Few things contribute to the final look and feel of your finished project as much as sanding. In the preparation work, between coats of paint or varnish, and sometimes even after the final coat, sanding is a key ingredient. I personally think it's fun (though some people might tell me to "get a life"), especially the results it creates.
"All right," you say, "so I'm becoming a convert to sanding. How can you tell when it's time to change the paper?" It's time for a change when either one of two things happen: the sandpaper is clogged with whatever you're sanding (wood dust, varnish, paint, etc.) or the abrasive gets worn down. Oftentimes you can clear the paper of wood dust or dried paint-type products with a stir stick or stiff brush (old toothbrush is good) or even another piece of sandpaper, and keep going.
When you're sanding fresh finishes, especially water-based products, there is a tendency for them to form slightly sticky "balls" on the surface of the sandpaper. These are a real pain in the neck, because they often form quickly, after just a few swipes across the surface, and just a couple of them hold the surface of the paper away from the surface you are trying to sand so that nothing is happening. You can pick at those first few miscreants with a fingernail and pop them right off, but it will rub your nail down to nothing soon enough, so perhaps it's better to keep a stir stick or old screwdriver handy to pick at them instead.
The newer, green sandpaper seems to be a little better with latex at staying free of clogs. Otherwise, start with a new piece when these little clogs overwhelm the current piece, or whenever your patience wears thin, whichever comes first. When you have the time, the problem can be minimized by letting the finish dry longer before sanding.
The other situation is knowing when the sandpaper has just plain worn out from use. When you are wet-sanding you will not normally get any clogging because the water carries away the slurry before the paper gets loaded up.
(A small aside here: If you're still getting those little sticky balls of finish on your sandpaper when you're wet-sanding a "fresh" finish, try adding a drop or two of liquid dishwashing soap to the water you are using; this will usually keep them from forming. If it's still happening, the coat is not dry enough and must have more time to dry before sanding.)
The easiest way to tell when it's time to change the sandpaper is to run your finger lightly over the part of the paper you've been using, and do the same over a part that is still new - the part that is wrapped around a sanding block, for example.) When you can feel a difference between the rough new part and the smooth old part, it's time to change it.
The abrasives on the more expensive papers (silicon carbide, aluminum oxide) last longer than the lesser papers (flint, garnet) and so are often less expensive in the long run. I find that sandpaper never adds up to much of an expense on any given project and I am quick to change it rather than rub longer. (Another aside: I love sanding - by hand, not with a machine - because it's a great time to listen to music; you can really get into both at the same time! It can also be a great time to shoot the breeze with someone you like, as it's easy to pause or stop or move around when necessary for dramatic effect.)
This helpful article was provided by DoItYourself.com community member David Sorg