How to Prevent Sanding Swirl Marks

There is more to power sanding than you might think. Here, pros discuss the fine points. December 26, 2006

I have read all the Knowledge Base articles I can find on WOODWEB and elsewhere regarding this - some with great advice, which I have tried. But I still get swirl marks using pneumatic (Dynabrade) random-orbit sanders. I spent years using electric sanders of every make and model, and rarely ever had a problem with swirl marks. Early this year, I purchased a few Dynabrade pneumatic sanders and the air compressor to run them.
A 3/16" Dynorbital Spirit
A 3/16" Black Dynorbital (the "heavy duty" one)
A 3/32" Dynorbital Spirit
2 Dynabugs

It's my understanding that the 3/16" orbit should be used for the first sanding (in my case either 100 or 120 grit, depending on the wood, usually 120), followed by the 3/32" orbit in the next grit up (150 or 180).

The initial sanding puts such deep swirl marks that the second sanding will not remove them.
Air pressure? 90 psi AT THE TOOL.
Downward force? I've tried everything from the weight of the sander only, to bearing down quite hard.
Sanding speed? I've tried everything from zipping back and forth, to going very slowly in one direction (a couple inches per second)
Number of sandings? Anywhere from two to four. Never skipping grits.
Clean the surface before and after sanding? Yes, before the initial sanding and in between subsequent grits.

The closest I can come to a swirl-free finish with these little devils is to use only the 3/32" sander and sand at least 3 times. This is way too much work - way more work than the old cheapo electric sanders to achieve the same swirl-free finish. With the electric sanders, I could slap a piece of 120 on, sand, slap a piece of 150 or 180 on, sand again, stain it and it would be swirl-free.

I'm not a newbie at this, Iíve been doing it for 20 years, but only just switched to the pneumatic sanders earlier this year. My main problem is (of course) on maple. I'm at my wits end here, and really don't want to go back to the heavy, loud electric sanders. But I swear these air sanders are working me to death and have cost at least $2500 so far, for an inferior end product. Is there something I'm missing here, or is sanding - as I've often said - really a black art?

Forum Responses
(Cabinetmaking Forum)
From contributor A:
Have you played with the sandpaper? What brand of abrasive are you using - cloth backed, paper backed, film, any kind of hook and loop? Film will provide the best result, paper next. Cloth on the other hand will last longer. Also what is the density of the pad on the sander? This is worth looking into. The sander should work as long as it is receiving the correct amount of PSA.

From the original questioner:
I use both cloth and paper backed. Iíve been using cloth backed mainly (it lasts a long time and doesn't load up) by Mirka - Hiolit J, I think they call it. My supplier told me of a stearated paper, also Mirka, that he said should reduce swirl marks. It does seem to reduce them, but didn't eliminate.What do you think of Mirka? Could it really be simply crummy paper? I've always pretty much assumed that sandpaper is sandpaper, particularly with the bigger names such as Mirka, Klingspor, 3M etc. The sanding pads are the "standard" (medium density, I think) pads that come with the Dynabrade sanders, 100 grams.

I'm very interested to hear anyoneís experiences with any of this, be it the differences in sandpapers, or whatever magic words I may need to utter to keep the swirl marks at bay. I forgot to mention, it's all PSA paper.

From contributor B:
What about application? Do you start the sander while it sits directly on the wood or do you start the sander above the wood and then apply the sander while it is running? The start up torque/weight of sander may have some effect causing swirls, etc.

From contributor C:
Contributor B is the right track. If you use waterborne finishes, be careful with stearated paper. Some leave a residue that can cause fisheyes.

From contributor D:
I do not know if this will help or not. I have a wide belt sander and I finish sand with that at 150 grit usually, then to get rid of the cross sanding in the rails I actually will sand with 180 grit on the 3/16 to speed up the process of removing the cross sanding and then I will use 180 grit on the 3/32 to get rid of the swirling from the 3/16 sander (both sanders are Dynas). If I am sanding maple I will wide belt sand that to 220 and then use the 180 grit on the Dynas. Like you stated that the coarse grit is placing deeper scratches in the product. Then you are asking a finishing sander with a finishing paper to remove just as much material to get rid of those deeper scratches.

