Written by Stucco Champions — Southern California’s Authority on Exterior Plastering.

The Permeability Test: An In-Depth Guide to Stucco Paints

Selecting paint for stucco is not the same as picking a color for your living room walls. Stucco is a masonry product; it is hard, alkaline, and critically, it is porous. The coating you choose acts as the final layer of your home’s weather barrier.

If you choose the wrong product—specifically one that seals the surface too tightly—you create a "vapor barrier" on the wrong side of the wall. This traps moisture inside the framing, leading to the two nightmares of stucco ownership: blistering paint and dry rot. This guide explains the chemistry behind the three main coating options.

1. The Golden Rule: Permeability

Before we discuss brands or types, you must understand "Perms."
Permeability measures how much water vapor can pass through a material.
The Goal: You want a coating that is watertight (stops rain from entering) but vapor-permeable (allows internal humidity to escape).
The Standard: For stucco, you generally want a rating of 10 Perms or higher.

2. Option A: 100% Acrylic Latex (The Standard)

This is the most common choice for residential repainting.
Profile: Thin film, highly breathable.
Pros: Excellent permeability (often 15+ perms). It lets the wall breathe freely. It is cost-effective and retains color well.
Cons: It creates a very thin film. It will not hide hairline cracks or texture imperfections.
Verdict: Best for newer stucco that is in good structural condition.

3. Option B: Elastomeric (The Problem Solver)

Elastomeric coatings are thick, rubberized paints applied at 10-20 mils (5x thicker than standard paint).
Profile: High-build, flexible membrane.
Pros: It bridges hairline cracks and moves with the building during thermal expansion. It offers superior waterproofing against wind-driven rain.
Cons: Lower permeability. If applied too thickly or over damp walls, it can blister. It is also more expensive.

4. Option C: Masonry Paint (The Specialist)

These are formulated specifically for high-pH surfaces (new stucco/concrete).
Profile: Alkali-resistant binders.
Pros: Prevents "Saponification" (chemical breakdown) and efflorescence (salt stains). Excellent bond strength to raw cement.
Cons: Limited color ranges compared to acrylics.

5. The Sheen Debate: Flat vs. Satin

We are often asked: "Should I use Satin for better washability?"
Our Answer: No. Always use Flat.
Stucco is imperfect. It has waves, trowel marks, and texture variations. A Satin or Eggshell finish reflects light, highlighting every single imperfection. A Flat finish absorbs light, hiding the flaws and making the texture look uniform and rich.

6. The Cumulative Effect

Permeability is cumulative. If your house has been painted 5 times in the last 40 years, the layers of paint are building up a thick plastic shell.
Adding another coat reduces the breathability further.
The Fix: If your paint is failing or too thick, we may recommend sandblasting or hydro-blasting to strip the old layers before applying new material.

Conclusion: Function First

Don't just buy the cheapest bucket at the hardware store. Read the technical data sheet. Look for the Perm Rating. At Stucco Champions, we specify coating systems based on the age and condition of your stucco to ensure the envelope remains breathable and dry.