very thin, so it should soak into the timber well, with many people saying they've used it on exterior plywood with good results.

No man => "no!, man"

Reviews can be astroturfed and too often they are meaningless "I unpacked it and it is great!". But you know that