DIY Finishing - Furniture - Finishing Techniques

7 varnishing or painting defects

There are days when we have to stay indoors and we need activities to make the time go faster and easier. DIY enthusiasts can make the quarantine more bearable by transforming old furniture, freshening up the paint on doors or windows, painting frames or other wooden objects. And those with woodworking workshops have the opportunity to learn more about finishing.

finishing defects

I wrote here and here about problems that can occur when finishing surfaces with wax or oil. I thought an article grouping together the problems that can occur when applying paint or varnish and how they can be solved might be useful to you. Especially as many of the questions I get from you are related to this topic.

Surface preparation before repainting

But first, a few tips on preparing objects before they are repainted. An old object gathers dust and grease over time, which settles on the surface. If we want the surface to look good in the end and the paint to not peel off at the slightest touch, objects must be cleaned before applying the new coat of paint.

Don't worry, it's nothing complicated. Cleaning is done with water and soap scum or dishwashing detergent. I recommend these products because they are very effective in removing grease. Make a not very concentrated solution and using a soft cloth (old cotton t-shirts are best) wash the surfaces. ATTENTION, the water must not be soaking wet. Squeeze the cloth before wiping the objects. Excess water can soak in deeply and dislodge parts of furniture or veneer.

After wiping all surfaces thoroughly, insisting where the dirt is more difficult to remove, wipe everything with clean water, still with a soft, well wrung-out cloth. Finally wipe again with a dry cloth and leave to soak for a few hours before applying the new coat of paint or varnish.

In the case of furniture, degreasing is mandatory. Commercially available furniture care products contain a quantity of wax. This remains on the surface and not only fixes the dust, but also greatly weakens the adhesion of the new coat of paint.

And no, water does not whiten the lacquer on furniture unless it is in excess and puddles (sits for a long time) on the surface.

finishing defects

Some of the problems that can occur when varnishing or painting and how they can be solved

Let's get back to the problems that can occur when painting or varnishing. I've limited myself to those that arise when paints are applied with a brush, trafalette (roller) or simple spray gun (cup gun).

1. Rough surface

There may be several causes:

  • the film has trapped dust or other impurities from the atmosphere,
  • the film has incorporated air,
  • if the application was made directly to the wood, it was not properly sanded beforehand

Dust is the enemy of varnish or paint films, especially when they are glossy. To avoid trapping dust in wet film, avoid working outside in open spaces and use varnishes and paints that dry faster. But not too quickly (don't force drying with heat sources like hot air blowers) because then the air in the film doesn't have time to escape and you end up with the same result. Air in the film is always there, regardless of the method of application, but it is least likely to get there if you use a stiff sponge mop.

If you apply paint directly to the wood, it should be sanded first with 180-220 grit sandpaper or medium abrasive sponges, then buffed off with a damp cloth. Do not apply paint immediately unless it is water-based. More about wood sanding (white sanding) find here.

In order to obtain a smooth surface that is pleasant to the touch, you will need to sand the rough surface very well with a fine sanding sponge or 280-320 grit sandpaper (or 400 grit if the varnish or paint is very glossy) until the roughness is removed (it can be felt by hand), dust very well with a cloth, allow any dust in the atmosphere to settle and carefully apply another coat of varnish or paint. Apply thin coats because a thick film is less elastic and can crack.

If the object you are finishing is glossy, you can remove roughness from the film by buffing. How to polish find here. It is not recommended to use the method for matt varnishes because it will increase the gloss.

If you have a special space where you do the painting, you can throw water on the floor (as you do in summer before sweeping). Water attracts dust and holds it.

2. Orange peel appearance

After drying, the film is not perfectly smooth, looking very much like orange peel. It is visible immediately after varnish application (the more visible the more glossy it is), but may disappear after drying. Lacquers and paints are viscous and take time to set. If this time is less than the drying time, the final film will not be smooth. Viscosity is controlled by solvents (water or organic solvents). If the varnish or paint is thick, it needs to be thinned in order to get the best possible laydown.

Orange peel can also occur if the lake and object are brought from an unheated warehouse and have not been allowed to reach a temperature above 15°C. If the application is done by gun, the distance between the gun and the object as well as the air pressure can lead to the defect. The distance should be at least 25 cm and the pressure 3-4 atm. Those of you who have a compressor for staple guns in your workshop, do not use the same pressure when applying varnishes. In addition to the orange peel film, you will also have a large loss of varnish.

Sanding and repainting also solves this problem. Sanding should be done more thoroughly, not just superficially, to remove any bumps in the film. After sanding, apply a thin, thinned coat of varnish and do not force dry.

3. Prick in the film

The defect may look similar to air bubbles left in the film, but it is different. If you look closely you'll see fine needle-like punctures, clustered together. They appear when varnishes or paints are applied by spraying.

