Avoiding Swirl Marks in Finish Sanding

Grit choices, sander type, and technique can all affect finish sanding performance. May 6, 2010

Iíve done tons of research on how to fix the dreaded problem of swirls and pigtails left behind only to be seen once you stain and still havenít found a consistent fix. When I get my rough stock in I plain it down to the thickness I want leaving it very smooth. Once I make the doors I sand with a porter-cable 6" random orbit sander with 120 grit. Then I had been going to 220 grit then stain. At times I can see the swirls once I stain.

This past time I went from 120 to 150 to 220 and after sanding. If I look hard I can still find some swirls. This is all with using oak. Should I be final sanding with a quarter or half sheet finish sander instead of the random orbit? I tried this but it takes forever and itís impossible to sand any profiles with them. Or would using the same 6" random orbit sander but with a smaller orbit like a 5/32" instead of the 5/16"? Would that make a difference? This is my only problem I have to making a perfect door. Itís not every door but enough that I want to get it fixed.

(Cabinetmaking Forum)
From contributor A:
6" Porter Cable RO sanders are junk. You should be able to start with 150 grit (Mirka, Klingspor, good 3M paper discs) on planed boards. We only drop down to spot sand joints, etc. with 100 grit. The scratches you are see are 120 grit not 220. It would take a lot of sanding with 220 paper to remove 120 scratches. I suggest you buy a new sander Bosch, Fein, Festool or Metabo.

From contributor D:
We use Festool ro sanders then use a half sheet sander with a high grit paper on it and we never have swirls. Try the Festool sanders they blew me away the first time we used them. Good luck.

From the original questioner:
I started out with a 5" Makita ro sander and a Delwalt 5" ro sander. Then a local cabinet guy told me to use his porter cable and it would cut my time down in half, which it did. But I didnít solve the scratch problems. Do the other sanders spin as fast at the Porter cable? Or is that part of the problem? I believe itís around 6000 rpm.

From contributor M:
You should not jump from 120 to 220. Try 120 to 150 to 180. 220 on oak is useless. 180 or even 150 on course grain is good but never skip three grits.

From contributor S:
I start with 150 and then use 220. Used 220 already has the larger sand particles off the paper. Even 220 will swirl but itís the larger pieces of sand that make swirls. The only thing i use fresh 220 for mainly is sanding first coat of primer.

From contributor G:
Random orbit or sheet sanders leave swirl marks. That is what they do and that is how they work. If you are talking about the occasional very noticeable swirl mark, most likely there was something that got stuck between the sandpaper and the wood. A stray piece of grit or a larger bit of dust. You have to move the sander slowly. Any fast movements can cause noticeable swirl marks.

Are you using a vacuum system to pick up the dust while sanding? This will help a lot. I use a DeWalt sander and don't have very many swirl issues and I don't use a vacuum pickup.

Vacuum, technique, a good sander and skill will keep the swirls down. If you want fast you will get swirl marks. Getting a fine sanding job done takes time. You can get better equipment to speed it up, but it still takes time. If you take a high powered lamp, 500w halogen, and place it at very low angle to the wood you may be able to see most errant swirls.

From contributor A:
Trust me it is the sander. I bet you feel like you are getting carpel tunnel after an hour of using it. The real test of a good ro sander is to put it on a table. Grab it by the cord like a leash and turn it on. It should sit more or less in place. The Fein do it as well as the Bosch.

Your PC cost $150. The Bosch is $250, Fein $350, etc. The cabinet guy told you to switch to a larger disc size. The 6" is 50% larger in area than the 5". They definitely sand faster. Add the dust collection and the paper lasts a lot longer with less scratches due to nibs.

From contributor F:
Whether hand sanding or using a random orbital you cannot skip grits. If you are using a RO sander after planing, you should start with 80 or 100 grit which will be required to get out planer marks, tear out etc. 90% of the work is performed with the first choice of grit and it must be sufficiently coarse to get the job done, the rest of the work is simply to eliminate the scratch pattern left by the previous pass. A common mistake is to start with too fine a grit or to move on before sanding out all the defects with the first pass. The dynabrade type sanders are great for the initial sanding but for the finish passes I have never found the equal of the Porter cable 505 and 330 sanders, old technology but still unmatched, and that goes for Festool as well (I've tried them). The major drawback is a lack of dust collection, but you can't have everything.

From contributor Y:
It's not so much the PC sander as it is the 5/16" orbit as you were suspecting. You should be using a 5/32" orbit for finish sanding period.

From the original questioner:
I think my biggest problem is jumping from 120 to 220. The next problem is the sander. Iím wondering if I couldnít get by starting out with 150 even. My planer doesnít leave any marks to much and itís about like glass but definitely not sanded smooth.

From contributor G:
With most softer hardwoods I will plane and start off with 150 grit. On joints and areas that need more aggressive sanding I will start with 120 grit. I rarely go any farther than 150. If I am using hard maple I will start with 100 to sand and 80 to grind joints on panels and such. But you need to go up through the grits to eliminate the swirls of the previous grit. If you are using a widebelt sander or belt sander you can skip a grit because of the power of the machine. Neither of these will leave swirl marks, just straight line scratches.

From contributor L:
Skipping grits and a cheap sander with a large orbit = poor result. We use Dynabrades on a down draft table. We grind our own carbide knives and keep them very sharp. Sanding past 150 is a waste of time. I've proved it in several tests. Cut up a sheet of 3/4" cherry veneer into cross strips about a foot wide. Tape off areas and use all the various methods, grits, etc. Wash coat, stain, seal, scuff sand, top coat just like you are doing a fine finish.

