That depends on what kind of paint is used. In the past when the plaster was on newly laid masonry it could take up to a year before it was painted. This was due to both the high pH of the lime in the hydroxide phase but also because it took that long for the masonry mass to dry out.
Today we are generally plastering old masonry or wood lath, so the drying time is much shorter. But the high pH (around pH-12 when first installed) still needs to be taken into account. Until the lime carbonates and the pH eventually reaches about pH-7, the pH can damage many paints. Applying oil or acrylic paint over too-fresh plaster can cause a reaction that creates a big soapy mess as the oil is saponified.
Does that mean that since lime plasters cure relatively slowly you can’t put anything on the surface for months?!
Obviously limewash due to its compatibility can be applied as soon as surface carbonation takes place, in as short as 24 hours. Ideally I would still recommend letting fresh lime plaster cure three weeks even before applying limewash as any coating will slow the movement of carbon dioxide from the air into the interior. That means the exterior of your plaster may firm up, but internal carbonation will be impeded which is not ideal long-term.
But are there alternatives to lime wash that can be applied to fresh plaster?
Namely, can casein paint be applied to relatively fresh lime plaster?
Since I hadn’t heard of anyone doing this, I decided it was time to do a small test. And given that we are now able to get lime putty that has much higher surface area than in the past, the rate of carbonation is improved, making that consideration a smaller concern. When I started in this business, people lauded 10 sq m/gram of surface area as a great feat. Now we wouldn’t accept anything less than 40 sq m/gram. I beat up a bucket of lime mortar that has been sitting around the shop for several months and spread trowelfuls on a bone-dry interior block wall.
Included at the end of this article are a few notes on plaster preparation steps.
An hour after install I used a wood float to open up the surface texture. By not leaving the surface slicked/smooth, the wood float increases the carbonation rate to the interior and compacts the mortar to accommodate for initial water loss. When I returned the next morning the surface was well carbonated, having a hardened crust.
That next morning I mixed up a batch of commercial powdered casein paint and brushed on a bright yellowy mustard stripe which took to the surface well. When I checked a few hours later it was quite dry without any chalking.
For those new to casein paint, it is casein from milk reacted with lime in water to create a pretty tough mineral-and-protein crosslink. Normally milk paint is sold in powdered form by companies such as The Old Fashioned Milk Paint Co (milkpaint.com), a company with a nice array of brick- and masonry-related colors and used primarily on wood. To make powdered milk paint into a casein paint for exterior applications, a small amount of 1% boiled linseed oil is added just before painting.
From 1-5% oil to the dry weight of casein is acceptable, but I find 1% is usually more than adequate without changing sheen. I appreciate the completely matte appearance this provides as it produces painted masonry that does not appear painted.
Many brick buildings of the Victorian Era and peaking between the 1870s and 1910s were painted with casein. This is a fact that has largely gone unrecognized because the masonry does not appear coated in paint, again due to the extremely matte thin applications of casein paint. Without knowing it, each of us has probably seen 130-year-old paint on many buildings and not realized what we were seeing.
By the next morning, I began testing with pH paper. Adding a bit of water to the surface helps draw forward the higher pH from the uncarbonated interior of the plaster. While surface pH was low, it was still quite high at the interior. Since any coating can alter moisture conditions, it is important to consider interior pH. Despite the higher pH the casein looked great and showed no deterioration.
I tested the plaster over several more days. By the 4th day, the surface was down to pH-7. But adding water to the plaster would draw more of the high pH lime to the surface and would eventually rise to pH-10.
The pH range is from 0-14 with most citrus products at about pH-3 and baking soda usually around pH-9.5. Since pH-7.5 is considered neutral, a reading of 10 still shows the lime putty’s original high pH of 12 has not been neutralized by complete carbonation. When fully carbonated, lime mortar should be at pH-8.
Keep in mind that because this test was done using a well-aged high-surface area lime plaster, it was already quite well carbonated at the surface at 24 hours. If this were an exterior application of stucco, the curing could have been increased even faster by exposing it to the rain after the first 24-36 hours. (see more below.)
This little experiment with me providing regular hydration mimics how rainwater can mobilize remaining hydroxides in the uncured lime beneath a cured surface to repair cracks (autogenous healing). At the same time, rainwater — which is more acidic than tap water — acting on carbonated portions of the lime can dissolve and redeposit lime at the surface repeatedly. This allows the carbonate crystals to realign and nucleate on one another until they create an increasingly tough calcite surface, again, all the while sealing up any cracks that may have occurred.
(For those interested in learning a bit more about the action of rain on mortar, you might want to check out this article on Lime Mortar Weathering Tests.)
Before we go, and just in case you are new to plastering with lime mortar, this seems a good spot to comment on the importance of preparing your hawk full of mortar properly.
Mortar for plastering, just as with most pointing, should be well knocked-up so that you can lift up to several inches with a pointing tool for a 1/4″ high joint 
Each time the hawk is loaded up, the entire mass should be scooped up and dropped from a foot high several times 
…and with that, the plaster is ready when the hawk can be inverted without any mortar being dropped.
The Preservation Science Blog 
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