Techniques for Plastering a Wall, FAQ Friday with Randy Ruth

Q: I have painted drywall in my home and would like a lime plaster finish. How can I achieve this?

A: There are many ways to achieve a lime plaster finish over painted drywall. The first issue to address is who is going to do the work? If you hire a good plasterer then they should know how to approach this issue. Even still, you could enlighten them with new products that they may not know about, that you have discovered in your research. If you are a more “do it yourself” kind of person with some trowel experience than you might be surprised what kinds of finishes you can achieve with a little bit of practice on some 12″ x 12″ sample boards.

How you ultimately approach, this task is dependent on three major factors… budget, authenticity, and texture. Depending on the budget allocated toward a lime plaster finish, a person can achieve a wide range of finishes. The polished mirror finishes often-associated Venetian plaster can but not always be associated with higher costs. This is due typically to higher materials cost and higher wage costs because of the skills required to achieve that level of finish. Speaking of wage costs it should be noted that ceiling applications are much more labor intensive, rightfully so and should probably left to a professional plasterer. A rougher coarser finish can hide slight imperfections in artisanship, thus it typically costs less than other finishes. Since we have now linked the relationship between budget and texture to how you can achieve a lime plaster finish over drywall, it is time to move on to authenticity.

Authenticity refers to the quality of the product. Is it lime, acrylic, or a blend? Sometimes authenticity does not matter however; with the ever-increasing customer demand for low or no VOC products, it may play a role in your decision-making. There are flexible “lime” plasters that have chemical additions as well as many acrylic based bonding agents available on the market. If you want a more real lime plaster system on your painted drywall, than has a solution for you.

You should begin with a clean sound surface, free of any soaps or detergents and of course with no peeling or flaking paint. A quality finish is only as good as the quality of what is beneath it. Then simply trowel apply Takcoat™ evenly over the entire wall 1/16th to 1/8th inch thick. This will act as a transition coat from paint to plaster. Takcoat™ uses hydraulic lime and natural additives to achieve a bond that can stick to glass, ensuring a good bond to the painted wall. Once the transition coat of Takcoat™ has cured for a about 3 days, a second coat of lime can be applied to achieve the finish. Depending on the what the kind of finish is desired another coat of lime can be applied, or even three, four or five. It all depends on what you or the client wants.

If a rough finish or soft-sanded finish is desired than Ecologic™ Mortar in either coarse or fine sand can be applied. If a fine polished, finish is desired than NHL 2 can be applied paper-thin in multiple coats to create depth to the finish, which is polished with black soap diluted with water. all of the products mentioned can be blended together to make the right finish for you or the client. Custom colors can be matched for you, or you can add your own iron oxide or natural lime proof pigments.

Because of all the case specific challenges, answers to this FAQ are just a basic overview. If you plan to tackle this issue head on, give a call or shoot an email to We can help you with choosing the proper products and application techniques. (FaceBook : Google+ : LinkedIn : Twitter : YouTube)

Differences Between Repointing with Natural Hydraulic Lime (NHL) Mortars vs. Cement Based Mortars FAQ with Randy Ruth

Q: Is pointing with lime different from pointing with cement? I’ve done work with cement but I want to know what the differences are for application and general use for repointing my old house. Can you help?

A: When you refer to pointing, I am thinking you are referring to repointing mortar joints. As with any masonry project, proper attention to preparation of the substrate is critical. Always follow proper preparation guidelines regardless of the mortar being used.

If you compare the application process of repointing mortar joints with lime-based mortars to cement based mortars, the only real difference is workability.

Lime mortars tend to have better workability than their cement counterparts do. This is because lime is used to add plasticity to modern cement mortars and thus, when you omit the cement you have the greatest workability. However not all lime mortars can be treated equally when it comes to their aftercare during the initial curing process. Like cement-based mortars, there are different grades of lime for different applications, with different characteristics.

Although there are many different types of limes used around the world, I will only address the four most common from softest and slowest setting to hardest and quickest setting. starting with High calcium lime putty, (Natural Hydraulic Limes) NHL 2, NHL 3.5 and NHL 5.

High calcium lime putty is the softest and slowest setting of the lime choices. Proper attention to curing procedures must be adhered to allow it to set properly, this may take six weeks. Lime putty based mortar has its place in the world, where trained professionals should be the applicators to ensure work is executed in way were the margin for error is limited.

Different grades of Natural Hydraulic Lime are followed by a number designation that indicates the minimum compressive strength at 28 days with a particular amount of sand in Newton’s per millimeter squared. The reason for this classification is that there are no Natural Hydraulic Limes currently produced in the United States and must be imported from where the metric system is used and there have been established standard for a number of years on NHL’s, primarily in Europe. As said before, the lower the number designation for NHL’s the slower setting and softer that type is. So, what does all this mean to you the mason or adventurous DIY homeowner?

Well if you have a conservator mindset then matching the new mortar as closely as possible to the old mortar in color, texture and physical attributes is the end goal. You might want to consult a professional for general advice on what type of mortar to use if you are unsure, after conducting your own research. In general, for most of the United States NHL 3.5 mixed with local sharp well graded sand, which should meet ASTM C-144 or a pre-blended NHL 3.5 and sand mix can be used for general repointing work of older brick and stone structures. This is because NHL 3.5 has an acceptable initial setting time and more importantly provides good vapor transfer in a wall. This allows repointing work to move along at an acceptable speed while knowing that in most cases moisture is not being trapped in the wall cavity. Lastly, the difference between cement mortar and lime mortars for repointing is aftercare. Even though cement based mortar should be damp cured it is not always practiced and is not typically the same length of time when dealing with lime. While working with NHL mortars it is important to allow the mortar to slow cure with high humidity or by misting with water, keeping the recently completed work damp. Although the length of time for aftercare curing of mortar will vary in direct proportion to the particular grade of lime used. Generally, 2-4 days of slow damp curing with either damp burlap misting for repointing work is acceptable. In some cases however, this curing period should be extended.

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FAQ Friday with Randy Ruth

Hooray, Today is Friday! Another rainy weekend will be upon us here in Pennsylvania, but now it’s time for another FAQ Friday with Randy Ruth. Today’s question is another very common question on the use of lime mortars vs. cement. After you finish reading please visit our new facebook page and “Like” us…

  • Q: Could I perhaps use cement and sand? What is the advantage of using lime on old stone structures
  • A: You should not use only cement and sand for a variety of reasons, the first being that in today’s mortars, lime adds workability and plasticity to the Portland cement mortars. Without lime in the mix or proprietary additives, the cement and sand mortar will have extremely poor workability. Secondly, modern-day cement is much different than early cement or lime, it is very hard, dense, vapor impermeable, and brittle. Cement can trap moisture inside the wall and erode the mortar behind the repair mortar, this can cause further unseen deterioration and masonry unit (stone, brick, terracotta) deterioration, thus resulting in a delayed and much larger/costly masonry repair.

The advantage of using an appropriate lime mortar on old stone structures deals with compatibility. There is a rule of thumb when approaching a restoration project and that is to repair in kind with like materials. By following this approach an individual can avoid unforeseen problems associated with trying something “new and improved” when there is such a well over 2,000 years of lime building history. There have been a number of studies done around the world on historic structures that conclude that even a small amount of Portland cement added to a lime mortar mix, can cause detrimental damage to the adjacent masonry and historic bedding mortar.
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