From contributor E:
We have run into similar issues. I believe you are on the right track. Pressure at the tool is important. I would bump the pressure to 100 psi. Don't forget to oil the tool daily. It affects the power. Blow off the piece and keep the paper clean between grits. Finally, we are trying the Dynabrade Supreme. I spoke to the sanding manager from a $20M cabinet company and he had the same issues, he said the Dyna supreme is heavier and does not jump around. Finally, we found scuffing the entire surface with a sanding sponge eliminates the stray swirls. You might also try paper from another batch, stray coarser grits could have missed it.

From contributor F:
Does your Dynabrade sander swirl the disk at a high speed like a disk sander and also oscillate or does it just vibrate around slowly and oscillate? To get a good result you need to have the disk swirling at a high speed and oscillate. Remove the muffler and inside are some felt pads. Remove them and see if you have an improvement. Yes they will produce a bit more noise but the results are worth it. Also take the sander apart all the way down and clean it out. Lube and put it back together again. You should have better results.

From contributor G:
Your sander is not the issue. We have over ten Dynabrades in the shop and do not have this constant issue. However, it does arise on occasion. We have always found it to be a worn pad or a lack of sufficient air pressure. As stated earlier I would bump up your air pressure. 90 psi is too low. You should be at 100 psi at the least. I would shoot for 105-110. Also, your paper could be an issue but I doubt it. If you get swirls when sanding finish then it could be the paper and most likely the buildup of finish on the bottom. This happens very seldom on wood. Lastly, how has this cost you 2500 bucks?

From the original questioner:
I'm very careful not to start it before it touches the wood, and not to stop it until just lifting it off the wood, and not to apply pressure to it during the start. Being as all of the sanders are brand new (as of the first of the year), I don't think wear is the problem, and they are oiled religiously. To contributor F: Your suggestion seems to be closest to what I'm arriving at. That is, 90 psi at the tool (even though this is expressly what Dynabrade specifies) isn't enough. Particularly with the 3/32" sander, it just doesn't seem to spin correctly (that is, fast and clockwise) unless I am running it at 105 or higher (this is at the tool, not at the air outlet, which I have found is quite a bit higher.) To contributor G: It has cost me over $2500 counting the purchase price of the sanders and the air compressor to run them. How do you think running it at 105-110 psi will affect the tool life? I have run the 3/32" at that pressure before, and it seems to really blacken and break down the oil quickly.

From contributor I:
A couple of scattered thoughts: The 90+ psi is with the tool operating. A few years ago I thought I had pressure, but some clogged filters were reducing the volume of air. The lack of volume did show in the pressure, but not as readily. Check those things.

From contributor J:
Something I just recalled - I saw a guy actually deform the pad by repeatedly banging into the sides of the box he was sanding. The pad cupped and only the outside edge was contacting the work. This is easily checked by looking at the sanding dust on the pad.

From contributor A:
Some stearated products can lead to some problems in finishing, but I haven't found that problem with Klingspor PS33. It is an A/O w/ stearate and seems to work well. It is available with a hook and loop backing and I believe it provides a good finish. I would also agree with the other posters on jacking up the psi on the sander. Dynabrade makes a nice sander and should work. Keep at it, you'll find the right combo.

From contributor K:
I had problems with swirls on electrics and thatís why I went to air. I think Dan is on the right track. I had a conversation with a guy who now does some finishing for me and he was talking about the fact that some sand papers are a lot worse than others. I found this out several years ago when I picked up a roll of 150 and started having a terrible time with swirl marks that I didnít previously have.

From contributor L:
What size air hoses are you using? I have Dynabrades, and switched from 1/4" ID to 3/8" ID with larger fittings and it made a difference. I'm thinking of switching over to the new poly air lines as they are less bulky than rubber and I've been told theyíre smoother inside for better air flow. I widebelt maple to 180X than back down to 150X using the 3/32" Dynabrade with little or no swirl marks.

