Bagpipe sections are fitted together by means of yellow string, known as "hemp." Hemp allows the joints to be optimally snug (hence, airtight) while allowing for wear due to friction. Here are some guidelines for handy hemping:
Hemp should be thoroughly waxed.
Hemp is subjected to cycles of wetting and drying and is therefore subject to swelling and shrinkage. This is particularly true of the stock joints, especially the blowpipe, since this joint is closest to the source of moisture. Swelling can cause the parts to jam, or it can even cause the stock to crack. You can minimize the swelling and prolong the life of the hemp by impregnating all but the final layer with beeswax before wrapping, and coating the outer strands with paraffin after wrapping. Paraffin not only repels moisture, but also lubricates the joint. For more on paraffin, see #7 below.
Joints should be snug, but not too tight.
You should keep the stock joints tight enough to stay put (so the chanter doesn´t blow out of the stock during play, for example), but loose enough that a normal person can get it apart with a firm twist. Firm is a relative term. Use common sense. Don´t hemp a joint so tightly that you risk cracking the wood, and be aware of the possibility of hemp swelling under the influence of moisture. A nicely hemped non-moving joint will "click" audibly when twisted in the stock. Properly waxed hemp will resist moisture and last for a long time (months, possibly years) with minimal maintenance. It´s okay to leave a bit of a gap (a millimeter or two) between the stock and the base of the projecting mount when assembled. This facilitates removal and also allows for a progressively snug fit as the the hemp gradually breaks down.
Tuning slides should slide.
The tuning slides should be loose enough that you can tune your pipes easily with one hand, but snug enough that your drones aren´t easily jerked out of tune when marching or in normal handling of the bagpipe. A common problem is inconsistent tuning slide surfaces, which means that the drone tops fit too tightly at some points, and too loosely at others. If you have this problem, make sure the joints are tuneably snug at the optimal tuning position. In some cases, the tuning slide bore may have to be gently sanded out using very fine sandpaper wrapped around an appropriately sized dowel.
Keep the hemp dry.
In addition to waxing, another way to preserve the hemp and prevent jamming is to disassemble the instrument and allow the joints to dry out after playing. This is essential in the case of hemped practice chanters, which become very wet very quickly. Wet hemp dries out pretty fast in the open air. If left sealed, the joint could take hours, even days, to dry out completely, especially if you live in a humid region or play a lot in the cold.
If a joint gets stuck...
What are you going to do? The prudent thing is to be patient and allow the joint to sit for a few hours. Don´t try forcing it with a pipe wrench or vise grips, and above all, don´t get in the habit of gripping the chanter at the bottom end when twisting; always twist the sections apart with both hands together at the joint, with one hand holding the ball-shaped portion of the top end of the chanter, and the other hand gripping the stock ferrule. Otherwise you can easily overstress the wood and break the chanter.
Anchor the hemp securely, and wrap it firmly.
When applying hemp, always apply sticky black wax (or tar) to the first few feet (the first layer or so). A very common and frustrating occurrence is to have the hemp work loose on the tuning pin, making it difficult or impossible to move the drone section up or down. Incidentally, most new bagpipes are not properly hemped by the dealer. If your pipes are new, plan on rehemping them immediately according to your own high standard.
Apply hemp evenly and carefully.
When you´re done hemping a joint, the parts should fit together consistently, with no obvious gaps or troughs. Take care to lay each strand directly against the previous strand, applying tension to the hemp and you go around. It´s not a difficult operation, but take your time and get it right. After you´ve built a few layers, check the fit frequently. Somewhere between the point of too tight and not tight enough, you can ensure a precise fit by spacing the last few strands farther apart. As you check for the final fit, twist the hemped piece around in the stock to set the strands tightly and consistently, and apply a couple more strands if necessary to snug it up just the right amount. Well waxed hemp should make a smooth, shiny surface when you´re all done. If you´re applying beeswax to unwaxed hemp, leave the last layer unwaxed (beeswax is too sticky for a smooth, snug-but-easy-to-disassemble joint); apply instead a good rubbing of paraffin to the surface after the wrapping is complete. For a really nice finish, you can melt the paraffin into the hemp by applying a cigarette lighter flame (carefully - not directly!) to the waxed hemp and twisting the optimally snug joint into place while the wax is still warm. Yum, yum, yum! You may need to add another strand or two after performing this operation, then wax and repeat. By the way, I always start my wrappings with a clove hitch, and finish with a half hitch or two, but this probably doesn´t matter very much if you´re waxing your hemp as described--the wax should hold everything in place well enough.
