WHEN TO USE EPOXY CEMENTWhen you need a strong rigid workable (e.g. sandable and/or paintable surface) adhesive, particularly for use on metal or hard plastic, epoxy is usually the best choice.  Most epoxies dry with little running or shrinkage, and so are good for filling gaps.  The major drawback to epoxies is that they consist of two parts (resin and hardener) which must be mixed in small batches, and then applied with your own applicator (e.g., a toothpick or popsicle stick). Some are packaged in a dual-syringe--but that is not an adequate solution, because the components must still be manually mixed in order harden to maximum strength.WHEN TO USE JB-WELDJB-Weld is a very strong epoxy cement.  JB-Weld is the cement/glue of choice when joining metal parts or filling holes in metals.  Otherwise, ordinary epoxy cement or other types of cements/glues are usually better choices.  Some other reviewers have complained that the descriptive name "weld" is inappropriate---to some extent that's true.  JB Weld can do some jobs welding can't.  Conversely some simple welding jobs like joining two thin metal rods at right angles are difficult with JB Weld.HOW TO USE JB-WELDGeneral Principles: Although JB WELD makes an excellent filler---and can even be used to cast small parts, it is not remotely as strong as real metal used this way.  Whenever possible, use JB WELD as an adhesive---a very thin layer of JB WELD holding two pieces of metal together.  Always reinforce with metal pieces when possible.  For example, rather than building up a massive flange with JB Weld, you might be able to use scrap peice of metal to form most of the flange, held in place with JB Weld.  The greater the relative surface area of contact, the strong the bond will be.Surface Preparation:  As with any adhesive, the surface must be clean and rough.  Clean with a solvent (according to another reviewer, acetone is the best solvent for this purpose, but rubbing alcohol will work), roughen the surface, then clean with the solvent again.  Use a clean cloth for each application of solvent.  I generally use a small grinding wheel in a Dremel tool to roughen the surfaces.  You can also scratch the surface with a knife, or use coarse sandpaper, or both depending on the material.The solvent cleaning is for removing the LAST TRACES of oil from CLEAN surfaces.  If the parts are oily, even multiple cleanings with solvent may not be enough---you may redeposit oil from your cleaning cloth or as the solvent evaporates on the surface.  So, if you can detect any oil on any portion of the part, or if you have any doubt, clean it with a degreaser first.  If you do not have a commercial degreaser handy, a paste of laundry detergent and a stiff brush may do the trick.  Only when you are certain that there are no traces of oil is it time for the solvent cleaning(s).Joining Broken Parts: If you are joining broken parts, with a clean but irregular matching surfaces, limit the roughening to scratching with a wire brush or etching with an acid (but be sure to wash off all traces of the acid). Apply a THIN uniform layer of JB WELD, lightly clamp together hard enough to squeeze out any excess. Sometimes a thick rubber band is perfect. Often you can arrange pieces so that the weight of the top piece will hold the joint together until the JB Weld cures. Sometimes you can place a weight (such as a brick) on top to apply gentle pressure.  Do not remove the clamp or pressure until the JB Weld has cured completely.  After an hour or two, after the glue has set but is still soft, you can cut off any excess with a knife or razor---but if you don't need to cut the excess off, don't.  Often you can build-up the surfaces around a break for added strength.  Sometimes blue painters tape can be used to help hold pieces together while the glue sets, and/or to make a temporary stand to hold the pieces in the best orientation.ScotchBlue Painter's Tape for Multi-Surfaces 2090-.75A, 3/4 Inches by 60 Yards, 1 RollJoining Dissimilar Parts:  Minimize any gaps if possible, e.g., by sanding if possible so that the contact as perfect as possible.  If you will be filling substantial gaps, sometimes it is helpful to cover openings with blue painters tape to prevent the JB Weld from flowing away and leaving gaps.Joining Long Thin Parts (rods or sheets): Try to overlap if possible, or reinforce them with a similar overlapping peice of metal glued in place with JB Weld.  