What type of Urushi are you using? Is it actual Urushi or is it Cashew Lacquer? As with anything we seem to endeavor to do....it is the foundation in Urushi that makes all the difference. Shitaji must be applied,sanded smooth, reapplied to fill in any surface irregularities from the first coat, then you can move on to preparing the surface. I find a 70% thinner 30% Urushi to work well, making sure to place a lightly dampened cloth next to the work piece for proper polymerization. If you are using Urushi...its hard to believe how many people try to use dry heat to cure it. It just won't work.Sand, with 400 grit and reapply in a 60/40 ratio, (thinner to Urushi) sand with 600 grit, then move up to 60% Urushi and go over it with 1200 grit between coats. After that I usually use a high grade aluminum oxide based polish designed to remove 2000 grit swirl marks.
Getting an actual set of Urushi specific brushes is also very helpful. What can you say? There must really be something about girls hair that seems to work.