I cannot find the original question about removing stratches from Titanium which prompted the reply about the DA seat post, but....
As an owner of two Ti bikes, I have become a sort of expert on the topic, though Joe Bell could give us more input on this (hi, Joe). Basically, to match the original finish seamlesslessly, you will need to know the process used by the manufacturer, of which there are many. If you use a combination which isn't like the one you are touching up you will see the difference and it will bite a little more every time you see it thereafter. The only fix to this is if you screw up, be prepared to refinish the entire piece (be it a frame, seatpost of fighter plane) to match your touch up.
For the purpose of this text, I'm gonna assume we're talking about a frame. If it's bead blasted, you're up a creek as far as spot refinishing. If the stratch is so bad that you feel it must be replaced, it's time to call the manufacture, get more decals, strip the frame and take it to place that has a bead chamber. There are a few different types of beads used. Lightspeeds use a relatively large diameter, Moots uses a smaller one which gives a much richer finish.
If it's a brushed frame, Like some Hollands, all Merlins, some Litespeeds - your task will be easiest. Usually, it's a series of degradating Scotch Bright grids. Start with the roughest, then go finer, then finer. Again, the combination will be determined by what the frame maker did in the first place. I have had excellent results with my Holland starting with Red, then Green, then White Scotch Bright. If you have to do the whole frame it shouldn't take more than an hour to do the whole thing. Litespeed does not use the white - you can tell because the finish isn't all that smooth. Merlin's process is listed on their website someplace.
Make sure to GO AGAINST THE GRAIN OF ATHE METAL.
If it's polished, like all Sandviks and most some Litespeeds, very careful use of a buffing wheel with polishing rouge should take care of it. If you use the wrong wheel though you'll be in trouble - it won't be seamless. I've done this before, but I've srewed it up more times than i've nailed it.
Hope this helps.
Up to my neck in 3.25 in Santa Monica,
Peter Grenader