What are these and should I worry?
This is called corning and yes it is something to pay attention to.
Corning is distinct from normal clogging — notice the dark colour, as opposed to dust clinging to the paper which is usually very light.
Corns are generally formed of either resins from the wood itself or a finish in or on the surface, such as varnish of course. All finishes that dry hard can lead to corning, including drying oils, shellac, varnish and lacquers. And as well as that glue residue can cause it too.
The reason you don't want to continue to use corned paper is that the corns 1, interfere with proper sanding (because they sit higher than the surrounding grit they reduce sanding efficiency since less grit is in contact with the wood) and 2, they can actually scratch or bruise the wood surface in the worst of cases.
Prevention is better than cure
It's best to try to avoid corns forming although this is not always possible.
Corns tend to form most when you press down hard on your paper — which experts tell us not to do anyway, regardless of sanding method; "let the paper do the work" as the saying goes — and/or if you sand very quickly. Both pressure and speed means the paper can generate a lot of heat which softens or actually melts resins, which can then cling to and harden on the paper.
The problem with speed as a cause is that it is inherent to most powered sanding operations, which is why it's not always possible to completely avoid this happening. So it's vital that you not press hard, no matter how tempting it is! For vibratory or rotational sanders you want the weight of your hand resting on the sander to be the only pressure, no muscle power should be used to press the sander onto the surface.
Also try to move the sander in a measured, even way, not exactly slow but definitely not fast.
What to do when you do get them
Corns can be robust enough that they're very difficult to dislodge from paper. One old technique recommended in older guides is to drag the sanding block (or the pad of the palm sander in your case) sharply over the edge of a piece of wood, hardwood preferably. This is worth trying still but I've found it isn't that effective.
A better method is to brush over the paper with a wire brush. Brass-bristled or bronze-bristled brushes are sufficient for this, but you can use steel brushes if that's all you have. At best this brushing will remove all or almost all corning and you can then continue sanding.
If brushing doesn't remove enough it's worth trying to scrape the remainder off using the edge of something like a 'beater' chisel that you keep around for rough jobs, or with a stiff-bladed putty knife (probably the better too for the job). Either way there is a chance you'll tear or cut through the paper, in which case you should toss it and apply a fresh piece.
Although they're not always available for power sanders I should mention there are papers that are purpose-made to help prevent clogging and these will tend to be more resistant to corning as well. These papers generally are of one of two types:
- lubricated (usually with a wax-like material, these are called stearated papers)
- open-coat abrasives, which have a wider spacing of the grit particles