The Woodworking Network started a new blog series on Finishing. Here's the portion of the article that's most useful for our readers of The Sandpaper Blog:
Why do you scuff sand?
• Modern coatings get hard and some get chemically resistant within hours. Subsequent coats will not rewet and blend with the layer below. The only type of bond that is possible between coats may be a mechanical bond created by sanding scratches.
• It’s true, in my experience, that most first coats make you look like a monkey’s uncle. They look horrible! But if properly scuffed, a good seal coat will make you look like a rock star after your second coat.
• The lacquer makes wood fibers stand up.
• Surface tension in the lacquer attracts it to those fibers and it builds up around them creating “pimples” in the lacquer that must be sanded away.
• Surface tension causes lacquer to pile up around the pores.
• Surface tension creates fat edges that need to be attended to.
• There’s always dust that appears as if by magic or “spooge” out of the gun that lands in the first coat
• There’s always a bug willing to commit suicide in your wet lacquer.
• Heaven forbid! You just created a run or sag.
All of these and more are the enemies of a glass smooth finish. And lest I forget, it is über-important to get rid of the sanding dust that you create. Again, it may not be rewetted by the next coat. Dust and debris are your enemy!
At the same time, the guy who sands his work and perspires onto the surface is the first one to call and say that he has fisheye issues. Wear at least a short sleeve shirt. The oils of your skin and certainly the deodorant that you wear will contribute to fisheyes. That’s doubly important with water-borne coatings. Remember, oil and water don’t mix.