Craters and Fisheyes: Double Trouble

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Jul 22, 2021

Craters and fisheyes are visible imperfections on the surface of wood floor finishes that resemble small circular indentations. The rounded dips in the floor are known as craters (pictured top), while the small bumps are known as fisheyes (pictured bottom). Craters and fisheyes are common issues with hardwood floors, but they can easily be avoided by following proper procedures. The root causes are problems with finish, application, and contamination.




Check the expiration date of the finish and be sure it has been stored in temperatures and an environment recommended by the manufacturer, or else it can increase the risk of these 2 finish flaws. Consult the manufacturer for recommended thinners, dry-time extenders, bond-enhancers, and other substances, if additives are needed. The wrong kind of substance or too much of 1 substance can contribute to these phenomena. Craters and fisheyes can also stay visible if existing ones were not removed before applying subsequent coats.


An inadequately cleaned applicator is another reason craters and fisheyes appear. If the applicator was cleaned with a solvent that is incompatible with the finish, it can increase the risk of imperfections appearing on the surface, or excess water from washing the applicator may imbalance the finish defoamer system.

Craters and fisheyes can also form when finish is applied over a not-yet-cured finish or sealer, or if trapped air or solvent bubbles burst but don’t level out in time. Read more about the curing stages of finish here. It can also be a problem if a finish is coated over an incompatible finish or sealer such as:

  • Perfluorooctane sulfonate
  • Wax finished with urethanes or oils
  • Natural oils finished with incompatible urethanes
  • Incompatible finishes from different manufacturers

Click here to read more tips for applying finish.


Wood floor surfaces can become contaminated when foreign substances such as maintenance products, grease, oils, polishes, soaps, sweat, and other contaminants are present before coating or have been introduced between coats. These contaminants can be dripped, spilled, or over-sprayed onto the floor surface prior to or during finish application. Contamination can also occur during mixing or packaging. 

Resolution and Prevention

If craters and fisheyes are discovered on the wood floor, there are a few options that may help resolve them.

  • Screen and recoat the floor with manufacturer recommended applicators and spread rates.
  • With a coarse to medium-grit abrasive, scrape or sand the area of the wood floor then move upward in grit sequence. If necessary, recoat and consult the finish manufacturer for final abrasives suggestions.
  • A contaminated floor or finish may require sanding in order to remove the contamination and start the finishing process from raw wood. In cases where the floor cannot be resanded, replacement of the affected areas may be necessary.
  • Fill the floor to block contaminates in voids or between boards with a trowel.

To prevent craters and fisheyes from forming, follow all manufacturer directions and recommendations prior to and during application, including conditions for applying finish and storage. Ensure the applicator is thoroughly cleaned before applying and avoid contact with substances that are not supposed to be on the floor.

To get in contact with a Basic Coatings representative with questions about preventing craters and fisheyes, please visit this link. For more information about the advantages of water-based finishes and sealers, click below:

Source: NWFA’s “Problems, Causes, and Cures 2018”

Photos courtesy of NWFA’s Problems, Causes, and Cures (3rd Edition, 2018).