Finishes and Finishing

When applying the stain, I accidentally left a streak of stain across the grain. Now, I can see that wipe mark. What can I do about it?

I was given some maple boards, but how can I tell if they are hard or soft maple? Also, what’s the best finish for each?

What finish would best protect the front and sides of a bathroom vanity and medicine cabinet?

I am in the process of refinishing wooden casement window frames. What kind of finish should I use?

I picked up a stunning piece of birch with an exceptional flame pattern grain. How can I finish it to show off the patterns while retaining its light color?

Years ago, I knew of a vinegar solution that blackens oak. I remember the vinegar part, but what else goes with it?