Why is my varnish full of bubbles and brush marks?

Bubbles in varnish come from four places, and shaking the tin is only the first. How to stop them forming and how to remove the ones already in the film.

Difficulty: intermediate
Time: 40 min
Read: 4 min

Short answer

Never shake the tin — stir it slowly. Do not work the brush back and forth, load it by dipping only a third of the bristles, and lay off in one direction with the tip. Bubbles that have already cured come out by de-nibbing between coats, or by cutting the final coat back once fully hard.

Varnish is unforgiving in a way paint is not: it is clear, so every bubble and every ridge is visible from every angle for the life of the finish. The good news is that the causes are few and specific. Air gets into varnish in only a handful of ways, and each has a direct answer.

What you'll need

A good quality brush appropriate to the varnish type
A clean container to decant into, and a strainer
Very fine abrasive, 400 grit and finer
A foam applicator or a wiping cloth, for a wipe-on approach
Cutting compound, for correcting a cured finish

Step by step

Stir, never shake.Shaking whips air through the whole tin and it takes hours to come out. Stir slowly with a flat stick, lifting from the bottom to redistribute the flatting agents in a satin or matt varnish, and let the tin stand for a while before you use it.
Decant and strain.Pour what you need into a clean container. This keeps the main tin free of contamination, avoids skinning, and lets you strain out any lumps. It also stops you overloading the brush by dipping deep.
Load the brush shallowly and do not wipe it on the rim.Dip only about a third of the bristle length. Wiping the brush hard across a tin rim forces air into the varnish held in the brush. Tap the brush gently against the inside of the container instead.
Do not brush it out.The instinct from painting is to work the coat back and forth until it is even. With varnish that whips air in and drags a partly set film. Lay it on, spread it once, then stop.
Lay off with the tip, in one direction, with the grain.Once the coat is spread, go back over it with the very tip of an almost unloaded brush in long single strokes with the grain, from one end to the other. This is what pops the remaining bubbles and levels the film, and it is the whole technique.
Control the conditions.A warm surface releases air as the varnish is applied; a cold room slows levelling; a draught skins the surface before bubbles can escape. Aim for a normal room temperature, gentle ventilation and no direct heat or sun on the work.
De-nib between coats.Once each coat is hard, a light pass with a very fine abrasive shears off any cured bubbles and nibs, and the next coat flows over a flat surface. Two or three coats treated this way give a far better result than one careful coat.
Consider wiping instead of brushing.Thinned varnish applied with a lint-free cloth in very thin coats introduces almost no air at all, levels beautifully and is nearly foolproof. It takes more coats, and on a difficult piece it is the reliable route.

Tips

Horizontal work bubbles far less than vertical. Where a piece can be laid flat, lay it flat.
Foam brushes and foam rollers can generate their own bubbles in oil-based varnish. A good bristle brush laid off properly usually beats them.
If the varnish has been shaken by accident, leave the tin standing overnight before using it and the air will come out on its own.

Common mistakes

Shaking the tin to mix it — It fills the varnish with air that then transfers straight into the film. Stir slowly and let it settle.
Brushing the coat out until it looks even — Working a setting film introduces air and leaves drag marks. Spread it once, lay it off, and leave it alone.
Applying a thick coat to save doing another one — Thick varnish holds bubbles that cannot rise out before the surface sets, sags on vertical surfaces, and cures slowly and unevenly.

If it doesn't work

Fine bubbles all over the surface, cured in

Cause: Air whipped into the varnish by shaking or over-brushing — Fix: Once hard, cut the surface back with a very fine abrasive until the bubbles are gone, clean thoroughly, and apply a further coat laid off properly.

Bubbles only over a particular area

Cause: Air escaping from porous timber or an open joint as the varnish is applied — Fix: Seal the porous area first with a thinned first coat or a sanding sealer, then varnish over it.

Ridges and drag marks rather than bubbles

Cause: Working the coat after it has begun to set, or a room too warm — Fix: De-nib once hard and apply the next coat faster, in a cooler room, laying off with the brush tip only.

Questions people ask

Why does my varnish bubble when I brush it?

Almost always air introduced by shaking the tin, by wiping the brush on the rim, or by brushing the coat back and forth after it has been laid on. All three are avoidable.

Can I sand bubbles out of the final coat?

Yes, once the film is genuinely hard. Cut them back with a very fine abrasive used wet, then restore the sheen with a cutting compound — or simply apply one more properly laid-off coat.

Is a foam brush better for varnish?

It can be for small flat work with water-based varnish, but foam can create its own bubbles in oil-based varnish. A good bristle brush laid off with the tip is the more reliable method.