LimeStong plasters are simple to mix and apply, but the adhesion, curing, and performance characteristics of pozzolan-lime plasters need to be understood and proper preparation and curing techniques used.
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Preparing the Surface Correctly is Critical to Successful Adhesion. The effort to properly prepare the surface directly reflects on the quality and durability of the finished job.
Lime-pozzolan plaster has natural adhesive properties and can be applied directly to wood lath, metal lath-clad plywood or OSB sheeting, straw bales, pumice-crete (a honeycomb-like insulative concrete) stone, kiln-fired or sun-dried brick, earthen structures like rammed earth, and hempcrete.
LimeStrong can also be applied directly to poured concrete walls and cinder or concrete blocks, but unless the surface is fairly rough (usually not the case when the concrete has been consolidated with a vibrator) it is necessary roughen or key on the surface before apply plaster. This can be done a couple of ways.
1) Use tile thin-set gaged (mixed and thickened) with a little LimeStrong, then spread over the wall with a quarter-inch notched trowel (or throw it on) to create the needed tooth/key on the surface.
2) You can also use a common concrete bonding additive. (It looks like thick milk). Again, gage with LimeStrong to thicken to a sandpaper-like texture. Dampen the masonry wall with water first then, apply the gaged bonding additive.
Taped gypsum drywall (like Sheetrock®), makes a good surface for a LimeStrong finish, but must first be primed with a prep coating of a polymer-based primer (like Kilz® or any PVA primer/sealer). First, gage the primer liberally with LimeStrong to give it a gritty tooth, then roll it on drywall or OSB sheeting. No presetting necessary before applying the gaged primer/sealer.
You can also use LimeStrong over EPS foam with an acrylic-modified adhesive base coat, like Primus®. Again, gage the acrylic-modified adhesive with some LimeStrong (thicken to the consistency of a thin-set tile grout) and use a quarter-inch notched trowel to apply it to the foam.
If the interior drywall or concrete is painted, sand to rough it up, then prime with a LimeStrong-gaged PVA primer.
BONDING. It is critical that a good bond is established between LimeStrong plaster and the base surface. This grip is achieved both from suction as well as physical purchase on surfacing substrate. A good "key" or surface roughness is necessary. PREWETTING (damp, not dripping wet) is typically needed to enhance bonding between substrate and wet plaster, as well as subsequent plaster layers. Prewetting/damping before application will also avoid the dry surface beneath sucking the water from the new layer above and affecting the quality of the cure. In low-humidity conditions and when going over wood lath or straw bales, the substrate can be wet thoroughly the day before, then dampened again right before application.
Between plaster coats, a large brush or sprayer will do the job.
OLD SUBSTRATE. Beware of simply coating over old plaster (or any other failing surface substrate)...if that plaster is failing, it will take the LimeStrong off with it when it goes. Also, synthetic and Portland cement stuccos and plasters have different characteristics than pozzolan-lime plasters, and will adversely affect the lifespan and performance of LimeStrong.
REPAIRS. If the base coat is cracked, those cracks will eventually work through a surfacing coat of plaster. Cracks must be gouged out and filled. Wide cracks, joints, holes and deep gouges need to be filled with multiple layers of LimeStrong and allowed to cure.
CLEAN SURFACE: The surface must be free of dust, organic growth, oils, paint, sealants, etc. Once clean, prep and, if necessary, prime and key the surface before applying LimeStrong.
EVEN SURFACE: LimeStrong can be applied to uneven surfaces, but ideally, you want to main base coat to go on and cure at a consistent depth. That means some preliminary application may need to be done to fill low spots and bring the surface up to a more consistent evenness.
Preparing the LimeStrong for Application. Mix LimeStrong with clean water and let it rest for 20 to 30 minutes, then re-stir and apply.
Preserving Unused Mud. Because lime-pozzolan plaster cures naturally—without the ticking-clock of a hot chemical reaction, unused plaster can be kept covered to keep out the air, (a lid; sheet-plastic sealed around the edges) then uncovered, stirred and adjusted to application consistency. Best used within 3 days of initial mix for optimal workability.
Application Basics. The following is by no means a thorough detailing of proper plastering techniques and processes, but rather serves as a set of general guidelines for correctly applying LimeStrong pozzolan-lime plaster.
Laying up three coats of LimeStrong plaster is the typical approach, allowing proper cure between each coat. The thickness of the first two coat is approximately 1/4 to 3/8 of an inch. Going thicker than an 1/2 inch will invite curing problems. Key (score) the surface lightly to receive the next coat. On interior applications over primed-and-keyed drywall, two plaster coats will usually suffice.
