Jim,
I'll add a few bits to Pat's good advice. As to your specific questions,
Tips: Yes, you saw that I used a chisel-shaped tip for the water hatch. My general rule is to use the largest and shortest tip that can fit where it needs to go. The reason is indeed for best heat transfer from the iron. Short and fat is good to conduct heat faster from the heating element, and the flat of the chisel shape can, depending on the shape of the work, give a bigger area of contact of tip to work than a round cylindrical or conical tip. The flat also holds a small amount of solder nicely to feed into the joint. But, I use a half dozen tip shapes including conical of differing sizes. They are changeable on my iron in a few seconds while it is hot, and I use the one that gives me the best contact area to match a given piece of work.
Tinning: I didn't do a good job of defining tinning in my first post. Tinning a surface just means coating it with a nice, continuous, not spotty, layer of solder. (The term comes from old-time tinkers who coated iron or copper pots with tin to make food taste good. Fancy French copper pots are still tinned). Tin an iron tip by feeding it some solder to cover the surface. The iron tip needs to be tinned to protect it from oxidation that prevents it from wetting, and I hope I have made it clear that a wetted tip is an absolute prerequisite for good soldering. The liquid solder itself conducts the heat from the tip to the work (except for very tiny jobs that need very little heat). To tin the tip it must be clean and not oxidized. If you keep the tip tinned all the time it is hot it will stay good. If you cook it for any time while dry, it will go bad. Using rosin core solder for tinning is okay, you clean and tin the tip at the same time so the clean tip is never bare. Those tinning pastes can do the same thing. If you need more cleaning power, there are numerous methods. You can use paste flux, I mentioned a very good one in my first post. But in all cases, whatever flux or other cleaning agent you have used must be removed right away. You need to get it all off, or it will cook on the tip and wreck it eventually, and also you don't want any to get on the work. Wipe flux, and excess solder, off completely with a folded up paper towel and immediately re-tin. Lots of people including me use a damp sponge to wipe on after flux has been applied- the steam helps dislodge it. Or wipe on one of those copper things that look like a pot scrubber. I like those too, but not needed unless you cook flux on. Use a sal ammoniac (ammonium chloride) block - this is amazingly wonderful for reconditioning an oxidized or not-wetting tip. Do all of the above as necessary. Keep the tip tinned all the time, then no worries!
So the answer to your question- tin before, during or after?- is: Yes.
Attaching castings- I don't have experience with resistance soldering so I have no opinion about it. I like to use the butane microtorch for several reasons. First it can provide heat at a very high rate so you can solder at one spot before the rest of the assembly gets too hot. Second, no physical contact means that parts don't get nudged out of position. The flame can be directed to heat the casting, but if you don't heat the mating part adequately too you won't get the solder to flow on that surface and a bad joint will result. The torch flame can be directed to do both simultaneously. Another important reason for fast heating is to avoid evaporating or burning away all the flux and oxidizing the surface before it's hot enough to flow the solder. Sometimes it's necessary to re-apply flux to a large workpiece as it heats.
Think about how heat will flow within and between the parts you're working with. Generally you want to apply most of the heat to the part that has the larger thermal mass. For instance, to attach a grabiron to a hefty end beam, tin the grab, then heat the beam. As the beam heats the grabiron follows along, and when the tinning melts the job is done. I illustrated this with the hinges and the handle on the water hatch. With an air pump, you'll have to heat both it and the boiler. The boiler tube may be thin, but it's conducting heat away over a large area, from the joint to the larger surrounding mass: a heat sink. Another important reason for fast heating is to avoid evaporating or burning away all the flux and oxidizing the surface before it's hot enough to flow the solder. Sometimes it's necessary to re-apply flux to a large workpiece as it heats.
I like tools that have more than enough power to do the job. That's why I use a 36-grit 7 inch disc grinder to do initial fairing on boat hulls. One slip and you've destroyed it, but the job gets done fast! The torch is a bit like that for soldering. I don't know about the resistance tool, but your description sounds hopeful to me. Lots of heat with that in a well localized area, yes? But does it apply heat to a very small area, and is that prone to overheating and oxidizing? The torch is very flexible, you can move it around fast to cover exactly the area you want. It takes practice but it's a great instrument, you can play any tune on it.
Locating pins are good, as we agreed before, to make accurate and strong mounting. I'm usually too lazy to do that though. With the torch, and suitable orientation of the work, good old gravity can often hold the part happily in place, as long as (here I go again) you don't give it too much solder to float around on.
Thin wire solder- yes. As I mentioned, I use .020" solid core tin-lead solder (actually 63-37 tin-lead, I somehow came upon a one-pound roll ages ago, a supply for the next few lifetimes). The Stay-Brite and Tix are larger diameter, but as Charles or Pat pointed out, you can flatten them out thin and cut off tiny pieces easily. If you have small solder you can always use more of it. If you have larger solder it's harder to use less of it. In any case, I seldom feed solder from the roll onto the work, too hard to limit the amount melted just right. The solder is either already on the part, or on the iron tip before it's applied to the work- both of which I illustrated with the water hatch fabrication. The plumber's "flux it and feed it" regime is not for us makers of small things. Unless maybe you are modeling a foundry floor, with great puddles of spilled molten metal.... That sounds like fun! I visited an aluminum foundry once, it was dangerous because molten aluminum is not red hot, it looks just like cold aluminum, so you couldn't tell whether spilled stuff was liquid or cold. Don't try to pick up an interestingly shaped drip!
Filling holes- Pat's got the best method. If you just try to fill it with solder, you get an untrustworthy dimple, exactly as he says. And especially note his advice to solder from inside- or whichever side can't be seen. Then you can make a nice big strong fillet of solder to hold the wire best.
Finally, I beg to differ with Derrell. Au contraire! Fingerprints and other such nasty surface-befoulers are what keep solder from wetting. Cleanliness and flux-iness are the answer! You've never known how attractive brass can be to solder until you've used Stay-clean flux (or Tix or other similar). Instant-flowing solder and complete happiness will follow! Why am I using all these exclamation points?
and, yes, do practice, and do it on parts of the same thickness and size as the real thing, to learn how to manage heat as you want. I should have said right in the beginning that a proper solder joint is defined by the surfaces being wetted. Just like how water wets a surface rather than beading up. That's what you're aiming for. If it's not happening, then the surface isn't prepared or fluxed properly.
John
John Greenly
Lansing, NY
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