From the original questioner:
The air hose I'm using right now is 1/4" poly/plastic/whatever, you know, those yellow retractable reels that Grizzly makes. I love the fact that the hose is so light and doesn't catch on things or boss the sander around. I have thought about going to a larger diameter hose, but all I've been able to find is rubber hose, which I don't want to use because it's so heavy, and leaves marks on the wood as it drags across.

I definitely am going to try another brand of paper, probably Klingspor. Ironically they were supposed to send me some samples after IWF, but all I've received so far is a coffee mug in a sandpaper box! Based on responses here, I will try cranking up the air pressure. I'll get with Klingspor and try to get a selection of papers from them and see if that helps. Does anyone know of a place I can get that same smooth plastic hose such as the Grizzly reel has, only in a larger diameter?

From contributor I:
Iíd definitely go to 3/8" hose. Go on-line and order the urethane as someone mentioned. They're light and smooth.

From contributor M:
Get the cheap stuff from Abrasive Sales Inc. You won't go back to Klingspor.

From contributor N:
I run mine at line pressure from the compressor. My Dynabrade salesman said it wouldn't harm the tool as there is pressure line loss because I am using 1/4" hose. Itís been working for 10 years. We do have some issues with swirl marks, mostly on harder woods like maple.

From contributor O:
This might sound stupid, but try replacing your quik-disconnect air fittings. They actually seem to be one of the first things to go. I'm not sure what happens, but whatever wears inside them seems to affect the sanders before they leak or affect any other tool I use.

From contributor P:
Try not to go to 100 grit or 80 grit - these are very coarse. If itís necessary to remove planer marks on solid you need to do 100, 120 150. Good plywood gets 150 only. Never start or stop the sander on the wood. For the best results 1" per second is the recommended rate of hand motion, especially on the coarse papers. Maple is the least forgiving, obviously when stained.

From contributor Q:
If anyone hates swirl marks of any kind itís a finisher like me. Now donít get me wrong but are you going overboard on the subject of swirl marks. After all a machine of any kind that goes round and round just might leave a slight mark .About the best sander Iíve used is the Porter Cable 525 and itís a "finishing sander", but I wouldnít use it just by itself as itís too gentle. I use the 525 after the more aggressive sanding I do with the Dynabrade. Do I still get a mark now and then? Sure, but someone would need a NASA microscope to view them.

From contributor R:
I own two Dynabrades 3/32" orbit. I switched from a Dynabag to hooking up to a dust collector (there was a conversion kit at the time) and get much better performance and the paper lasts longer. Less of the abrasive that falls off the paper during sanding gets between the pad and the work. Also, the speed (hand motion mentioned above) is a huge factor. Two different cabinetmakers using the same sander get vastly different results. The person who moves his hand more slowly gets a swirl free finish, and when the other guy goes slower (it is hard for him) he gets a much better finish.

From contributor S:
We use Dynabrades and they work very well. We have found that itís the paper. Experiment until you find the right one for you. We use all 3M paper and lots of it. We change very often and it costs us more in paper but less in labor and we all know what labor costs.

From contributor T:
Are you using and open or closed coat paper? Open coat won't load as bad and closed coat will give you a better finish. Just take it easy with it, don't try to remove defects with it, and use a light touch. I go from 60 to 220 to 320 and rarely have swirl issues.

From contributor U:
We use the Dynabrade 3/16 with stock sanding pad up to 150, then switch to the 3/32 with a soft contour pad for the last pass at 180. Make sure the edges of the paper don't get dinged. If they do, change it right away. We get good results with this.

From contributor V:
I think you are really just now for the first time using a true sander. Drop all that 100-120-150-180, and just start out with 180 and sand the thing. The sander will do it with 180 or even 220, and thatís where the time you save, returns the 2.5K you laid down for the whole setup.

From contributor W:
We switched to pneumatic sanders to cure our swirl problems and they never seemed to be much better. Then we bought three Bosch random orbit electric sanders. I start with 180 grit hook and loop and only sand once. Not a swirl since. We love these sanders. I know they're electric. Give the compressor a break for a day and try one of the new Bosch variable speed random orbitals.