Random Orbit...

[Frank Rizzo]

184050184049I am finishing a black wanut table top and have run into a problem, seems that whenever i run my ROS I get marks... scratch marks that wont go away, i have taken a few pictures and posted them for you to see. I can't figure out why all the sudden i am seeing these scratch marks. I ran through the different paper grits 80-220 but it seems every grit does some sort of marking like these....any thought would be appreciated

[Erik France]

Are you doing any dust collection during sanding? Do you brush or vacuum between grits?
Quite often the swirls are from loose bits from your sandpaper.

[Mike McCann]

It looks like your finish is melting on the paper and leaving the swirl marks

[Steve Ryan]

Are you doing any dust collection during sanding? Do you brush or vacuum between grits?
Quite often the swirls are from loose bits from your sandpaper.
+1
Large grits left behind and getting cought up in the finer paper. Use dust collection and vacuum surface between grits if necessary.

[Howard Acheson]

I can't tell from the picture. Have you applied a finish? If so, do not use a power sander for between coat sanding. A ROS is particularly a problem.

First, be sure the finish is fully dry. Between coat sanding should be done with 320 grit paper on a felt covered or rubber sanding block. Sand in the direction of the grain.

[Van Huskey]

Looks like finish building up on the paper, barring that it looks like the paper is loaded and scratching. If it is the former finish plus ROS usually leaves heartache.

[Glen Blanchard]

Frank - If those marks were there BEFORE the finish was applied, you may be moving the sander too fast. Slow and steady is the name of the game with these sanders.

[Tyler R. Jones]

Moving too fast will cause these. The number one thing that I see cause this is guys pushing down too hard on the sander. Pushing down negates the random function of the sander. Also make sure you keep it flat to the workpiece. After that I would say never use any grit larger than 120. The scratches are just too deep. Make sure you sand enough with each new grit before moving on.

[Steve Jenkins]

another possibility is that there is a chip or something between the pad and the sandpaper. If you are using psa paper it may be a small ball of glue or if you're using hook and loop it could be a chip of wood. If it's somting on the psa pad it will clean easily with lacquer thinner and a rag.

[Dan Hahr]

80 grit- way too coarse for ROS unless you are shaping wood. I like 150 or smaller grit. Even 180-220 will leave those swirls if the ROS is moved too fast. It looks like you skipped some grits or didn't stay long enough at each.

If the wood is really hard, I would finish with a scraper or sanding block with the grain.

Everything else suggested is a possibility, too. I suggest removing the finish and surface of the wood with a card scraper and starting over. If the card scraper works well, you don't need anything coarser than 320. You shouldn't get visible swirls with that.

Good luck, Dan

[Peter Elliott]

The picture shows small pig tails.. common on ROS.. if your sander is "chattering" across the board, that will cause these swirls. If you apply pressure to the outside of the pad, cause swirls. Keep a 1-2 second pace, per inch and direct center pressure on the sander that is light, not forcing down.

Grit wise, not sure what the other guys are working with but I use 80 grit all day long on hardwoods like maple, oak, etc.
Unless the board is fairly clean coming off the planer, you spend way too much time on a higher grit like 120.. Fact, you will burn out the 120 and lesson the chance of getting out finer scratches. Nothing is cut/dry either. Look at the board and decide where to start.

Always use a set of grits. Everyone has there style. On festool ROS - maple: 80-120-180-220-400.. and I don't spend a lot of time on each grit. If I get a clean board from the planer, I might go 120/180.. I also throw a card scraper in too for trouble spots.

Looking at your pictures. my guess is that applying pressure to one side of the ROS.

Do some sample boards with suggestions here and figure it out. Last thing you want is to ruin all that work with a bad finish.

Good luck
-Peter

[Jim Rimmer]

Lots of good suggestions on what may have caused it and how to fix it. Just wanted to add a comment for future projects. Before you start to put the finish on, wipe the project down with mineral spirits. That will make scratches like these, mill marks, other sanding problems, and glue spots show up. The MS will not raise the grain and it gives you a good look at what the finish will do when you apply it and an opportunity to react without having to deal with the finish. Good luck.

[bill schmoott]

Let me guess, DW 4120 sander? What is the age of your sander? I threw my ROS away and have gone to hand sanding and or scraping. Your sander may need re-built. Check the movement of the pad, then go to a big box and check a new one. Just my 2 cents

[David Hawxhurst]

i dumped the ros and now do it by hand. sanding blocks automotive type like these (http://www.lakeshorehardwoods.com/index.php?page=shop.product_details&flypage=flypage.tpl&product_id=51&category_id=7&option=com_virtuemart&Itemid=68) and psa paper works great. i found that i can sand by hand faster than with a ros.

[Roy Fleming]

Make sure that you clean the surface between grits or you are just regrinding the previous junk

[Rob Sack]

Lots of good suggestions on what may have caused it and how to fix it. Just wanted to add a comment for future projects. Before you start to put the finish on, wipe the project down with mineral spirits. That will make scratches like these, mill marks, other sanding problems, and glue spots show up. The MS will not raise the grain and it gives you a good look at what the finish will do when you apply it and an opportunity to react without having to deal with the finish. Good luck.

I'd be careful about wiping down with mineral spirits when using water based finishes. When using water based finishes, I wipe the piece down just before finishing with a damp (water only) blue paper towel from Costco. It there is any raised grain, it is usually very slight if at all, and a quick, light pass with 320 takes care of it.

[Darrin Davis]

I'm a high school woodshop teacher and see this all the time. Simple answer: You're applying too much pressure while sanding with 80 grit! During the production of sandpaper, there will be an occasional over-sized piece of grit that makes it on to the paper and stands proud amongst the rest. This is the piece of grit that's giving you the deep scratches in the wood when applying too much pressure. You need only apply just a touch more pressure than the weight of the sander does on it's own. To get rid of the swirl scratches you will have to spend a lot of time with 80 grit and light pressure. Hope this helps!