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Most times it's from the operator
The dynabrade is an excellent sander, I don't know about the Master Craft.
I'm assuming it's an air powered sander. They do wear out. Especially if you skimp on the oil. The bearings become worn and the RO pattern becomes distorted.
But the likely answer is what Jim said, operator error. It sounds like you are using a vacuum on them which should remove any particles that might cause the swirl marks.
Ask them to slow down. Usually my sanding procedure is go against the grain in 1/3rd steps. Then do a circular pattern overlapping 50% and then with the grain in 1/3rd steps.
If you are only willing to do one of them choose the circular pattern motion. And tell them to change the paper like its free. Over using the paper is a waste of money. Causes more problems then it's worth.
with my guys it is usually caused by them pressing to hard on the sander
As important as the quality of disc is the type of abrasive, hopefully your using an Aluminum Oxide (open coat) If your using an orbital sander go with @ 3/32 orbit and progress through your grits properly such as 100,120,150 ect; also the type of stain plays a role. if there's dye in the stain solution your more apt to notice those swirls if its a pigmented stain only not so much.....good luck!!
This is why National Detroit straight line pneumatic sanders are still being manufactured and sold.
Orbital marks or swirls are a normal part of using random orbit sanders. I have written extensively about methods involving wide belt sanding to try and minimize them, but they are a fact of life.
Go to your local football stadium and plow the field without leaving a furrow.
The best you can do is create a surface that requires as little random orbital sanding as possible and get it done quickly with as little pressure as possible.
A wide belt does the flattening for you so you just remove scratch pattern. You will have very little swirl until you have removed the scratch. Once you are sanding a clean flat plane you are driving the grains into that surface and creating swirls. The more you sand the worse it gets.
If you are relying on your random orbit sander to flatten the surface and stepping down grits, you really can't help but end up with swirls on some level.
A properly maintained sander running 90 psi at the tool, a clean, flat back up pad, and a quality disk help. Use very light pressure and smooth, slow passes on your finishing pass to help minimize them.
Beyond that, hand blocking after random orbital sanding is the only sure way to remove swirls.
My Mirka Ceros sander is virtually swirl free. I don't recall the last time I've seen a swirl since I've gotten it.
I don't have any swirls,
Lots of good advice Thanks !
Sometimes the vacuum system, especially if a high powered shop vac, is too powerful and literally sucks the disc down onto the surface and can give swirls (same process/effect as pressing too hard). Happened to me when I got a new, more powerful shop vac.
Veneer: Dynabrade 180
Right to stain, no visible swirls
Swirls are from either pushing way too hard or running a sanding disc until the end of time without changing it
Also could be cheap vacuum discs, I don't like Vacuum Sanders personally
My 2 cents
If you ever watch a TV woodworking show