Greg Mihaich, Application Engineer, Norton | Saint-Gobain Abrasives
This article appears in the April/May edition of Hardwood Floors Magazine. Reproduced with permission.
Every contractor has come across this situation. You’ve just finished working on a floor, taking the time to make sure your sanding, staining, and finishing were as close to perfect as possible. Then you get the call from the homeowner that the floor is damaged from moving in furniture or appliances or something leaked. It’s disheartening. While sanding and finishing a floor is hard enough on its own, doing a repair on a small area and trying to have it blend in seamlessly with the rest of the floor can be a daunting task.
Repairs are an unfortunate part of a contractor’s daily routine. Aside from damage to new work you just completed, you probably also will come across situations where you need to do repairs on older floors or spot repairs while in the middle of staining. Doing repairs on older floors can be especially difficult because of faded stain color, darkening of the wood, or yellowing finish. While most of the focus is spent on how to match a color or sheen, an often-overlooked key component to achieving a seamless repair is the choice of abrasives, sanding sequence, and the process for the repair.
One thing many people overlook about the repair process is that you are not just trying to match the color and finish sheen. You also are trying to match the texture of the floor. Even if the color and sheen match perfectly, if the repair is much smoother than the surrounding floor or the spring grain isn’t as exposed, the repair will stand out. That’s why abrasive choice and sanding process matters so much.
Ultimately, the final color of a floor, the quality of the finish, and appearance are dependent on the sanding process that was used. Ask 10 contractors about their sanding process and you will get 10 different answers. If you are doing a repair on one of your floors, then you’re ahead of the game because you know how the floor was sanded. However, when dealing with a floor that was done by another contractor, you’ll have to look for any visual cues and take your best educated guess.
Make samples and experiment with different sanding processes and colors until you get a match. What you don’t want to do is experiment on the floor, applying color and sanding it off until you get the right match. Over-sanding the area will create a depression, which will be visible when the repair is completed. You want to do the minimal amount of sanding on the floor to ensure the repair is seamless.
It can help to understand some of the differences between abrasive products. The minerals on all abrasives aren’t the same. Contractors have many choices in the minerals used on abrasive products today, including silicon carbide, zirconia, ceramic, and aluminum oxide grains. All are different in shape and size, and leave a different sanding profile, which affects the overall finish of the floor. If you are doing a repair on a floor you worked on and know the products you used, it is best to stick with those abrasives for your repair. If you are doing a repair on a floor that was done by someone else, you will have to take an educated guess on the abrasives used and experiment with samples. In either case, it’s best to stick with actual floor sanding products for the repair instead of using standard hand sanding sheets or orbital discs.
Most hand sheets and orbital discs are made with aluminum oxide grain, which is blocky in shape and leaves a much more shallow scratch than silicon carbide, zirconia, or ceramic grains, which are the primary grains used for floor sanding products. Using products with aluminum oxide grain can sand the surface too smooth, affecting the color and appearance of the floor.
Another thing to consider is the difference in sanding profile between machine sanding and hand sanding. Because of the weight, pressure, and speed of sanding with a belt sander, edger, buffer, or multi disc, the abrasive grain on the belt or disc is pushed into the surface of the wood more than you can achieve with just hand sanding. This means you can get a different sanding profile from the sanding process during a repair, opposed to when the floor was originally sanded. Though it may sound counter intuitive, you may have to drop down a grit when doing the final sanding on the repair than what you typically would have finished the floor with. A coarser grit used under lower sanding pressure can help match the sanding profile of a finer grit. In some cases, the process of water popping can help to negate minor differences in the sanding between the repair and the existing floor, but even doing that will not always work and may make the repair area more noticeable.
For most smaller repairs, the first step in sanding will be to use the edger to flatten out the repair. It’s best to use the finest grit possible to avoid dishing out the floor. Typically, the next step would be to use an orbital sander to remove the marks from the edger, dropping down a grit or two from what the floor normally would be sanded to, followed by hand sanding with the grain of the wood using a folded edger disc. Mimic the sanding profile of the surrounding floor as close as possible.
In the case of floors that were final sanded with a screen, some people go the extra step to use a cut piece of screen on a maroon pad backer to sand with the grain of the wood in an arc, to mimic the sanding direction of a clocked buffer, to impart a similar scratch pattern to the rest of the floor. If the floor was finished with a multi disc machine, then the orbital sander can be your last step in the process, but use the same type of floor sanding abrasive typically used on the multi-disc sander.
One area that you do want to sand as fine as possible is where the repair meets the existing finish. There are two types of repairs that can be done. Some contractors will tape off boards, then sand the area and scrape up the edges of the existing boards to make their repair, while others will sand out into the existing floor and use the process of grain chasing to help blend in the repair. With either method, you want to make sure the edges of the existing finish are feathered out and very smooth so there won’t be visible lines when the repair is coated. Use a very fine abrasive, typically 400-600 grit and in some cases finer, to make sure the edge is feathered with no ridges and no fuzz.
With the growing popularity of natural oil finishes and how integral buffing pads are to the application process, it can help to understand some of the differences when trying to do repairs on these types of finishes. Floor pads have varying degrees of aggressiveness and many are coated with abrasive grain. The two most common pads used in the process are white and red pads. White pads have no abrasive in them; they use a light denier fiber, and have less resin than other pads, so they are ideal for polishing. Most red pads don’t contain any abrasive, but use a heavier denier fiber and more resin to make them stiffer and more aggressive. Similar to the sanding process, using a pad by hand is going to give different results than using the pad in a buffer with higher speed and pressure. When doing spot repairs by hand, you will have to change your process and the types of pads you would have used on the buffer to get the sheen to blend with the existing floor.
Just like with wood sanding where you need to drop to a lower grit, you most likely will need to use a more aggressive pad on the repair to get the sheen to match to the existing floor. This could mean using a green pad, which for most manufacturers is the next most-aggressive pad to a red pad and contains fine abrasive. Some contractors even will use a very fine steel wool or synthetic steel wool to blend the repair.
No two floors or finishes are the same, so the repair process always will involve a little bit of experimentation. Achieving seamless repairs can be an art, but understanding a little about the science behind abrasives and the sanding process can help you get that perfect repair.