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Refinishing Old Wood Floors- Sanding A Wood Floor

 

Q. I sanded and finished an older wood floor and applied four coats of water- based urethane. Now the floor has cloudy or white stripes along the separations of the boards. Why?  

A. Remember that the floor has had a lot of wear and tear and may have been abused-improper maintenance products may have been applied for years. There could be wax, furniture polish or other household cleaners that are not intended for hardwood floors present on the floor. If left behind in the seams or wood cells, these contaminants could cause white lines to appear in the finished floor. This could occur throughout the entire surface or only in heavy traffic areas. The white or cloudy stripes or lines that you now see can happen to any refinish job that has a surface finish, whether it's water-based, oil-modified or another type. When sanding and refinishing an older or existing wood floor, there are many precautions to take. Proper choice of sandpaper and proper sanding procedures are critical. Knowing what grit of sand-paper to start and finish with could prevent problems in the finishing stages. The floor must be cut properly each time it is sanded.sanding wood floors Different grits of sandpaper (coarse, medium and fine) should be used on succeeding cuts. If these procedures are not followed during the sanding process, problems can occur. Most people get into trouble when the floor sands easily and the old finish cuts right off, because they then take shortcuts by eliminating a sanding step. When you eliminate a sanding step (i.e., skip a grit), you leave contaminants on the floor. Following all sanding steps ensures sufficient wood is cut to eliminate or at least minimize the amount of contaminant left in the floor. In some cases, trowel-filling with latex filler between the medium cut and fine cut, or grain filling after the fine cut and before screening may eliminate some of the problems. Improper screening between coats may also contribute to the problem. Inter-coat adhesion is very important. In the finishing stages of the job, it is critical that you follow the finish manufacturers' recommended procedures for applying finish and preparing the floor between coats. Too often when a floor is coated, there is not enough time spent on preparation between coats.

screening (light sanding) wood floors
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Buffing with an abrasive pad or a screen to smooth and abrade the finish between each coat is a very important part of the finishing process. Some manufacturers' finish products can be coated without padding or screening between certain coats. The finishers must have good judgment up front. On all finishes, dry time is very important to the coating process and how the job looks and performs after it is complete. If you know that you will be using a water based product on a refinish job, you must consider extra dry time. Just because a finish is dry enough on the surface doesn't mean it will be dry in the seams or side matches of the hardwood floors. Finishes or stains will aggravate or loosen any contaminants that are present, so pay particular attention to finish in end-joints and side-matches. Remember that contaminants are not seen or detected until after the first, second or third coat is applied. Old floors also move more than new floors. They have been exposed to the elements longer and the structure has settled over the years. A floor that is tight, with minimal cracks, is deceiving. Even though it looks tight, it may move up and down. In this case, it will cause the finish to stretch at the seams, resulting in the white or cloudy lines in the floor. This appearance can worsen over a period of time. The best prevention against this stretching of the finish is to keep a constant humidity level, thereby keeping floor movement to a minimum. Dark floors such as mahogany will show the effects much more than a lighter floor. When sanding and refinishing an older floor, keep in mind what problems that may occur. Good job-site inspection should be done at the time of the estimate and all findings written on the contract or job order. When a floor such as this is refinished, it is the responsibility of the wood flooring contractor to educate the customer. Don't hang yourself out to dry in this situation !

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 Technical questions answered by NWFA's technical director(1994-2000)
Daniel Boone

Related Pages:

~Types of Finishes                                      
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FinishingWoodFloors.com- Complete Step by Step Finishing Procedures
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Technical Questions - We got answers - FAQ's
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Trouble Shooting Finishes-Understanding Problems that may occur

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