The second plaster coat is the straightening coat, wherein you're going for consistency by eliminating surface irregularities and undulations and floating to the desired line.
The finish coat is the thinnest, and can be left somewhat rough, or worked semi-smooth to smooth with a steel trowel once it has cured green-hard. If you want a tight, glass-smooth finished surface, the ultrafine paste grade of LimeStrong is recommended for the final coat.
The cure time between coats depends on the thickness of the coat and the atmospheric conditions...anywhere from a couple of days to a couple of weeks or more. A functional cure is typical in 48 to 78 hours. Remember, a proper pozzolan-lime plaster cure takes time.
One tactic to determine proper cure before applying the next coat is to press with the pad of the thumb—if it makes no indentation, you're good to go. This is the green-hard stage, and it will be still be slightly damp.
Loose grains of plaster should be lightly brushed off before the next coat is applied. If it has cured/dried to the point that it is no longer damp, clean water can be used to dampen things up.
Color Tint Finishes. Color pigment (mineral-derived power or liquid) can be mixed into the finish coat or working into an almost-cured finish coat as a lime wash. LimeStrong cures to a natural whiteness and so accepts and shows off tints and color washes extremely well. A skilled applicator will work in both color and texture, applying thin finish coats (washes) to build deep, rich, fascinating color and texture combinations that are not possible with conventional paster finishes and certainly not typical painted surfaces. The process is a true fresco technique that penetrates the plaster and becomes part of it, preserving the plaster’s ability to breath. The color will mellow and soften with time, arriving at a beautiful patina that has character and interest arrived at in no other way. Best to work out the particulars of the color washes on sample boards before going after the finished wall.
Do not paint a LimeStrong wall, as it fills the pores in the plaster and wrecks the breathability of the finish.
Finish Textures. The finish texture possibilities are limited only by the skill of the applicator. Exterior applications range from a rough, flicked-on old-world look to various types of troweled-in textures. Interiors can be worked to various textures—rustically rough to glass smooth, with course, fine, and paste LimeStrong grades available to easily meet the desired final texture.
Adjust the Wetness the LimeStrong Mud to Meet Local Conditions. The stiffness to which your mix the LimeStrong mud depends on the atmospheric conditions. For example, in the western United States, were dry air is the norm, a wetter mud is needed to enhance workability and adhesion as well as set up proper curing on the wall.
Assuring the Proper Cure. A pozzolan-lime plaster cures slower than the chemically-charged rapid cure of Portland-based cement or synthetic stuccos. Pozzolan-lime plasters cure by both reacting with atmospheric carbon-dioxide and via a pozzolanic reaction, and that takes time—it’s a curing process, not a drying-out process. And it is this gentle air-cure that ensures the desirable breathability, flexibility, and autogenous healing characteristics of pozzolan-lime plasters.
The surface needs to stay damp to insure the plaster cures completely through. If the surface is allowed to dry too quickly, it will inhibit the curing process on the interior material. Allow adequate curing time between plaster coats—how long depends on atmospheric conditions. In dry climates and/or windy conditions, it may be necessary control the drying rate by misting with clean water. Do not over-wet the surface (if water droplets form, you are over-wetting).
Avoid freezing curing plaster. In freezing conditions, provide adequate protection.
Proper Building Design is Critical to Protect an Exterior Lime Pozz Plaster Finish. If your house or building is still in the planning stages, contact us for advice about structure design best-practices that insure optimal performance of a LimeStrong finish. What you want are adequately deep eave and gable overhangs to prevent water from flowing freely down the plaster surface.
The Case for a Rough Exterior Finish. As a rule of thumb, the rougher the exterior finish, the more durability is inherent in the finish. Rough textures are not only quicker and easier to apply, they softens the look of the structure, as reflected light dissipates and defuses. Rough finishes, such as a harling or thrown finish, are durable, easy to repair, show less imperfections, and are quick to apply. Unlike smooth finishes, rough finishes are quite suitable for sever climates, as evidenced by the traditional use of rough lime plaster finishes throughout Scotland.
Preserving Unused LimeStrong Mud. Because lime-pozzolan plaster cures naturally—without the ticking-clock of a hot chemical reaction, unused plaster can covered to keep out the air, (a lid; sheet-plastic sealed around the edges) then uncovered, stirred and adjusted to application consistency.
Protect Yourself. The alkalinity of lime can cause skin irritation and burns. Long sleeves, gloves, and eye-protection is recommended. You can keep a bucket of water and vinegar handy to neutralize the bite of plaster on skin and tools.
Contact us for more information, availability, and training in the use of our LimeStrong products.