I've been using a Hakko FX888D for a few years and I'd consider myself intermediate-level soldering skill, but when I use really fine-point tips (J-Tip [TR18-BR02] or Narrow-pitch soldering [T18-S4]), it seems like there is almost no heat at the tip.
Is this normal or is this improved with the newer T12-style tips?
Can I "upgrade" my FX888D with anything to help the heat transfer to the tip?
Also, how can I "repair" older tips that I might not have taken 100% care of? (I always leave solder on the tips, etc, but sometimes they seem.... old?)
I bet it's completely normal. Small tips have low thermal mass and worst heat transfer, and they're serious limitations.
I also own a tiny T18-BR02 tip which I purchased for soldering Micro USB connectors. But while it looked like a good idea on paper, in practice, i found it's not useful at all due to its low thermal mass. When the iron tip meets the metal connector body, the solder joints often have no reaction at all and remain solid.
To successfully solder something, the problem is not temperature but heat. A large tip like a conical tip or a chisel tip has high thermal mass, which means it can store a lot of heat and dump that heat to the joint at once. The large size also means heat transfer is more efficient. In other words, it has low "output impedance", and the thermal mass acts as a capacitance to improve your "transient response". If you use a small tip, the performance is going to be worse.
From experience, I found even soldering something trivial under ideal conditions, like a tiny SOT-23 chip over ground planes, with a standard conical tip (not a fine tip), can be a challenge. Even when using standard Pb solder and conical tip, at 350 °C, the ground plane acts as a huge heatsink, the solder barely melts. Often I have to turn the temperature up to 400 °C. For lead-free solder the situation is worse, temporarily turning it up to 450 °C is necessary. Now imagine if you also have to use a tiny tip for this job, the situation is going out of control...
The same thing happens when you're solder a metal connector, or a QFP pin with vias to a ground plane. If the tip is tiny, the heat transfer will be worse, so you basically have to:
1. Turning the temperature up to eleven. It works for a while, then the tip oxides within minutes and no longer works well.
2. Tinning the tip with a large drop of solder. Molten solder has the best thermal conductivity and also increases the thermal mass of the iron tip. However, the physical size of the solder drop removes any precision-work benefits when using a tiny tip.
Thus, while I must definitely acknowledge other people may them useful under some circumstances. But my own conclusion is that a tiny tip is not personally useful. My recommendation: Just learn drag soldering with a standard tip, not a fine tip.
Or if you are like me who's not very good at soldering, get a stereo microscope, then you can solder the pins one by one and clearly inspect each joint. Use desoldering wick to rework the bad joints. Just rinse and repeat until all pins are perfectly soldered.