Choosing the right filler for a loss is one of the most frequent decisions in restoration. A mistake here can compromise the mechanical stability of the treatment, alter the visual readability of the work, or, in the worst-case scenario, damage the original layers. In this guide, we review the most commonly used filler types in conservation-restoration, their properties, and when each is appropriate.
What is a restoration filler and what is it used for?
Filler is the material used to volumetrically reconstruct losses, missing areas, or damage in pictorial layers, supports, or polychromy. Its function is twofold: to mechanically stabilize the affected area and to create a base surface on which to apply chromatic reintegration.
A good restoration filler should meet the following criteria:
- Mechanical compatibility with the original support
- Reversibility: able to be removed without damaging the original
- Long-term chemical stability (no volume changes, no acidity)
- Suitable surface for retouching paint
Most commonly used filler types
Traditional protein-based fillers
The classic formula of animal glue (rabbit skin glue or hide glue) with mineral filler (calcium sulfate, calcium carbonate, or whiting) remains the reference for old works on panel or canvas with traditional preparation. They are compatible with the original materials, respond well to changes in humidity, and allow for work in multiple layers.
When to use them: works from the 13th to 19th centuries with gypsum or carbonate preparation. Ideal when maximum historical compatibility is desired.
Limitation: they are hygroscopic. Not recommended in environments without climate control.
Modostuc
Commercial putty based on calcium carbonate and polyvinyl acetate (PVA). Widely used for its ease of application and good surface finish after sanding.
When to use it: small to medium-sized losses in works on canvas or panel where maximum reversibility is not required.
Limitation: PVA ages and becomes less soluble. Avoid in high-value heritage works.
Polyfilla Fine Surface
Powdered putty based on fine-grain acrylic gypsum (also commercially known as Polyfilla Interior). Good dimensional stability, low shrinkage upon drying. It can be applied in thin layers and sanded to achieve very controlled textures.
When to use it: flat losses in contemporary paintings or where a very precise finish is needed. A good general option for C&R.
Lascaux Gesso
High-quality semi-absorbent acrylic ground formulated for fine art and conservation. Porous and matte surface, with good adhesion to any support. Once cured, it forms a firm base that accepts oil, acrylic, or tempera retouching without migration.
When to use it: contemporary works where a base with controlled absorption is needed for retouching. Especially suitable when working with aqueous techniques or watercolor retouching.
Lascaux Primer
A denser and less absorbent acrylic ground than the Gesso from the same brand. Firm surface with less porosity, designed for supports with higher technical demands or when minimal absorption is required in the final retouching layer.
When to use it: losses where the original work has a very dense ground, contemporary acrylic works, or when retouching is done with oil-based media and no absorption is desired.
Key difference from Gesso: Primer is more impermeable; Gesso is more absorbent. The choice depends on the intended retouching technique.
Quick comparison
| Filler | Base | Reversibility | Ideal for |
|---|---|---|---|
| Animal glue + filler | Protein-based | High (hot water) | Old works, panel, historical canvas |
| Modostuc | PVA + carbonate | Partial | General use, small losses |
| Polyfilla Fine Surface | Acrylic gypsum | Good | Flat losses, fine finish |
| Lascaux Gesso | Semi-absorbent acrylic | Partial† | Contemporary works, aqueous retouching |
| Lascaux Primer | Dense acrylic | Partial† | High technical demands, synthetic supports |
† Cured acrylics are not water-soluble; they can be removed with specific solvents (acetone, acetone/alcohol mixtures). Do not confuse "partial" with "non-reversible."
Key application points
- Substrate humidity: slightly dampen the loss before applying protein-based fillers to prevent them from absorbing too quickly and shrinking.
- Thin layers: multiple thin layers are better than one thick layer. This reduces shrinkage and provides greater control over the final level.
- Finish level: the filler should be slightly below the original level so that the retouching does not sit on top of the paint.
- Isolate before retouching: a layer of varnish or consolidant between the filler and the retouching prevents color migration and facilitates its future removal.
In summary
There is no universal filler. The choice depends on the support, the age of the work, the conservation environment, and the type of retouching to be applied. The key question is always: what level of reversibility do I need, and what original materials am I working with?
At Innovart, you will find the main filling materials used in professional conservation-restoration. If you have any questions, write to us at info@innovart.art — we respond with technical expertise within 24 hours.