How to apply plasters in showers

How to apply Venetian plaster and other stuccos to shower walls

Our customers often ask us if they can apply plasters in shower stalls. Certainly, the application of stuccos to showers is among the most ambitious challenges of plaster finishing. Many are the issues connected to it, while, as we will see, the solutions are few. Continue reading to learn how to apply plasters inside a shower with stunning and longlasting results.

A worthy challenge

The application of natural lime-based plasters inside showers can easily transform an ordinary bathroom into a high-end, luxurious environment. Marmorino Venetian Plaster, Tadelakt and Pastellone are all examples of elegant, eco-friendly finishes that can be used in shower boxes.

However, shower stalls are subject to daily water flows and low air circulation, which leads to the accumulation of damp. Consequently, when you apply natural lime-based plasters to shower walls, they rapidly develop irreversible wet marks and molds. Yet, after years of plaster application in showers and steam baths, we have found the solutions to all the main challenges. In this post, we share with you the techniques to achieve durable and flawless plaster finishes inside showers, step by step.

1. Prepare a resistant plaster substrate for the shower

If you want to apply stuccos to the shower’s walls, it is crucial to prepare a resistant yet flexible plaster substrate. This will prevent the stucco from cracking. Let’s see why and how to do it.

I. Applying a plaster to a waterproofing membrane

First of all, in some countries, it is mandatory to cover the shower’s substrate with a waterproofing membrane. This membrane is often a rubbery and flexible finish. However, these membranes are designed for tiling, not for plaster finishes. Plasters are too thin to be applied to such a soft surface, and will surely crack if applied directly on it.

Moreover, covering it with a coat of a fibrous adhesive plaster before applying the stucco is not enough to prevent its cracking. This is because every shower box has a weak point: the faucet handles. When fixing the handles, a ring is screwed into the wall exercising strong pressure against it. As a result of this pressure, the plaster around the handles cracks and breaks. Furthermore, if the plaster coat is not strong enough, cracks can eventually occur all along the walls. 

The simplest solution to this issue is to apply a first coat of fiber-reinforced plaster all over the shower-box surface (such as our AntiCrack enriched with the Strong Adhesion Agent SAA). Once dry, apply a polyester or fiberglass mesh (180 gr per square meter) on the tap and faucet handles area, and fix it to the surface using the same fiber-reinforced plaster.  When this second coat is dry, apply once again the same mesh to the entire shower wall, fixing it with the fibrous plaster. Finally, when the third and last coat is dry, you can directly apply your stucco on the shower box walls.

II. Applying plasters over tiles

Sometimes, you may want to apply plaster over old tiles in a bathroom or shower walls. When you apply stucco on a tiled shower wall, you must prepare a leveled, gripping substrate first. In other words, you want to fill the gaps between the tiles to create a uniform, flat surface. Yet, the majority of substrate plasters tend to shrink when the water contained in them dries off, leaving the gaps unfilled.

In this case, you should cover the tiled wall with a non-shrinking, adherent substrate plaster, such as the Gap leveling plaster (GLP). This plaster has an extreme adhesion capacity and doesn’t shrink when it dries off. Moreover, since it is very adherent, you can use it as a substrate layer on very smooth surfaces, such as marble or tiles, without the need of sanding them first. Just clean the tiles to remove the greasy substances before applying it.

On the contrary, if there are neither waterproofing membranes nor tiles, you can directly apply your plaster finish over a first layer of Universal Primer.

2. Waterproof the shower surface and its critical points

Waterproofing is the second most important step in the application of stuccos in shower boxes. As we have said, the accumulation of damp inside a shower will rapidly cause marks and molds to appear and to permanently damage your plaster. Unless, of course, you know how to choose the right waterproofing treatment for your finish. Furthermore, you must take extra care in protecting those areas where water drops hide or dry slower. Let’s see them one by one.

I. Choose the right final treatment for your plaster

First of all, you must waterproof the shower walls, but also make them easily washable. Indeed, you may not only get stains provoked by a prolonged stay of damp, but also molds caused by the deposits of organic material coming from skin and soaps. Likewise, lime plasters suitable for exterior surfaces also tend to deteriorate in a shower area. This happens because they struggle to dry in the corners, in the lower parts, and in general in an environment with little air circulation such as the shower. The prolonged wetting of a plaster generates irreversible dark stains due to the accumulation of salts in the surface layer of the stucco. 

