I have a 'knotty alder' front door that was refinished by a contractor last August. However, because the lower panel of the door has already deteriorated due to sun exposure, I decided to sand the existing spar/urethane for a smooth finish before applying 2 fresh coats. However, after smoothing with a sanding block using 220 grit, there are several streaks where more of the urethane was sanded down due to unevenness of existing application (see photo). I'm hoping that fresh coats of urethane will blend in and eliminate these streaks. I plan to use a 320 grit sponge between the two coats. Just hoping that my initial use of the 220 grit had not created a major problem!
The streaks should disappear with the first new coat of urethane assuming you haven't sanded into the stain or surface of the wood. The wood is probably stained or has darkened due to sun exposure, and if you sanded into the wood it will need to be restained or given time to darken.
As far as recoat time, follow the instructions on the can. Typically you can recoat as soon as the first coat has lost its tackiness, but read the label to be safe.