There are two causes of the defect:

  • accumulated water in the compressor which, for various reasons, ends up in the gun,
  • spraying of water-based and solvent-based products is done with the same gun.

In the first case, stings occur if the varnish is solvent-based. Moisture from the atmosphere accumulates in the compressor or on the hoses and reaches the gun where it is converted into very small droplets and mixed with the lacquer. As water and solvent don't mix, the fine water droplets reach the surface and form those little pinprick holes. The problem is solved by putting filters on the trails, checking and emptying the compressor more often, especially during periods of high humidity.

In the second case, the stings also occur because the water and solvent do not get along. If solvent-based varnish is sprayed after water-based varnish and the gun has not been washed very well, these stings occur. It is recommended that the gun is rinsed with acetone after washing. Acetone is compatible with both water and solvent.

Removal of the punctures is also done by vigorous sanding and application of a new coat of varnish or paint.

4. Craters (fish eyes) in the film

They can be seen immediately after the varnish/paint is applied or immediately afterwards. Looks like when water gets on a greasy surface. This is due to grease, oil or wax marks on the surface because it has not been properly washed and degreased or silicone particles in the atmosphere. Greasy substances can be on hands or on cloths used to wipe surfaces. Silicone particles get onto the surface from the air, where they can linger for days after being used to insulate or fix windows (even a long distance from where the finish is applied).

Unfortunately, in this case, the entire layer must be sanded off, the surface thoroughly degreased and the finish retouched. When it comes to silicone, there are special thinners that can be used in mixture with solvent-based varnishes to counteract its effects.

5. Shadows (white clouds) in the film

They occur when drying has been too rapid and the solvent has not had time to fully exit the film. It is more visible on water-based products, but can also appear on solvent-based products. Humidity in the atmosphere is also very important. When it is high, the chances of film whitening are higher. Air circulation during drying is also important for water-based products. If the temperature is high and there is no airflow, the film dries on the surface (shrinks) before all the water comes out.

If there is not a large amount of solvent inside, the problem can be solved by heating the film slightly. The method is valid for both water-based and solvent-based products without hardener (nitrocellulose varnishes).

In outdoor water-based products, the defect may appear even a few days after finishing. For example, if water-based exterior varnish is applied to window frames and a few days afterwards a rainy period comes, the varnish may whiten even if the raindrops have not reached the finished surface. It's not a big deal, the film will return to its original appearance once the sun comes out. This is natural with water-based varnishes, which continue to have moisture exchange with the environment for another 2-3 weeks after the varnish has been applied.

6. The film scratches very easily

When the film scratches very easily and the scratch is a broad whitish streak, it is not a lightly scratching varnish but a poor adhesion to the old varnish layer. It happens when solvent-based varnish is applied over an old water-based or gloss varnish coat. Because of the low adhesion, when mechanical action (knocking, scratching) occurs, the varnish peels off and air gets underneath (hence the white colour).

If you want to apply solvent-based varnish to an older, glossier surface, sand after degreasing. It is good to test the compatibility of the materials beforehand in a more hidden place. If they don't separate immediately or the solvent doesn't attack the bottom layer (looks like it's blooming), they are compatible. Sanding is done with fine abrasive sponges or 280 or 320 grit sandpaper. Lacquer is applied after sanding.

finishing defects

7. Cracks in the film

Cracks in the film can occur due to the varnish/paint and the way it is applied, or due to the natural movement of the wood as the humidity in the air increases or decreases. In the first case the cracks are uneven over the whole surface, in all directions, without following a certain pattern. In the second case, the cracks will be along the grain of the wood or along the joints.

When we talk about cracks due to varnish and the way it is applied, the causes can be too thick a layer, poor adhesion, brittle varnish due to too much hardener, application at too low a temperature, choosing the wrong varnish for the way the object is used, water-based varnish that has frozen. To remedy this, the entire varnish layer must be removed by sanding and the whole thing re-sprayed using a suitable varnish and following the application instructions.

Cracks due to wood movement are caused by wood with too much moisture, too little moisture, or dried wrongly (too quickly, at too high a temperature, without checking the temperature inside the wood, not just the surface temperature), low elasticity of the varnish or adhesive. In this case, too, the varnish layer must be removed, the wood humidity checked (it must be 8-12% also on the inside, not just on the surface) and a varnish with higher elasticity applied.

I hope you find these explanations useful. More can be said about each of the flaws, but I have chosen to present the main ideas. If you would like to know more or have encountered other film defects that you would like to clarify, write below in the space provided. I'll certainly reply.

Stay optimistic! It will pass and it will be fine!

About the author

Mihaela Radu

Mihaela Radu is a chemical engineer but has a great passion for wood. She has been working in the field for more than 20 years, wood finishing being what defined her during this period. She gained experience working in a research institute, in her own company, as well as in a multinational. She wants to continuously share her experience with those who have the same passion - and more.


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