Wood – Reapplying spar varnish/urethane to exterior door

When you apply multiple coats of urethane, any dust or small flaws will be amplified with each new coat. If you want the smoothest finish possible, then wait until it is dry, very lightly sand with very fine paper (220 or finer) to remove any dust bumps, wipe it clean with a slightly damp cloth or tact rag, let it dry or buff it dry with a dry cloth, then apply the next coat. There is no reason to use mineral spirits, actually I would advise you not to use mineral spirits.

I see no advantage to applying successive coats with, then against the grain. As long as you apply an even coat with a good quality brush or foam brush and don't overwork it, any brush marks should disappear. The finished product should be so smooth that you couldn't tell which way you brushed the urethane on. Also, if you are using a satin finish urethane, be sure to mix it well, stirred, not shaken. The dulling agents tend to fall to the bottom, so it is important to mix it well. No matter how many coats you apply, it should not become "glossy".

1] Wait. It may take days or weeks for the varnish to harden enough to sand without it "balling up" or being "gummy". This is the most important thing. 2] Test it gently from time to time with 120 or 150 grit until it sands cleanly, no gumminess. Then scuff the whole thing down gently, just knock off the high spots and ridges, avoid breaking through to bare wood. 3] Lay on another coat, try to brush in one direction, "flow" the varnish on and don't worry about small streaks, varnish is "self levelling" if it's not too cold. 4] Follow the same steps using 220 grit. This should be a decent finish. 5] Depending how far you want to go you can work up in stages to 1200 grit and polishing compound, but that's extreme. 6] When brushing varnish, what you did wasn't entirely wrong. Cross brushing then linear brushing to "work" the varnish onto/into the wood is good but you have to do it fast before the varnish starts to "set up". Good luck and enjoy.