Our experience taught us that there are two solutions that give the best results. The first is to apply a solvent-based paraffin wax (such as Stucco Italiano’s Protective Wax), better if administered after treating the surface with a water-repellent (like Hydrocalce). Yet, if you choose this paraffin protection, the treatment must be repeated several times every few months. 

The most effective solution is instead to use a polyurethane varnish, which leaves the color intact and, if of good quality, also respects the aesthetic aspect of the finish. First, apply two coats of Universal Primer, diluted 1:14 with water. This treatment ameliorates the penetration of the varnish in the plaster’s micropores. Then, apply two or three coats of polyurethane varnish, such as the PU1C or PU2C. Make the first coat slightly more liquid than suggested by the manufacturer, so that it penetrates well into the plaster’s pores. The water percentage should be between 10 and 15%.

II. Protect the shower’s tab area and the shower tray seal

The shower box has two delicate points. The first, as we have seen before, is the faucet handles area. When we apply lime-based plasters in showers, we must prevent the water from slipping behind the rings of the faucet handles and tap. When it penetrates, it slips inside the plaster substrate creating spots around and under the handles.

You can easily avoid this issue by applying silicone behind the handles’ rings when fixing them to the wall. It is sufficient to put the silicone in the upper arch of the ring.

Create a shadow gap

The second sensitive point is the sealing between the wall and the shower tray. You should never overlook the seal when you apply plasters in showers, since damp and dirt accumulate in this area more than in any other. This will provoke severe wet and mold stains. If you start from the construction, we recommend creating a shadow gap between the two surfaces, as in the picture below. When designing the shadow gap, it is important to make sure that the wall’s surface falls inside the shower tray, so that the water does not stagnate at the point of contact between the two. Then, apply an anti-mold silicone inside the shadow gap.

Example of shadow gap in a shower box to prevent the accumulation of damp between the wall and the shower tray

Other typical mistakes

If you chose to protect your plaster finish with the paraffin treatment, be careful to apply the silicone seal before you apply the paraffin. Indeed, silicone does not adhere to paraffin, and will surely peel off when applied on top of it. The picture below shows this typical mistake: the wall had been first treated with paraffin wax, and only then the shower tray had been sealed. The water infiltrated and the silicone came off.

We avoid applying plaster finishes in the shower tray. You can do this, but you must take extreme care in preparing the area of ​​the drainpipe so that it can be sealed well.

Suggested plasters and treatments for showers

Over the years of application of stuccos in showers and steam baths, we have designed and ameliorated the following products in order to make this ambitious challenge easier for the applicators.

Substrate

• AntiCrack: Stucco Italiano’s fibrous AntiCrack plaster is a perfect substrate plaster for showers. This fibrous plaster must be enriched with our Strong Adhesion Agent (SAA) to be applied to waterproofing membranes. This enriched plaster is extremely strong, while its fibrous nature prevents it from cracking. This material, combined with the use of the mesh, constitutes an incredibly strong inner surface for the application of stuccos on waterproofing membranes. When using our AntiCrack, it is not necessary to apply a Universal Primer before covering the wall with the plaster.
• Gap leveling plaster (GLP). It is a plaster with an extreme adhesion capacity, that doesn’t shrink when it dries off. Because of this quality, we use it to fill gaps between tiles in walls and floors, before applying Pastellone or other plasters. Moreover, since it is very adherent, it can be used as the first layer on very smooth surfaces, such as marble or tile floors, before applying lime or cement-based flooring products. If our GLP is used, no sanding of the tiles is needed: just clean the tiles to remove the greasy substances before applying it.

Plasters suitable for showers

You can use only smooth plaster finishes inside showers because the coarse ones retain the organic residues, which cause molds. Here are some smooth plasters that our clients love to apply inside showers.

• Marmorino Medio: our polished Venetian Plaster, perfect for a greater glossy, classic effect.
• Pastellone: our reinforced Venetian Plaster. You can apply it to bathroom walls, shower and also to the floors. We particularly recommend it when you want to recreate the natural elegance of a minimal, seamless environment.
• Tadelakt: this finish is traditionally used inside Moroccan steam baths (hammams). It is particularly waterproof thanks to its application method. The material is repeatedly pressed with a special stone, that causes all the micropores to close, making the surface impermeable.

Final protective treatment

• Universal primer + PU1C (1 component polyurethane varnish).
  or
• Hydrocalce + Protective Wax

How to apply stuccos inside showers – summed up

FAQ – Can I apply plasters in showers?

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