Hi Fred,,You stated that you are having problems sweating copper and I am sure this is a topic that would be of interest to all, so instead of offering a quick answer on flux, how about I do a quick primer on sweating copper? I think with a little training and practice you will find that sweating copper pipe is fun and easy.
Before we go into technique lets take a few moments to discuss tools and supplies for running copper pipe.
TUBING CUTTER: When working copper pipe or tubing the first tool you will need is a good quality tubing cutter. Atempting to cut copper pipe or tubing in any other manner will reault in leaving the pipe or tubing out of round which makes fitting and soldering nearly impossible. There are many varieties of tubing cutters on the market, and basically any of them are suitable to the task, but from experience I have found that a "Minni-Cutter" that is capable of cutting up to 3/4" tubing will be the most practical for residential plumbing work. The mini-cutters have two distinct advantages. 1. They are small and fit in a pocket so its always handy, and 2. they have a very short turning radious so they work well in tight places. This is one iten where quality counts. You will see some cheap red colored mini-cutters that are made in Taiwan in all the hardware stores for about $5, in my opinion they are little more than junk. You will also find a gray colored mini-cutter made by"Genenal Tool" or "Pro-Line". The are moderately priced at about $10 and would be fine for a homeowner or DIY'er who only demands occassional use, in fact, I have one that I used to completely plumb about ten houses and it is still working fine. If you want the industry standard trade quality cutters made by "Rigid Tool Company" or "Imperial-Eastman" are top of the line at about $20 each. One of the distinct advantages of the higher quality cutters is that you can buy replacement cutter wheels or rollers, which greatly extends the service life of the tool.
CLEANING TOOLS: If there is any single trade secrete to sweating copper, it surely must be in how you clean the copper. DO NOT be fooled into thinking that your copper pipe and fittings are new and shiney, therefore they dont require cleaning. The surface of copper will oxydize within hours and even a miniscule layer of oxidation makes soldering more difficult. Even though new fittings look nice and shiney, once you start cleaning them you will see a noticable difference in color. The easiest way to clean the ends of copper pipe is with a strip of emery cloth about 2" x 10". You fold the emery cloth over the end of the pipe and work it back and forth like a shoe shine rag until you have the pipe cleaned about an inch or two from the end. You can find 2" x 30' precut rolls of emery cloth called "A plumbers roll" in the plumbing dept or all hardware stores. You will note that there are two varieties of "Plumbers Rolls". One is plain cloth backed emery cloth for about $5 a roll, while the other is an open screen type cloth sometimes called "Sand screen" for about $8 a roll. The abrasive on the open screen type does not load up like orindary emerycloth or sandpaper, therefore although it is a bit more expensive it outlasts the common cloth back about 10 to 1.
Cleaning the inside of small fittings with emery cloth is challanging at best. The simple solution is to use an ID wire brush. Here again, there are number of varieties. Some have just a plain twisted wire handle, which others have a wire handle with a plastic filler grip. I suggest, rather than buying an expensive one, but two or three of the cheapies, that way if you loose one, you have a spare in the toolbox. And when you have a whole pile of fittings to clean you can snip the loop off the end of the handle, then chuck the brush in a drill motor and power clean the whole pile in a few minutes.
The small round OD brushes are not required, but they certainly are handy if you have a lot of pipe to run, personally I love em.
Some people prefer the 4-in-1 type cleaning brushes that have both ID & OD brushes for 1/2" & 3/4" pipe. I find that you will wear out the 1/2" ID brush long before you even use the 3/4" brushes and due to their overall size they can be very awkward in tight places.
Now to answer your original question regarding flux. Technically speaking, tinning flux will spread much faster under heat that what soldering flux does. Tinning flux is primarily used in the sheet metal trade although it can be used interchangably with soldering flux for sweating copper. You will also note that there is two types of flux, Petroleum based and Water based flux. In some local jurisdictions water based flux is required when soldering potable water supply lines, but, unless you are required to use water base I would recommend sticking with petroleum based flux until you gain some soldering experience. As far as brand names, it is all pretty much the same but I personally prefer the "Oatey" brand when available. (Oatey is a well respected trade name for making chemicals and supplies for the plumbing industry)
FLUX BRUSHES: Although you will see many people apply flux with their finger tip, that is a bad habit to get into. First of all, you run the risk of contaminating the joint surface with dirt or debris that may be on your finger and in some instances even the natural oils in your skin will have a detrimental effect on soldering. Also, getting into the habit of applying flux with your finger is a good way to end up severely burning a finger on a hot piece of pipe. Look around in the plumbing section of your local hardare and you will see some very inexpensive "acid brushes". They are just a piece of rolled metal for a handle and some bristles on the end. The normally sell for about $.10 each and i would suggest grabbing a half dozen, that way if you drop on one the floor and contaminate it, you have a clean one in the toolbox. Also, you will be burning them up as you go, so make sure to have spares.