To join a 1/8" steel rod, to another 1/8" steel rod, forming a "T" (a common and easy welding job, almost impossible with just JB-Weld), cut two 1"-long "L's" from a heavy-paper clip, (or make L's from similar wire), and embed those with JB-Weld on opposite sides of the joint.  You can further strenthen the joint by wrapping fine steel wire around the rods holding the "L"s in place---before  covering it all with JB-Weld.  Be sure to thoroughly clean the rods and wires first (see below).  For small parts, or when wire wouldn't work well, unwaxed dental floss is remarkably strong, and can do an excellent job of reinforcing joints (when embeded in JB Weld). Ideally, apply the JB Weld, then wind the floss tightly into the soft JB Weld.Filling Holes:  Don't forget to prepare the surfaces as described above---even if you can only roughen the surfaces with an ice pick or awl.  JB Weld shrinks very little as it cures so fill the hole completely.  If it is possible to sand the surface when the JB Weld has hardened, then overfill the hole slightly.  Since JB Weld flows a little until it sets, the surface should be horizontal.  If not horizontal, you can apply the JB Weld and then cover the hole with blue painters tape until the JB Weld sets.  Depending on the job, it is often a better strategy to fill the gap as much as possible with a piece (or pieces) of metal embedded in JB Weld that a single mass of pure JB Weld.Reinforcing a leaking pipe:  Work JB Weld into a piece of fiberglass cloth or with a spatula, apply, and then add more JB Weld to any thin spots and to feather out the edges. You may have to apply several layers of the JB Weld-saturated fiberglass cloth. Sometimes wire cloth (window screening) can be used.Holes in car body panels etc:  If you can cover at least one side of the hole with a piece of sheet metal (held in place by JB Weld) that is a good strategy---the combination of pop-rivets and JB Weld often works especially well, even if you ultimately sand the heads of the pop rivets away.TEKTON 6555 Rivet Gun with 40-pc. RivetsMixing:  Follow the instructions.  A paper plate can be a good mixing surface; popsicle sticks are the best mixing tools.  Beware that you must use the same amount of resin and hardener, and they MUST be VERY thoroughly mixed.  Mix until you are certain that it is completely and thoroughly mixed, and then continue mixing for twice that time.  Seriously!!!Curing:  Follow the instructions, but wait longer than recommended: the longer you wait the stronger the bond.  A a day or two at 70 or 80 degrees is a reasonable MINIMUM.  Note that epoxies do not "dry"--there are no solvents to evaporate, instead a chemical reaction hardens the cement (a plastic resin).  Chemical reactions are proportional to temperature (the rule of thumb is that for each 10 degrees, the reaction speed doubles).  Using the rule of thumb, if it takes 1 day to cure at 80 degrees, then it takes 16 days to cure at 40 degrees, and that's only to minimally cured.  Okay, I know that waiting this long is not practical in many circumstances---you can probably get away with curing for 4 hours at 90 degrees (but don't go much higher than 90 degrees), and "take it easy" for a couple of days.WHEN TO USE OTHER EPOXIESFor materials other than metal, particularly for rigid plastics, other epoxies are usually better than JB Weld.  Some epoxies are clear, or white, or specialized for particular purposes.  For exampleCLEARBOND H-3S Clear Strong Epoxy Adhesive Syringe - .85ozWHEN TO USE QUICK-EPOXIESOrdinary (slow) epoxies (whether JB Weld or other epoxies) dry harder than quick epoxies and are generally the best choice.  Maximum strength is the whole point of epoxies--if you don't need maximum strength---you probably do not need epoxy.  Use quick epoxies when 1) the hardening speed is essential, 2) when the item cannot be supported in the required orientation while the epoxy sets (at least several hours), or 3) when you have to absolutely minimize flowage.  Super-glue is often a good alternative in such situations.WHEN TO USE OTHER CEMENTS OR GLUESWhile epoxies are ideal for a few specific uses (primarily when you need a rigid cement for metal or rigid plastic)--there are many situations when other types of glue are better choices.  Epoxies are generally NOT recommended for flexible surfaces, such as leather, wood, or flexible plastic, and generally do not adhere well to glass, etc.  