How do you apply a coat of varnish to wood?
Do it yourself · Finishing
FinishingApplying a coat of varnish
Three thin coats with light sanding in between always beat one thick coat. That is almost the whole story — the rest is about dust.
- Three thin coats
- Sand between coats with 240
- Dust is the enemy
Why thin beats thick
Varnish cures from the outside in. A thick coat seals itself off while it is still soft underneath.
A thick coat of varnish seems efficient but produces a soft, tacky film that still shows marks months later. Three thin coats, on the other hand, each cure fully and bond to one another, together giving a tough finish that stands up to use.
Between coats, sand lightly with 240 grit. That does two things: it removes the dust nibs and air bubbles that end up in every coat, and it gives the next coat something to grip. Skip it and every imperfection shows through the top coat.
The biggest enemy is dust. Varnish stays wet for a long time and catches everything floating in the air. Work in a room that has not just been swept or sanded, close windows against draughts and wipe the workpiece with a damp cloth or tack cloth just before varnishing.
Shaking puts air into the varnish, and that air ends up as bubbles in your coat — which you can't get out again. Stir gently with a stirring stick and let the tin stand for a few minutes afterwards.
- Number of layers
- three thin coats is the standard
- Sanding between coats
- 240 grit, lightly, then remove all dust
- Brush
- synthetic bristles for water-based, natural bristles for solvent-based
- Direction
- always lay off along the grain
- Temperature
- between 15 and 25 °C, not in the sun
- Water-based
- does not yellow, dries fast, little odour
- Solvent-based
- flows out more smoothly, harder, but yellows
To be fair: varnish is the least forgiving finish when it comes to maintenance. If the coat gets scratched or worn, you can't touch it up locally as you can with oil: the transition always shows. Repair means sanding back the whole surface. For a tabletop in daily use, oil is therefore often more practical.
In pictures · Why thin beats thick
Schematic drawing.
From bare wood to third coat
- Sand in stages120 grit, then 180, optionally 220. Each stage removes the scratches of the previous one; skip one and you will see it under the varnish.
- Remove the dustVacuum, then wipe with a slightly damp cloth or a tack cloth. Wait until the wood is dry again.
- First coat thinnedThin the first coat slightly according to the instructions. It soaks in deeper and acts as a sealer coat.
- Let it dryFollow the time on the tin and add some extra in damp weather. Carrying on too soon is the most common mistake.
- Sand lightly between coats240 grit, with a light hand, just to knock off the nibs. Then remove the dust again.
- Second coat unthinnedBrush it out thinly and lay off straight away along the grain, in one stroke from edge to edge.
- Sand between coats and apply the third coatSame method. Do not sand after the third coat — that is your final surface.
- CuringDry is not cured. Treat the surface gently during the first week; most varnishes take several weeks to reach full hardness.
Water-based or solvent-based?
Acrylic varnish
The standard for indoor work, especially on light-coloured wood.
- Does not yellow — stays clear
- Dries fast: several coats in one day
- Little odour, tools clean up with water
- Brings out the grain less
- Less hard than a good solvent-based varnish
Alkyd varnish
For surfaces that take a lot of wear.
- Flows out more smoothly, less prone to brush marks
- Harder and more wear-resistant
- Gives the wood a warm glow
- Yellows over the years
- Long drying time, strong odour, needs solvent
High gloss shows every imperfection and every speck of dust; satin is the safe middle ground; matt hides the most but can develop shiny patches with heavy use. For a first project, satin is the wisest choice.
Five problems and their causes
| What you see | Cause | What to do |
|---|---|---|
| Air bubbles | varnish shaken instead of stirred, or brushed too quickly | let it cure, sand back, recoat calmly |
| Dust nibs | dust in the air or surface not cleaned | sand between coats with 240 and let the dust in the room settle |
| Streaky finish | dried too fast or too little material | lay off in one direction, don't keep brushing back |
| Still tacky after days | coat too thick or worked too cold | patience, and sand back if it stays tacky |
| Finish peels | applied over grease, wax or a glossy old finish | degrease and key the surface before you start |
Frequently asked questions about varnishing
How many coats of varnish do I need?
Three thin coats is the standard. More thin coats are fine; one thick coat never — it does not cure fully and stays soft.
Do I need to sand between coats?
Yes, lightly with 240 grit. That removes dust nibs and bubbles and gives the next coat something to grip. You don't sand after the final coat.
Why are there bubbles in my varnish?
Usually because the tin was shaken instead of stirred, or because you brushed too quickly. Stir gently and let the varnish stand for a while before you start.
Water-based or solvent-based varnish?
Water-based does not yellow and dries fast — the standard for indoor work. Solvent-based flows out more smoothly and is harder, but yellows and dries slowly.
When can I use the furniture again?
You can usually touch it after a day, but full curing takes several weeks. During that time, don't put anything heavy on it or slide anything across the surface.
Can I touch up a scratched varnish finish?
Not invisibly in one spot. Unlike with oil, the transition always shows; repair means sanding back the whole surface and building it up again.
Or would you rather choose oil?
Oil is less hard but can be touched up locally. Which finish suits which use is explained separately.
Have a question about your project? Just ask — we're happy to help.
Photos via Wikimedia Commons: Wood block with no finish: matt and pale — Simon A. Eugster, CC BY-SA 3.0 · Same kind of block with varnish: glossy and warmer in tone — Simon A. Eugster, CC BY-SA 3.0 · Wooden stair treads with a glossy tinted varnish — Editor B, CC BY 2.0