Varnishing Acrylic Paintings: Isolation Coats, Finishes and How to Apply Varnish
Picture of Jessie Guerrero
Jessie Guerrero

Varnishing Acrylic Paintings: Isolation Coats, Finishes and How to Apply Varnish

Varnish is the last decision in an acrylic painting, and it is easy to treat it as an afterthought. It should not be one. The right varnish protects the surface and sets the final sheen of the painting. The wrong one, or the right one applied badly, can leave cloudy patches, brush marks or a finish you cannot undo.

This guide explains the sequence Golden Artist Colors recommends: let the paint dry, seal with an isolation coat, apply a removable varnish in thin layers, and choose a finish on purpose. Most of the specifics below come from the published guidance of Golden Artist Colors, a manufacturer of professional acrylics. Other brands publish their own instructions, so always read the label on the product you actually buy.

Why varnish at all?

Golden's application guidelines open with a blunt statement: varnish should "never be considered as merely an afterthought to the creative process." There are two practical reasons to care about it.

  1. Sheen. Varnish lets you choose a gloss, satin or matte finish for the whole painting, and Golden notes that a surface that needs to stay reflective should be finished in gloss.
  2. Future care. Golden's varnishes are removable, and Golden recommends an isolation coat "for future conservation and varnish removal purposes." In other words, the varnish is meant to be taken off one day without harming the paint. That only works safely if there is a barrier between the paint and the varnish. That barrier is the isolation coat.

Step 1: Let the painting dry first

Do not rush. For thin acrylic layers, Golden recommends waiting a day or two before applying the isolation layer, followed by another two days to a week before varnishing. If the painting has thick, impasto areas, wait a week or two before applying either one. Oil paints are a different case entirely: Golden notes they generally need six months to a year of drying time before varnish.

Step 2: Apply an isolation coat

An isolation coat is a permanent, clear layer that sits between the acrylic paint and the removable varnish. Golden describes its purposes this way:

  • It seals absorbent areas so the varnish sits evenly.
  • It protects the paint from the solvents used when the varnish is removed in the future.

Golden recommends using an isolation coat before varnishing, and says highly absorbent surfaces must have one before a matte or satin varnish goes on. Golden says this is to prevent a cloudy or "frosted" appearance.

A few details from Golden's instructions:

  • Brush mix. Golden's own isolation-coat page describes mixing its Soft Gel (Gloss) with water at 2 parts gel to 1 part water, to roughly the consistency of heavy cream. Mix slowly to avoid foam. A ready-made isolation coat product is also available.
  • Brush choice and coats. Use a soft, thin synthetic brush and apply one to two coats, brushing each coat perpendicular to the last.
  • Spray application. Two to three coats are suggested when spraying.
  • Check your work. Once the coat is dry, look at the painting at an angle. An even sheen means it is ready for varnish. If not, add another coat.
  • Drying. The coat should dry to the touch within roughly 10 to 30 minutes, and Golden says it is best to let it dry one day before varnishing. If two weeks or more pass, wipe the surface with a soft, lint-free damp rag first to remove residues that could hurt adhesion.

Cautions that matter

Because an isolation coat is permanent, Golden advises testing on your own work first. It is intended for acrylic paintings only and should not be used on oil paintings. Golden's isolation-coat page also lists works in water-soluble media, pieces with intentionally varied sheens, and artwork containing metallic leaf among the cases to avoid. It warns that brushing can add brushstroke texture, change or flatten surface texture, or shift color, and that diluting beyond 2 parts gel to 1 part water increases haziness. Golden recommends gloss products for this layer, because satin or matte versions can cloud the piece.

Step 3: Choose gloss, satin or matte

Golden's guidance on finish is specific:

  • Choose gloss when you want a surface to stay reflective.
  • When applying satin or matte varnish, never apply more than two coats.
  • To build up multiple layers, start with gloss and then finish with one or two coats of satin or matte.
  • If the result is too glossy, apply a satin or matte coat of the same kind of varnish that is already on the painting.

The last rule is worth remembering: when correcting a surface that is too glossy, use the same kind of varnish already on the painting. Golden makes polymer and MSA varnishes and gives separate instructions for each, so check which one you have.

Step 4: Thin the varnish

Golden states that its varnishes "must be thinned before use." For the polymer varnish, distilled water is the preferred diluent, and a ratio of 3 parts varnish to 1 part diluent should flow on well with a brush. For satin or matte finishes, Golden advises thinning only the amount you will use that day. For MSA varnish, Golden says "odorless" mineral spirits should not be used, and that mineral spirits should only be used after testing compatibility on a small sample.

Step 5: Apply thin coats, flat

Golden prefers brush or spray application and cautions against sponges and rollers, which can cause foaming, uneven coverage and sagging.

By brush:

  • Work with the painting on a horizontal surface to reduce running and sagging.
  • Use a high-quality bristle brush. Golden mentions brushes from Purdy and Wooster as examples.
  • Apply two or three thin coats with enough drying time between them, rather than one thick coat. When building multiple coats, Golden suggests 3 to 6 hours between coats.

By spray:

  • Apply three to four light, even coats instead of one or two heavy ones.
  • Keep a uniform distance from the surface and turn the painting 90 degrees between coats.
  • Golden says spray application is best for impasto and is required for fragile paint films such as gouache.

Step 6: Mind the room

Varnishing is sensitive to the room. Golden says the ideal temperature is above 65 F and below 75 F, with relative humidity between 50% and 75%. Excess humidity or cool temperatures may cause "bloom," a cloudy whiteness caused by trapped moisture.

Step 7: Let it cure and protect it

Golden recommends letting varnish cure for several days before packing or transporting a painting. Because its varnishes are removable, they should not be painted over. Golden says never to stack paintings, and to keep packing materials, including glassine, bubble wrap and other plastic, off the surface.

A quick varnishing checklist

  1. Paint is dry (a day or two for thin layers, a week or two for impasto).
  2. Isolation coat applied thinly, checked at an angle, and left to dry for a day.
  3. Varnish thinned according to the manufacturer's instructions.
  4. Two or three thin coats, with the painting flat, in a room within the recommended temperature and humidity range.
  5. Satin or matte varnish limited to two coats.
  6. Painting left to cure for several days before packing.

Finally, test first. Golden advises trying a varnish on a test piece similar in composition to the artwork before committing.

Sources

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