While epoxies may work with porous surfaces (paper, wood, ceramics, etc.), other adhesives are usually better.White Glue: For most light-duty indoor household applications which do not have to be waterproof, ordinary household white glue is safe, cleanup is very easy, and in most cases repairs are redo-able if you make a mistake.  White glue shrinks a great deal as it dries, and so is not good for filling holes or large gaps.  For example:Elmer's All Multipurpose White Glue, 7 5/8 oz. (E379)Glue Stick: Think of glue sticks as thick white household glue in stick form.  Glue sticks are great with paper, but have few other applications.Avery Glue Stic, 0.26 oz, Pack of 6, (98095)Wood Glue: Carpenters's wood glue is essentially water-proof (when dry) white household glue.  It penetrates and strengthens the wood better than any other type of glue. It is safe, and cleanup is easy. If the joint breaks again (because the wood is weak), it can be re-glued. For example:Elmer's E701 Carpenter's Wood Glue 8-OunceGoop:  Goop will bond virtually any material, including leather, plastics, glass, and metal.  The bond is clear, VERY strong, waterproof, and hard but flexible.  Since Goop is clear, if a little extra oozes out, it is barely noticeable. I've used Goop to repair ceramic plant pots, water hoses, and shoes; to seal holes in air-mattresses, to close small holes in window screens, as plumbers paste in assembling plumbing parts, to  I also use it to seal rust spots on my dishwasher rack--the Goop it also makes a "cushioned" surface that will prevent scratches to dishes.  Goop is particular good for repairing damaged electrical cords.Amazing Goop All-Purpose Household Goop, 3.7-Ounce Tube #130012Silicon Glue: Silicon glue is best for glass, and when you want a truly flexible connection (for example, to absorb vibrations). It is a good alternative for Goop for general household use on glass, plastic, and metal when you do not need the maximum possible strength. It is safer than Goop, and cleanup is easier. Like Goop, silicon glue takes days to reach reasonable strength, weeks to reach maximum strength. For example:GE Silicone II Household Glue, 2.8 ozFabric Glue:Beacon Fabri-Tac Permanent Adhesive, 4-Ounceworks amazingly well on fabrics, saturating the surfaces enough to provide a very strong bond, but without soaking through.  Depending on the application, fabric glue can be better than sewing.  It remains very flexible, e.g., it would probably be great on leather or heavy vinyl (e.g., women's handbags, shoes, luggage, etc.)Super-Glue (Krazy Glue): Super-glue is very strong, rigid, and fast. In many ways it is similar to 2-minute epoxy---except that no mixing is required. Although very hard, super-glue is brittle, and so, is not recommended for flexible objects. Super-glue is hazardous in that it can easily glue fingers (etc.) together.  Previously, I used super-glue frequently on objects (e.g., jewelry) when a clear bond was required---however, Goop is better for such applications, safer, and more conveneient. Opened super-glue keeps poorly, even in tighly sealed glass containers---so I recommended packages of multiple very small tubes.BAZIC Super Glue, 3 grams 0.10 ounces, 6 Per PackLiquid Nails/Construction Adhesive:  When you need to permanently join a large amount of surface, of almost any kind, Construction Adhesive is usually ideal. If you need to join 2 2x4s to make a 4x4 Construction Adhesive is better than nails.  Most construction adhesives are slightly flexible, and so can even be used on leather (i.e., to repair a loose shoe sole), etc.  Construction adhesive may leave a stain on porous surfaces, and may stain-through thin surfaces.  For small projects, Goop is usually a better choice.Caulking Compound:  You can think of caulking compound as white household glue with alot of inert filler--to use when you need to fill holes.  As an adhesive, it is the weakest on this list, but is strong enough for many purposes when spread over a large surface, such as the back of a ceramic tile.  A few years ago, I made a brick column to support a mailbox.  Traditionally, I should have used mortar--which would have been very time consuming.  I could have used construction adhesive (albeit any excess that squeezed out would have been ugly)--but caulking compound was quick, cheap, looks good, and was more than adequate for the job.