Understanding Support Induced Discoloration (SID) and Its Effects on Painting
Grounds
While watching several colleagues priming a number of Masonite panels with Acrylic Gesso (Acrylic Dispersion Primer), I noticed one of the panels had some odd marks on it. At first, it looked like someone touched the newly applied “gesso” with dirty hands—but that was not the case. Upon closer inspection, I realized it was something I had only ever seen once prior: Support Induced Discoloration (SID.) Let’s take a look at exactly what SID is and see how we might avoid it happening.
Introduction
Support Induced Discoloration (SID) is a phenomenon specifically associated with acrylics where water-soluble compounds from a substrate migrate into paint layers, causing unwanted staining and discoloration. This issue is particularly prevalent in acrylic dispersion grounds and mediums, but similar forms of discoloration can also occur in oil grounds and traditional gesso due to different mechanisms. Understanding the causes, prevention strategies, and impact of SID across different painting surfaces is crucial for ensuring archival-quality artwork.
Causes of Support Induced Discoloration
SID occurs when moisture interacts with water-soluble impurities in a painting support, drawing them into the overlying acrylic layers. Some of the most common compounds responsible for this effect include:
- Lignins and Tannins – Naturally occurring in wood fibers, especially in MDF and hardboard. These compounds can cause yellowing or brown stains in clear acrylic mediums.
- Formaldehyde and Synthetic Resins – Found in engineered wood products, these binders can react with acrylic binders, leading to staining or adhesion problems.
- Acids (Acetic Acid, Humic Acid, etc.) – Present in some wood sources and can alter the pH balance of acrylic films, leading to subtle discoloration over time.
- Surfactants and Processing Chemicals – Additives used in MDF and some panels may continue to leach out, interacting unpredictably with paint layers.
- Water-Soluble Salts (Sulfates, Chlorides, etc.) – These compounds can cause efflorescence (white, powdery residues) or texture shifts in acrylic surfaces.
SID in Acrylic Grounds and Mediums
Acrylic paints and grounds are particularly vulnerable to SID because they are water-based and porous. When applied to unsealed supports, water from the acrylic dissolves and mobilizes contaminants, drawing them into the film as it dries. Over time, residual moisture or humidity can continue to activate these compounds, making the staining effect worse.
Preventing SID in Acrylics
- Seal the Support: Use barrier layers like Golden GAC 100, PVA size, or shellac to block water-soluble contaminants.
- Apply Multiple Coats of Acrylic Gesso: After sealing, a properly applied gesso can further prevent staining.
- Use Pre-Sealed Supports: Consider archival-quality wood panels or non-porous alternatives like Aluminum Composite Material (ACM).
Discoloration in Oil Grounds and Traditional Gesso
While SID is a term specifically associated with acrylics, similar forms of staining or contamination can occur in oil grounds and traditional gesso, though the mechanisms differ.
1. Oil Grounds and Oil Paint
- Oil-based grounds are hydrophobic, meaning they do not interact with water-soluble contaminants as easily as acrylics do.
- However, unsealed supports can still lead to discoloration as oil-based mediums oxidize and darken over time, pulling in tannins and other compounds.
- Prevention: Applying a shellac or PVA size before priming with oil-based grounds can help mitigate staining.
2. Traditional Gesso (Rabbit Skin Glue & Chalk)
- Traditional gesso is water-based, meaning it can absorb and activate tannins and acids from wood supports, leading to discoloration.
- Rabbit skin glue is hygroscopic (absorbs moisture), which can cause long-term instability and make the ground susceptible to staining.
- Prevention: Multiple layers of glue size should be applied to seal the surface before applying traditional gesso.
Key Takeaways
- SID is most common in acrylic grounds due to their interaction with water-soluble contaminants.
- Oil grounds are less affected but can still experience long-term discoloration from tannins and oxidation.
- Traditional gesso can also pull contaminants, but proper sizing helps prevent this.
- Best universal protection: Always seal porous supports with a proper barrier (e.g., GAC 100, shellac, or PVA size) before applying any ground.
It is important to note that Support Induced Discoloration (SID) can impact oil paints applied over acrylic grounds, though the effect is less direct than in acrylic layers alone. If the acrylic ground is not properly sealed, water-soluble compounds such as tannins, acids, and formaldehyde (from MDF) can migrate into the acrylic layer before the oil paint is applied. Over time, some of these contaminants may interact with the oil paint binder, leading to subtle color shifts or long-term yellowing, particularly in lighter oil colors. Additionally, acrylic gesso applied over an unsealed, porous support may retain staining agents that remain dormant until they affect the oil paint film as it ages. While oil paints are less porous than acrylics, lighter colors and transparent glazes are more vulnerable to staining, and as oils oxidize over decades, any discoloration that was initially subtle may become more pronounced.
To prevent SID from affecting oil paintings, artists should first seal the support with a barrier (such as Golden GAC 100, PVA size) before applying an acrylic ground. Multiple coats of high-quality, oil-compatible acrylic gesso can further help block contaminants. Choosing non-porous supports or properly sealing MDF and hardboard before applying any primer is also essential for avoiding long-term discoloration. While SID is primarily an issue for acrylic layers, it can still indirectly impact oil paintings if the underlying acrylic ground is not properly prepared, making sealing and surface preparation crucial for archival stability.
Conclusion
Support Induced Discoloration is a significant concern for artists working with acrylics, though similar forms of discoloration can occur in oil-based and traditional gesso grounds. By understanding the causes and implementing proper sealing techniques, artists can prevent unwanted staining, improve the longevity of their work, and ensure better color stability over time. Choosing the right supports and ground preparation methods is essential for maintaining the archival quality of any painting, regardless of medium.
