Drying Plaster

How are you measuring pH? The range of pH is 0 to 14 and you mention 30 and 20.

Mike

When they built The White House, the drying time of the plaster was sped up by closing the draft on all the fireplaces. It probably seemed like a good idea at the time.

And your point?

I've seen your avatar before. He was a neon sign in front of the electric company in my hometown. His name was Reddy Kilowatt. I didn't know that at the time and I called him "The Frustrated Little Red Man". I was not yet 2 years old.

At least that's what my mom says.

Mike

I am meeting with the contractor's superintendent this morning and see what instructions he has in using the meter he has. I am sure it is a moisture meter. What guidance does he have that states the reading needs to be 13? The paint spec said the ph should be no greater than 13. Is there something in the instructions that will equate the two? All the reading I have done, and my inspector, give answers all over the map, as far as drying time for plaster before painting.

N5EBA,

I should have been more specific in my first post. In my chemistry background, pH has always represented how basic or acidic an aqueous solution is; 0 being most acidic and 14 being most basic or alkaline. Furthermore, I have always seen humidity represented as RH (relative humidity), hence the confusion.

I would be interested in knowing what he's using to measure the moisture. When you find out could you let us know?

Mike

I suspect that the OP is talking of "percent humidity" and the paint supplier is insisting that the surfaces be sufficiently dry before paint is applied.

Applying direct heat to the plaster/cement would not be a good idea, however maintianing a low humidity environment may help substantially. Dehumidifiers to draw moisture out of the air should help with this and drying rate is influenced by the difference in humidity.

A warm environment will also assist moisture removal as it will assist the molecules of water to move to the surface to evaporate.

I suggest to go back to whoever placed the "no mechanical means" requirement and get them to specify rather a maximum rate of change of humidity. I suspect the material supplier is attempting to avoid cracking and such caused by excessive differential rates and has found it easiest to take the high ground and specify NO DRYING as opposed to some more scientific approach. (Ask them to nominate a temperature and RH condition.)

For instance if this was in central Australia, the ambient RH could be as low as 5% at 35 DegC, while in the tropics it could be 95% at the same temperature, so "NO DRYING" seems a poor and unscientific approach. Might be suitable for a home handyman who is only doing one repair per year, but not for the trade.

Be careful though as when you measure the plaster humidity, the actual temperature may influence the readings.

Hi Just an Engineer,

I suspected as much as well. By my question, I guess I hoped to get MORE INFORMATION!!!

Still, there seems to be something else missing. How do you measure the percent humidity? There are sensors and meters that will measure percent RELATIVE Humidity (RH). In this application, how would you get below 18% RH if the ambient RH is more than 18%? Unless you are in a very arid region, the RH will not get that low.

Mike

Not sure how it happens, but I know that when we harvested wheat we could get "water content" readings below 11% while ambient air was around 45% RH.

Maybe with plaster and such, the solid grains account for a significant portion of the total volume and so percent water content can be less than ambient condition.

OK, but what's the test????

Question, how many times do you want to come back and repaint?

What is your experience with_{: blisters, buckles, check cracking, dry outs, efflorescence, excessive pinholes, and similar defects}

_{Warm-Weather Requirements: Protect plaster against uneven and excessive evaporation and from strong flows of dry air, both natural and artificial. Apply and cure plaster as required by climatic and job conditions to prevent dry out during cure period. Provide suitable coverings, moist curing, barriers to deflect sunlight and wind, or combinations of these, as required.}

Why not use cement board?

I suspect the paint manufacturer intends the specification to be <13% water by weight before painting. Obviously this can be achieved faster when the ambient temperature is high and the RH is low. However, rapid removal of water can cause cracking, so they warn you not to use mechanical drying means. I suggest you open windows or use available ventilation means to remove water vapor from the room.

I just got the instructions for the EXTECH moisture meter the contractor is using. All it shows is the meter reads from 13% to 100% relative moisture for concrete and other non-wood material. The contractor is assuming that at 13% it is safe to start the top coat, scratch and brown coat and paint. Anything over 13% and he will not stand by a warranty. The question remains-how dry should dry be? I do not think there is a good answer. Several articles say 30 days and others have something different. I am beginning to think there is no good answer. I could be brave and tell the contractor to paint at 20% and if the paint falls tell my boss the contractor did something wrong. I don't think that will work. Unless someone can come up with something more I will hang my hat on the 13% to 16%.

13% is the point at which lumber is considered to be dimensionally stable. The paint manufacturer may have just extrapolated this to plaster. For an accurate measure of M content in wood we are advised to cut a section and measure the center.

28 days is the amount of time in which both concrete and epoxy may be 'effectivly' cured, though both will continue curing for years. Epoxy needs warm and dry conditions, gypsum needs warm and wet. Dimensional stability will be achieved when the molecular bond is as tight as practicable, thus forcing moisture out. You may acclerate the plaster cure by spraying with warm water, the heat generated by curing will 'dry out' the plaster. You may have to spray 2-3 x/day. Experimentation will verify.

Carl

The confusion in earlier posts seems to be related to the abbreviation ph. In your case it means percent humidity. To others it means acidity. You could always measure the moisture content yourself by weighing fresh plaster and then storing the sample in the room and weighing it periodically to determine the actual moisture %(ph) over time. I suggest you contact the engineering staff at Extech (1-800-884-4967) and request assistance. You should know the model number of the meter being used and you should verify that it is in calibration. Whatever you do, make sure you think about it, and be prepared to defend your actions.

This will be my last comment so I will not bore everyone. I called the moisture meter manufacturer and the meter is accurate within 5%. So a reading of 13% could be between 12.35 and 13.65%. The paint manufacturer said the 13pH referenced in their spec sheet is the alkalinity the plaster must reach before priming and painting. The company has a measuring device that can be used to show the alkalinity. It is like a litmus tester. I will get my contractor to get some. A combination of both meters should tell the contractor when to paint. As far as drying is concerned, natural drying is the way to go in this area and it looks like a rule of thumb is 30 days. With all three in mind the contractor, and my inspector, can make a good educated guess when to paint. Thanks all for your comments and help.

N5EBA,

Thanks for coming back and letting us know what is going on - so many OPs don't.

Anyway, Good Luck!

Mike

Cure!

Don't dry!

The cement in the plaster has to cure to reach it's design strength. As it cures, the water combines with the cement. Then it seems drier. If you dry it before it cures, it will reduce the strength.

Curing can be accelerated by applying steam or raising the temperature in a 100% relative humidity environment.