For nearly 20 years I have been dealing with this on 400 feet of track (some pieces are over 30 years old/LGB and Aristo) in a stone ballast base and have evolved to this process:
1. Using a motor tool with a fairly aggressive stone, polish a finger-size portion of the top of the rail foot after the first sleeper past the joint you are jumping. (I found that a wire brush is no match for the grindstone.)
2. Put a dab of paste flux on the polished surface. (Not all fluxes work equally well, a bit of search/experimentation/luck may be needed.)
3. I use a 250 watt trigger soldering gun (a.k.a. Weller D550) to create a pad of rosin core solder where the flux was applied.
4. I then press the end of 2mm (or larger) desoldering wick into the solder pad with the gun's tip and then unreel the wick until I reach the next pad, solder, and cut, leaving a jumper across the joint about 2 inches long and snug (if I was careful) against the rail. I initially experimented with the desoldering wick because I liked its flexibilty and its eagerness to attach to solder (no tinning). I was dubious about its durabilty, but time has erased all doubt. Other than the occasional poor solder joint from earlier evolutions of the process and rare damage by rodents or birds (misidentifying a worm , perhaps) performance and durability are quite satisfactory, and I couldn't operate my low maintenance dc railway without it.