SOLDER: The plumbing codes require that all copper joints be made with "Lead Free" Solder. I would suggest you grab a 1/4lb or 1/2lb roll of solid core lead free solder. That should provide you enough to last for months. (Typically i can solder all the joints in a whole house with 1 lb or less.)
SOLDERING TORCH: To the uninitiated selecting a soldering torch can be a real challenge. You will find mini-butane torches that will fit in a shirt pocket, although I have yet to figure out why one would want a soldering torch in a shirt pocket. You will also find neat little kits with a Universal torch head, propane tank, spark igniter, some solder, flux and even a bit of emery paper all in a nice little metal carrying case for about $20. Examine the torch tip closely and you will see the combustion air intake holes are in the tip, about a 1/2" from the flame. Due to the short combustion area those torches produce a flame that is a little too cool. It is possible to sweat small diameter copper with that type of torch, but it can be difficult because it takes too long to get the pipe and fitting to the correct flow temp for the solder and in many cases you end up oxydizing or annealing the copper, which makes soldering very difficult.
Turbo Torches have the air intake ports at the base of the burner tube. Inside the tube there is a spiral of metal which causes the air and gas to swirl as it passes through the tube to the burner. This creates a much better fuel air mix which results in a slightly hotter flame which is better suited for soldering copper pipe. You can buy a Turbo torch in the range of about $25 to $30.
My personal preference is the BernzoMatic TS-4000T which is the turbo torch i have pictured in the illustration. The TS-4000-T has a fuel regulator that permits the torch to be used at all angles, whereas the cheaper torches you must keep the fuel cylander upright at all times. This can be a very distinct advantage when soldering in tight places or where you are up tight against the floor joist and desire to point the flame downward to prevent catching the wood on fire. This torch also is a total one handed operation, pull the trigger and it turns the gas on and lights the torch, release the trigger and it turns the fuel off extinguishing the torch. This is a very good safety feature because you dont have to worry about things catching on fire when you set the torch aside for a moment. It also conserves fuel because the torch is only burning when you are actually heating the pipe as opposed to burning constantly between steps. To give you an idea of fuel consumption, I typically use one tank of propane to plumb a whole house with this torch. The TS-4000-T is also a multi-fuel torch that will burn propane or MAPP gas. Propane is the preferred fuel for soldering copper and MAPP gas should only be used when working outdoors in the wind such as during new construction or when working on larger diameter pipes 1-1/4 or greater.
You should also have a pair of channel lock pliers handy in case you need to move or reposition a hot pipe or fitting.
You should be wearing a long sleeve shirt and safety glasses when soldering, especially when working overhead like running pipes in a basement. I cannot emphasize this enough. Be extremely careful when working overhead and make sure that no part of your body is directly under the joint. By example, I have a permanent scar on the inside of my left forearm where hot solder dripped off a fitting, hit my wrist and ran down under the cuff of my shirt sleeve on my forearm. By the time i could set the torch aside and unbutton my shirt the solder had set in place on my skin. That is not an mistake that I am likely to repeat any time soon, and hopefully I can encourage you to learn from my mistake.
The last item you will need is a wet rag. Make sure it is not a synthetic material. I prefer a piece of an old cotton tee shirt or a cheap bath washcloth.
OK, we now have all our toys together so lets play with some pipe and fittings. Before you begin cutting into your plumbing system I would strongly suggest you get a short length of pipe and a dozen or so fittings to practice on. Look in your junk box, that old piece of brown and corroded copper pipe is excelllant for learning how to properly clean the pipe for soldering.
Begin by cutting a couple piece of pipe, clean them as described above, then apply flux to the outside of the pipe only. (Do not put flux inside the fitting.) Fit the pipe into the fitting and rotate it slightly to insure the flux has spread evenly in the joint.
Take your roll of solder and unroll about 10" but do not cut it off the roll. Bend the end of the solder into a J hook about two inches from the end. You can now use the roll as a handle to safely hold the solder.
Begin by applying the tip of your torch flame to the end of the fitting so the fitting and pipe will preheat evenly. After a few seconds move the tip of the flame to the fitting. When solder melts it will flow in the direction of the heat, by applying the heat on the fitting it will suck the solder into the joint.
While applying heat to one side of the fitting lightly touch the tip of the solder to the pipe and fitting intersection on the opposite side of the pipe. If the pipe is hot enough the solder will instantly flow into the joint and you will see it loop completely around the pipe. If not, pull the solder back and continue heating a few seconds and try again. As soon as the solder flows evenly remove the heat from the pipe and continue feeding solder until you see a little drop form on the underside of the pipe. Remove the solder and wipe the joint with a wet rag. You should end up with a bright shiny wash of solder completely around the pipe as if it had been chrome plated.
Wiping the joints if very important because it not only cleans up the excess solder, it removes the excess flux. Flux is an acid and if not wiped off while the pipe is still hot it will cause the joint to corrode rather quickly.
After you have done a couple dozen joints on the bench you should be ready to go out and work ont the plumbing with confidence.
Six months from now you will be teaching friends and family all your tricks of the trade.