Stop before you get to the wall.
A common problem is that, as the newly hemped, snugly fitting joint is assembled for the first time, the top layer of loosely wrapped hemp gets pushed down and bunches up in a sloppy pile next to the projecting mount or chanter tenon wall. This is especially a problem if, in the case of the blowpipe, you are applying a layer of teflon tape and you want to cover all of the hemp to seal it completely against moisture. Leave a bit of unhemped space on both sides.
To teflon or not to teflon?
Teflon is the enemy of things staying put. For joints that are supposed to stay put, I generally do not apply teflon (plummer´s thread seal) tape. I´ve found that I can get a precise enough fit and adeqate lubrication using wax on the hemp. The only joints where I do I use teflon is on the tuning pins and on the blowpipe. I use it on the tuning pins because it allows for minutely precise adjustments due to an amazing self-threading property inherent in the material: if you pay attention to the direction you twist when assembling the drone, and always twist the same way to respect the bias when shortening the drone, and the opposite way when lengthening, you can easily tune up or down in tiny incements with just a gentle twist. (Take care to set up all of your drones consistently in this regard.) I also use teflon tape on the blowpipe joint in order to seal the hemp against moisture. Again, notice which way you twist to put the pieces together, and always do it the same way, and twist in the opposite sense to remove. Using teflon in this manner to seal the hemp will allow for an optimal fit and dry hemp for months or even years with minimal fuss. (Note: if the hemp is wet to begin with, say, after you´ve been playing, allow it to dry thoroughly before applying the teflon.)
What kind of hemp is best?
Hemp comes in different colors (yellow or black), and can be pre-waxed or plain. The idea behind the black waxed variety is that it supposedly saves you the trouble of applying black wax for those applications where you want the hemp to be really sticky (making bridles for traditional cane drone reeds, for example). The pre-waxed black hemp is not as sticky as ordinary hemp with "cobbler´s wax" or tar applied to it, but it´s probably sticky enough. This pre-waxed black hemp works well, and it makes a really smooth finish with no need for paraffin or other lubrication. The problem with black hemp, in my experience, is that it disintegrates when you try to remove it, which means you practically have to cut it off with a knife, which I find exasperating, whereas the yellow hemp can be unwound easily enough, provided you can find the end. Generally, I prefer the pre-waxed yellow hemp for most applications. It´s almost as sticky (maybe even more so) compared to the black variety, and it saves the hassle of waxing your own. When using pre-waxed hemp of either color, I usually apply black wax (tar) to the first few feet anyway just to ensure that it´s securely anchored. Instead of conventional yellow or black hemp, some people use other materials. Dental floss can work, but it´s kind of stretchy and seems to mash and deform easily, which can lead to irregular surfaces and inconsistent fit. On the other hand, it comes in flavors and can make your pipes smell minty fresh. I´ve also know pipers who have had cork applied to the tuning pins, like that used on clarinets. Whatever material you use, it should not be be so hard as to cause exessive wear on the wood. I would avoid fishing line, for example, and metal wire would be an obvious no-no.
Take your time and be proud of your work.
Try to cultivate a craftsman-like attitude towards the apparently menial task of applying hemp. Of course your bagpipe will work more efficiently, but you´ll find also that this level of attention to detail and commitment to excellence will cary over into other aspects of your playing, and to life in general.