Can you ruin the wood if you sand against the grain?
Tim Inman: No, but you can certainly ruin the look you want. One stroke across grain takes 20 strokes with the grain to make it right.
Stephen Rosasco: Sanding against the grain leaves noticeable scratches in the surface; they can usually be sanded out afterward.
John Swanson: You may not ruin it, but the surface will have more tear-out. This can be overcome by sanding with the grain on your final, finest grit.
Richard Jones: You won’t ruin the wood, but sanding across the grain may spoil the appearance of the piece once polished.
Kevin Hancock: Absolutely not. Sometimes it is the only way it can be sanded. Use a finer grit than you usually would as cross grain sanding tends to cut faster. Sanding up to about 320-400 grit will make most of those sanding scratches disappear under the finish.
Michael Dresdner: Am I the only one who noticed the flaw in all this? Here’s the dirty truth: well over 95 percent of all woodworkers use random orbit sanders for sanding raw wood. As the name implies, they sand in small, random circles. Wood grain goes in more or less straight lines. Every time you use your sander, you are sanding across the grain. Of course it is all right, but I do agree that the very last sanding should be by hand, with the grain.