4 steps to a perfect polyurethane finish
Apply stain and poly
Move to a low-dust setting: somewhere without lots of foot traffic and with dust-free surfaces. You can create such an area by hanging an inexpensive vinyl shower curtain in a corner of your shop. With your workpiece wiped clean, apply stain, photo right. (Skip the stain if you intend to leave your workpiece natural-color.) Let dry.
Prepare the polyurethane by reducing it about 10 percent with mineral spirits, unless you're using water-based polyurethane, which doesn't need thinning. Thinning the polyurethane makes it flow on more smoothly and reduces brush marks.
Quick Tip! To reveal flaws (bubbles, brush marks, etc.) as you work, shine a light at a low angle across the surface.
Brush on three thin coats with a foam brush (previous slide). (You can use a natural bristle brush with oil-based poly, but keep an eye out for loose brush hairs.) Coat the entirety of your workpiece, but don't brush excessively or you'll create areas with too little polyurethane. Allow each coat to dry fully. To give the subsequent poly layers something to bond to, sand lightly between coats with 320-grit sandpaper wrapped around a hard block. Note: The first coat needs the most sanding to appear smooth; don't worry if it doesn't look as flawless as you'd like at first.
After the third coat, sand with 320-grit, then 400-, and finally 600-grit sandpaper. Sand in alternating directions with each abrasive to make it easier to see whether you've removed the scratches from the previous grit. Sand with the grain on the final pass. Then, wipe off the dust with a rag dipped in mineral spirits.
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