Hi Cruisinfornia
I use GasFlux C-4 Nickel Bronze rod to do all my fillet bronze work with - I normally buy it from Henry James who sell tube sets for bike frames. I also use their paste flux.
The Nickel bronze fillets do not normally need much work to clean the up, usually just a bit of a file to take any high spots off and a rub with abrasive strip ( soak them in water over night to remove flux )
The Mig welds are another story completely -- Even with the nicest most even fillet possible you end up with a weld which does not blend into the tubes - usually it looks like a small 45 deg chamfer between the two tubes. This means you have to remove a fair bit of metal to get a smooth blended look. I roughed out the excess metal with a die grinder and a 1/4" ball shaped carbide burr, then used a file to blend the radius into the tubes. Finished up with a rub with abrasive strip. Would not recommend this as its way too much work and takes heaps of practice to be able to Mig weld 48 thou tube and get a real smooth even weld fillet. If you have access to gas welding equipment stick with Nickel Bronze fillets since it is much more useful and versatile for bike frames. As a comparison I could have never built the front forks using Mig and made them look smooth and integrated.
Graeme