The browning at the base of a stem/twig/branch is just part of the evolution to bark. It doesn't become bark, it is just the oldest stage of the scale that will fall off, and that's life. Don't mess with it or you will break more stems than you liberate. The solution is to do enough tip pulling and cutting back to induce growth thick enough to hide, if that's the word, to lower, inner stems. The amount of back-budding you can get is a function of time. Long bare branches = long time to drive the foliage back. On what I see above, you better be a young man. There is a solution: Buy tree stock that has foliage close to the trunk, or where you want it. Part of the bonsai education is Shopping 100, buying the inside view instead of the view of the outside. An experienced bonsaiist looks for healthy foliage and then spends the rest of his time looking under the lady's skirt. That's where the action is. Start with her feet. Big feet are good down on the farm walking on soft, newly plowed soil, and wide feet and wide nebari wins by a wide margin. The first couple inches of the trunk either has heft to it or not. Stocky legs are much more desirable than skinny legs. Movement in the trunk is wonderful and very individual. Some have something nice to look at with curves in all the right places and some don't. If they don't you have to ask yourself, "Self, can I put some action here, or am I doomed to accept her as is?" In conjunction with movement is the first three branches. The movement does not stand alone. If it is within the profile of the first three branches, it counts. If it is too high or too low for your prospective design, then a decision is called for.
You have to have designs in mind while you are looking under her skirt, before you pay for her. Sounds crass, but this is a practical relationship you are hunting. You can't change the quality of the pelt after you have paid for her. You're shopping at nurseries, but you don't want a young one that's just pretty skin-deep. If you can find one artificially aged by too much contact with life, handled or mishandled by too many to count, and with the scars to prove it, that's your candidate. If she's cheap, all the better, but don't limit yourself to just cheap because there are some that have been around the block a few more times and are worth more.
Too often, people start bonsai trying to make a silk purse out of a sow's ear. Eventually, they learn how to shop for silk.
