Anyone who has run a random orbital sanding machine is familiar with swirl marks, the radial scratches that stem from the orbital pattern of the sander itself. The random orbital sanding machine has two types of movements; the rotation at the main spindle and the orbiting of the main spindle which causes the orbital part of the action. This arrangement is supposed to randomize scratch pattern and it does to an extent. There are things we can do to help minimize the scratch pattern, although it is impossible to completely eliminate sanding scratches or we are no longer sanding.

We will talk about:
Technique
Oversanding
Polished wood
Over-pressure
Damaged back up pads
Poorly running orbital sanders
Poor quality abrasives
3/16″ vs 3/32″

Technique

If I had a dollar for every person I saw abusing a hand orbital sander I’d be very rich….. I see folks wiping the sander across the product so fast you cannot even see the brand of the machine in their hand. They move so fast that the rotation cannot possibly keep up with the orbit so they end up with long arcing swirls and horrible consistency. They push hard trying to remove more material with brute force instead of letting the cutting action of the tool have time to work. If the operators need to grind the heck out of the product, what you are sending them must be of very poor quality to require major work. Why?

Using an orbital sander is like using any kind of machine. Push it too hard and fast and you get poor results. The goal is to move at a moderate pace of around 3 seconds per foot. You do not need lots of pressure, although I put just a bit more pressure than the weight of my hand on the first pass to help speed up stock removal. Finishing passes are at nearly zero pressure or just enough pressure to keep the sander from vibrating. Overlap about 1/2 the width of the backup pad.

Slowing down the hand speed and using moderate to light pressure will result in less time per piece rather than more. Consistency is a by-product.

Oversanding in terms of wide belt sanded product.

When a wide belt sander is used correctly it will leave a flat surface with minimal scratch pattern that is open for proper stain penetration. The part should achieve proper color quality without any hand sanding at all. The scratch pattern will need to be removed in most situations, but the color should be fine.

After wide belt sanding product, your goal is simply to sand down the scratch pattern left from the wide belt and achieve the look you require. The orbital sanding should only have a mild effect on the stain penetration. The goal is to simple reduce the noticeable scratches and let the wide belt set the color consistency. The more you sand, the more you effect the color consistency and stain penetration. Usually more orbital sanding means polishing which causes less stain penetration and lighter color.

The other effect is swirls. As you start to remove the scratches from the wide belt sander you will notice there are remarkably few swirls in the surface. Once you remove the wide belt scratch pattern entirely you will see more and more swirls as your abrasive moves across a newly flat surface, with the abrasive grains plowing through the flat plane you have just produced. This is where oversanding becomes the source of swirls. The quicker you notice that the scratches are actually gone and stop sanding, the better for both color quality and swirl generation. Proper low angle lighting is extremely important to helping the operator to see when they are done sanding away wide belt scratch so they can stop sanding. This is also critical for sanding efficiency.

Oversanding without the wide belt

In this situation your hand sander becomes your flattening tool. Low pressure on a coarse disk is far, far superior to high pressure on a finer disk. Break the hard surface down with a very coarse disk like 80 grit and then work down the grits using less pressure on each successive grit.

Polished wood

If you polish the product with your wide belt sander then you have created a very tough situation for your orbital sanding operators. The moment your orbital sander touches wood there will be instant swirls. Take off your safety glasses and sand them with 180 grit and do not leave any swirls……you know it cannot be done…. you cannot sand glass and not leave tons of scratches.

If the wood is polished after the wide belt you are stuck using the orbital sander with a coarse grit to reopen the surface to accept stain and remove any polished scratches from the surface. This is a very bad situation. The coarse grit will leave swirls like crazy that you will have to fix with a series of finer grits and possibly hand blocking as well. Since you have human beings running the orbital machines they will hit some areas harder than others so the results will be patchy and inconsistent.

Over pressure

Make a black dot on the edge of the backup pad on your orbital sander. If you press too hard that dot will stop or be dramatically slower than it should. Make sure that dot is always moving quickly. If you stop the backup pad then there is no random to the orbital action. The only action you now have is orbit with no rotation, which causes horrible swirls.

The more pressure you use on the orbital sander the further the abrasive penetrates into the surface of the product. Coarser grains will bite even more causing more issues with swirls. Coarser grain always causes more swirls because the grains bite deep no matter the pressure and can even slow the sander a bit.

Damaged back up pads

The backup pads, be it PSA (pressure sensitive adhesive) or Hook and Loop, are a disposable wear item. Keeping them in good shape is very important. Spare back up pads should be kept on hand at all times so if the machine is dropped and the pad damaged or melted you can change it immediately.

If the operator is doing profile sanding work with the edge of the pad or tipping the pad up while sanding, the outside edge will take a set curve and not touch at all while flat sanding. This reduces the surface area and increases the penetration of the abrasive into the surface because only the middle is touching, causing slower sanding and more swirls.

Damage anywhere on the pad can cause swirls. Any material missing from the pad or anything stuck to the pad will cause pressure points that cause swirls. Pad wobble will slow the pad rotation and randomization causing more swirls.

Poorly running sanders

I see poorly running orbital sanders all the time. Lack of oil slows them down, ruins bearings, and wears out the cylinder. Removal of the exhaust filter causes over revving and vibration. Water from compressed air combines with oil to sludge up the air filters. Excess air pressure going to the tool causes over revving an vibration which damages bearings.

All of these issues harm the ability of the machine to randomize the scratch pattern effectively. Follow the maintenance guidelines set by the manufacturer of your tool. We recommend 3 drops of oil at the end of the day, blown into the tool by attaching compressed air and throttling the tool. I also like doing a drop or two at lunch time.

Poor quality abrasives

Every manufacturer of abrasives has their own quality standard. In a particular grit of sandpaper, let’s say 180 grit, there can be a certain per
centage of 150 grit and a certain percentage of 220 grit and still meet the ‘P’ grading standards. Some manufacturers put a higher percentage of the next higher grit to make their abrasives last longer compared to others. This may lead to more swirls with lower quality, downgraded products. If the grains are not applied properly they can be uneven and cause excess swirls.

Oversized grains can also cause major swirls but this will be very obvious as there will be one or two particular swirls far worse than the rest.

3/16″ vs 3/32″

This refers to the size of the orbit built into each sander. The larger orbit cuts far faster but leaves more scratch pattern and swirls. The smaller orbit leaves a softer scratch pattern and swirls although it does not magically make them disappear. It is still absolutely critical to use light pressure and slow, smooth, fluid motion with either sander to achieve the best finish possible.

I really like using both. I’ll use the 3/16″ to rip off cross grain really quickly and then run a 3/32″ finishing pass with the same grit or one grit finer to keep the swirl marks to a minimum. Remember we do not want to polish the product so we do not want to sand too much.

Proper low angle lighting can show the swirls the moment they happen so you can go back over that area an remove them using less pressure or slower movement. Light directly from above actually hides swirls and other defects because it fills in the entire defect with even light. There is no possibility of silhouetting the scratch and bringing out the details the operators need to see.

Happy sanding!

Adam West
Techdude@surfprepsanding.com