Brush Sander Break-Ins and Scratch Marks

Some detailed advice on the best way to break in and use brush sander brushes and sanding strips. July 23, 2014

Question (WOODWEB Member) :
I’m curious if anyone is using a Brush Sander (such as Quickwood) and what grit you might be using on hard woods such as maple? I am sanding to 180g on a widebelt then through the Quickwood at 150g but the Quickwood 150g is leaving noticeable scratches which show up when stained.

Forum Responses
(Finishing Forum)
From Contributor W:
I worked in the brush sanding industry for the last eight years. What type of brushes do you have on your Quickwood? Are they Quickwood brand aluminum cores with Quickwood brushes and separate replaceable sand paper slashes? Is the paper black? The scratches are perfectly normal with new brushes. The scratches you get from a brush machine are entirely different than what comes from a wide belt sander. You need to break in the brushes to start. Put a bunch of scrap doors in the machine and set the machine height just high enough for the support brush (the yellow Tampico). Drop the height down another 8 to 10mm or 5/16 to 3/8" lower than the surface. Run the machine at 300 rpm for three hours. After this initial break in you will see a huge improvement, but more might be necessary.

For production, run the height of the support brush about 3/8" above the conveyor at the slowest rpm the hubs can be set, usually about 200 rpm. The carousel rotation is your only means of consistency. Look at the edge break all around and increase/decrease the carousel speed to get the most consistent edge break possible. It will usually be pretty slow. If the corners trailing the direction of the carousel travel are not as broken as the lead corners, slow the carousel down a bit.

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From the original questioner:
We've had the machine for nearly two years now and I know about the break in process. The paper we are using is black and it is silicon carbide. It is the brush and replaceable strip system with the aluminum head just as you described. It seems we need to break the paper in to the point where you can no longer feel any abrasive on it in order to prevent scratch marks. At that point we find the wood is too smooth and the stains we use will come out much lighter that we would like. I don't understand the logic behind taking something like a 150 grit paper and wearing it down (breaking in) to the point where (in any other application) it is worn out. Why would we not use something like 280 or 320 grit that may actually sand a bit? Not saying you are wrong, I know that is the typical process. I just don't understand why. Can you explain that to me?

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From Contributor W:
It's not about taking away all the grain. It's about taking away the sharpness of the grains. Most people start by breaking in too much of the sanding strip to start with. They go often 3/4" deep, not realizing they are giving up huge amounts of brush life. I leave the brushes brand new above the last 10mm. There should be plenty of grit left on the paper once you are done.

As far as polishing, if the part isn't over polished going in, it won't be coming out if you run the machine as I described in my earlier post. The trick is slow head speed combined with a nicely open part going in. In Salt Lake City I fixed a color issue with a Slipcon brush sander by merely changing the first sanding head in the wide belt from a 120 grit to a 100 grit.

Also note that even an unbroken 320 grit will leave scratches that show in the stain. No matter the grit you use you will need break-in of some sort.