Finishing defects - Craters / Fish eye

Craters may appear in the film of a finished surface even during or immediately after application. They have the appearance of circular discontinuities, not very large in size. To give you an idea, it is like spilling water on a surface that has grease spots. What causes this and how the problem can be solved, find out below.

Grease, oil or wax stains on the surface

There are several causes that can lead to the defect. It most often happens because of grease, oil, wax or silicone stains of the surface to be varnished.

How do grease stains get on finishing surfaces? Sometimes, inadvertently, it is enough to touch the greasy palms (with hand cream, for example) to the surface to be finished and in that place the lake will "slide" forming the crater. At other times, greasy substances (especially oil) can get into the compressor and the pressurised air will become contaminated. By blasting the surfaces with compressed air or even by spraying the varnish, the greasy substance reaches the surface and causes the defect to appear.

What can we do to avoid this? Fitting filters to equipment and cleaning them thoroughly before use is one solution. Greasy substances can also get on the surface if dirty cloths with traces of grease, wax or oil are used for cleaning. If you notice greasy stains on surfaces or think they may be present, clean and degrease the surface with suitable solvents before applying varnish.

Silicon can cause problems even when working with it at great distances

If silicone has gotten on the surface, use a special solvent to remove it. Silicone is a material that can also cause problems if worked with at a great distance from the finishing area.

I remember one such problem years ago in a factory. At some point, seemingly for no reason, craters began to appear on the surface. As usual, the first culprit was the lake. I decided to go down there and see what was going on. As far as I knew, nothing had changed in the product formulation. The craters were as visible on the surface as could be expected. The first step was to look for traces of grease, oil or wax, but I found no trace of any greasy substance that could have caused the problem. I did notice, however, that at the other end of the hall windows were being fitted using silicone. Air currents brought fine particles of silicone into the spray area and the defect occurred. Since then, every time I come across this defect, the first question I ask is whether they are working in the silicone area.

Other causes and ways to avoid craters

Another cause of craters is the porous surface of the wood substrate. In this case it is recommended to use a pore-filling putty, and white grinding must be done very carefully.

Organic solvents that end up accidentally in water-based products can cause craters to appear. If working with both solvent and waterborne products it is recommended to use different lines or equipment. If this is not possible, equipment should be thoroughly cleaned when changing materials and allowed to dry. The spray booth should be well ventilated for a long time to remove any solvent vapours.

Incorrect viscosity or the wrong diluent chosen can lead to craters on the surface. Follow the indications in the technical and technological data sheets regarding these parameters to avoid their occurrence.

When refurbishing older furniture, the defect may occur if before furniture is not properly washed, degreased and sanded. Over the years, the furniture has been cleaned with "specific" commercial materials (Pronto type products). These contain waxes and without a thorough surface cleaning, the furniture cannot be refinished.

Solutions for crater defect remediation

When the defect has already appeared on the surface, there are not many solutions to fix it. If it has appeared after the primer has been applied, sand the dry primer coat thoroughly until the defect is completely removed, then apply a new coat.

If the defect is on the final coat and is caused by silicone, a thin layer of anti-silicone solvent or a thin layer of varnish mixed with anti-silicone thinner can be sprayed on top. However, no other finish can be applied over this coat.

Did you want to refinish an old piece of furniture and cracks appeared? Now you have a possible answer. If you've faced such problems, have questions or queries leave them below in the dedicated space. I'm sure I'll answer them.

Other finishing defects:

Orange peel
Air bubbles. Film punctures
Leaky lake
Cracks in the film
Lack of adhesion
Staining, discoloration, fading, color migration