Wood – the proper way to sand live edge piece of wood in preparation for a polyurethane finish

Pine is a difficult wood to stain under the best of situations. Pine has areas of different density, color, and resin (pitch) content. Using a product like Minwax prestain helps a bit. Follow the timing instructions carefully when using a pre-treatment. Little tips: i like to apply a liberal coat of oil based stain with a sponge brush. Let it set a few minutes, then rub it in and evenly distribute with a lint free rag. As one rag becomes saturated with stain, I use a second drier rag to finish the rub in. The wetter rag is great for distributing the color. You don't want any really wet stain remaining on your wood after the rub in. Once you get to the urethane, sand lightly between each coat with 220 grit paper to remove any dust bumps. Remove all sanding dust with a slightly damp cloth, let it dry and apply next coat. Again, I prefer a good grade sponge brush, but a good grade regular brush for oil/urethane works good for appling the urethane. Remember, thin coats, and sand between each coat.

The short ends you pictured are the cross grains of the board. What I see is that you have used a too coarse sandpaper on these edges. This should be easy to fix. You need to step down the grit of the sandpaper. Any final sanding should be done with 220 or 320 paper. On these end details, use fine paper, break its (paper's) back so it is flexible, and take your time working it on these ends by hand. It may take a few minutes, but it will smooth out nicely.