Floor Sanding And Floor Finishing Problems

Wood is Darker Around Edges

Wood Flooring has its own natural colour. When a coat of varnish  or oil is applied, the colour will change. The way the floor is sanded influences the final shade. A coarse abrasive makes the surface rougher. The varnish or oil will tend to penetrate deeper into the wood flooring than when using a fine abrasive and the colour becomes darker. That is why it is important to use the same sandpapers grit size for the main areas of the floor as for the edges and corners.

Practical tip: whatever grit you use for the main part of the floor, use the same grit for the edge of the room and corners

Score Marks Over the Surface

Sanding a wooden floor is a gradual step-by-step process. We recommend three steps or more as shown below:

  1. Coarse sanding. The purpose here is to remove dirt, old finish, and marks on the wood. The most common practice is to start with grit 36 or 40. In exceptional cases, with very uneven floors or pine coated in the old finish, you may decide to start with grit 24 or even grit 16.
  2. Medium sanding. Next, we recommend medium sanding with grit number 50 to 80, which will remove traces of coarse sanding
  3. Fine sanding. The fine sanding is usually done with grit 120. If you use solvent-based finishes, you can use grid 100 for the final sanding and still obtain a well-bodied surface. However, with a water-borne finish, the final sanding should be done with grit 120 or finer

Note: After fine abrasion, deep grooves remain if you miss out the medium sanding. To obtain good sanding results, never jump from a coarse to fine grit size.

Swirl Marks

The most common reason for these scratches is if the contractor has started to abrade too soon. At this stage, the surface of the finish is still slightly sticky. Most of the water has evaporated, but the coalescing agents are still evaporating. These evaporate quite slowly, so if you start to abrade the floor now, there is a good chance that the finish will fasten on the abrasive and scratch the surface.

A good way to check if the finish is dry enough to abrade is to touch the finish with the back of your hand. if it feels cold on the surface, then drying is still in progress and you will have to wait a little longer. If you open a window to create better air circulation, you will speed up the drying process.

Practical tip: let the finish dry properly before abrading.

Poor Adhesion

Most types of prefinished wood floors are suitable for overcoating but not all. With a few of them, the finish just won´t adhere no matter how carefully you abrade.

If you want to recoat a prefinished wood floor, you must check that the finish will actually adhere. Do a test on a small area. If the test is successful, you should abrade the floor very carefully, preferably using the Scrad system. Alternatively, use a grit 150 abrasive disc with a sanding machine to avoid marking the finish applied at the factory.

A new coat of finish will not adhere to a floor that has been waxed or polished. In that case, you have no choice but to sand back to the bare wood.

Practical tip: a coin is a way to test the adhesion of the finish to the underlying surface. If the finish starts to flake off when rubbed, adhesion is poor.

Long Scratches

Unless you are really observant, it is easy to miss scratches on the surface of the wood after it has been sanded. But once the finish or oil has been applied, these scratches become painfully visible for everyone to see.

The problem may be due to a poor abrasive with pieces of grit that become easily detached. Even if you vacuum clean the floor after each sanding step, you do not always remove loose pieces of grit. Some of them remain in cracks in the floor. The vibrations of the machine as it passes over can make them bounce up again and they could fasten under the back wheels or in the abrasive itself.

The sharp pieces of grit can fall off the surface of a poor quality abrasive. Bona Abrasives, on the other hand, are made with high-quality adhesives to hold the pieces of grit in position and minimize the risk of scratching.

Practical tip: after every change of grit number, vacuum clean the floor because any coarse grit lying around can damage the finer abrasive that follows.

Chatter Marks

This is one of the most common problems flooring contractors encounter and the cause of many complaints from customers. The chatter marks only become visible after the finish has been applied.

There are several possible causes:

  • Parts of the floor flex up and down as a heavy sanding machine passes over
  • The abrasive is not tightly fitted on the belt or drum sander
  • A badly balanced drum
  • A shuddering movement in the sanding machine
  • The abrasive is not clamped in properly around the drum and protrudes slightly
  • An overlapping seam on the belt rubs against the floor – this is easy to avoid by using a belt with a flat butt joint

Practical tip: use Bona abrasive belts which have a butt joint. This means that the joint does not have a different thickness to the rest of the belt

Black Dots

These dots are due to metal shavings or loose grit from the abrasive reacting with the finish. That´s why it is very important to vacuum clean carefully.

The dots are actually larger than a piece of grit and are almost black in colour, no matter what was the colour of the original abrasive grit. The finish reacts with the metal present in the grit to form a dark substance. This substance also discolours part of the wood immediately around the piece of grit. If you look at the spot through a magnifying glass, you may be able to see the piece of grit sticking up in the middle surrounded by a ring of colour.

The same happens with metal shavings. The shavings can be produced by sharpening a scraper or grinding the metal guard on a sanding machine with the abrasive.

Practical tip: to avoid black dots, vacuum clean carefully. Do not sharpen scrapers in  the room where you are going to apply the finish. Check regularly that the drum of your belt or drum sander is properly adjusted.


Secure
Trusted
Delivered