Let the panels dry for a day or two. Not letting anyone see the markings on the back that show the system used, have your most critical people look them over and say when it's good enough. Go at the panels just like you would in normal operations, not super slow, etc. If you have to, borrow assorted tools from other shops and let them in on the results. Any sander that has been dropped to the concrete may have a slightly bent shaft and not work as a finish sander. We keep three Dynabrades at the bench, each marked for a certain grit. No changing paper, just grab the next sander. Always blow the dust off before starting the next grit, gets rid of most of the stray bigger abrasive that falls off.

From contributor A:
I have had all of the above sanders in my shop and the single most important factor in preventing swirls has been the sandpaper followed by the sanding method used. Inferior sandpaper will leave swirls no matter which sander we use and if the operator either uses the wrong pressure or moves the sander too rapidly across the part there will be swirls. Also, as mentioned above, the size of the orbit plays a role in the size of the swirls.

The Fein sander is a great, powerful sander for knocking down the joints in a panel too big to run through the sander but on veneers it requires much more finesse to not burn through. The Sioux and Dynabrades are very great sanders to use if you have the CFM's to push them. Many smaller shops don't. We currently use a Bosch for much of our sanding and I prefer it to the Fein that we used to have and it is quite a bit cheaper. For Corian the Fein would be better but for wood I prefer the Bosch. We also use PC and Dewalt RO sanders and since I do 95% of our finishing I can say if they did not do the job they wouldn't be there.

I would suggest trying some of the purple 3m Cubitron paper with whatever sander you have and see if that doesn't solve the problem before you go and replace all of the sanders. There are other good ones but that type is excellent and widely available.

From contributor A:
The Mirka paper is a good product. We've been using them almost exclusively for about seven years. I've had the older version of this sander since 2002. The H&L pad lasted seven years. It's got more power than the PC. I still have my old PC which I had the bearings changed. I only use it for grinding paint of the outside of my house and shop. We cooked the bearings on 3 PC sanders when we were building my friends 40' fiberglass sailboat. The PC have poor dust collection even with the better vacs (PC, Fein, Festool). 90% of our casework is beaded inset ff. I always sand the faceframes after assembly with the 6" Bosch. We use tons of 1 1/2" wide mullions that have 5/8" sandable flats. This is about as delicate sanding as you'll find. I never hit the beads.

Contributor G and I essentially have the same sanding/finishing schedules. We pretty much agree on most everything in the finishing forum. The Fein sander is now $450 with less power. If I'm doing a stain job I always wipe the frames down with denatured alcohol to show the scratches. A couple of minutes with a piece of 150 hand paper takes those away. Depending on the wood sanding to 240 grit prior to staining can cause problems. I don't think a lot of guys realize the depth of a 100grit scratch. You've got to hit it for a bit of time with 150 grit - no quick pass. No way you could jump from 120 to 220.

From contributor K:
We use Dynabrade 5" air sanders and Klingspor hook and loop pape. Right out of the widebelt we use 120 then 180 without problems. 220 is too fine for staining in our experience. The Dynabrade sanders are really the key, they are the best we have found.

From contributor I:
I been using the Dynabrade sander with good results for years (air ). Your problem is the orbit size, the sander, and skipping grits. My abrasive supplier recently stopped stocking 150 grit and I tried to go from 120 to 220 and the swirls were there. I also agree with previous post about 220 being too fine for staining. I canít justify the cost of the Festool or the Fien but if I had the extra money I would swear by either. Does anyone know if they are faster than the Dynabride air sanders?

From contributor W:
Poor technique is often the cause of swirl marks. RO sanders leave swirl marks - they are designed to do that. The key is to keep those swirls dense, so that the eye can't perceive the individual marks. Most people sand too fast. That is, they move the sander too fast back and forth over the board, causing the swirls to stretch out and look somewhat like a coiled spring that has been over-stretched.

The sander should be moved at a rate of about 12" per 20 seconds. So, slower is really faster. It is possible to skip one grit size with ro sanders. Most consider 150g to be sufficient for oak. If the results are what you want, this is the point to stop. If not, ro sand one grit finer, then hand-sand with the previous grit. Some would argue that sanding is the most important step in the process. If it is poorly done, nothing in subsequent staining/finishing steps can correct it. Except paint it over.

From contributor F:
The sanders are not the problem in my opinion. What wood are you using? You must progress through the appropriate grits as others have suggested 80,120,150,180,220. You can start and stop at different grits with different densities in wood. I think your biggest problem is leaving the residue of the previous grit behind on the work piece. Make sure you blow off the 120 residue before proceeding to 150. Also wet the surface down after 150 and re-sand with 150 before moving on to 180.

From contributor F:
Thatís a range of grits. I don't use all five. On alder, start with 120 and progress to 220. On maple or red oak start with a 80 and progress to 180. I remove heavy millmarks with 80. On softer woods you can eliminate this step. Believe me if you skip to a higher grit and don't remove them, they'll bite you in the butt in the finishing process. Make sure you're sanding with a high enough grit that your swirl marks are small enough they aren't noticed. Skipping grits will not polish them out.

From contributor F:
I think you have enlisted some very fine help here from the other guys - very fine advice on consecutive grits and sander brands. One thing that has not been touched on though, is a technique issue. You have said that you don't notice the swirl marks on every door, and perhaps you know this already, but you should never start your ro sander off of the piece. It is extremely difficult to place a running sander on your piece without causing a swirl mark that won't sand out with the finer grits that follow. Also, I try to make my sander exit the piece with a sweeping motion that is as parallel to the surface as I can make it. This helps me to produce a great finish, and I hope it helps you too.