Did I say welding UPSIDE DOWN? Boy I'll bet that got your attention. Really, these tips are for all welding done with the MIG. In fact, there really is no difference in welding upside down or laying flat, it is all in the mind where the difficulties come from. I have to say that I use to cringe when I saw that I had an inverted weld to do. Then all that changed I took the ICAR (a nation wide organization for standardizing repairs) welding test. It included two each of a plug weld (where you weld through a hole to the sheet below) and a "bead" weld. Both had to be done in a horizontal, vertical and inverted. That inverted was the only one I was worried about. Well I knew I would have to practice a LOT. The funny thing is I found the answer not so much in practice, but in studying HOW a MIG works (this can be found in my basics of MIG welding) and I found out how important clean metal was. Don't get me wrong, the practice did it for me, however the understanding of how the MIG worked helped me more than anything. I had a basic idea how the thing worked, I mean come on, it is a direct short where a wire coming out of the gun melts into the metal because of resistance. I studied the science as well as the art of welding, so I really got a grasp on what exactly the wire speed did and what exactly the voltage did. For the first time I understood what was happening when that wire hit the metal. On the first official practice, I set up the work area just like it would be at the test site. I took sheet metal "coupons" about the size of a playing card, punched holes in a few for plug welds and started welding. The thing I noticed right off was that it really wasn't that hard, why? I was welding really nice welds including the inverted. These welds were much nicer than I do at work. It hit me that the reason was, I was sitting properly and the metal was clean. That was it, that was the magic. By the time I practiced a few hours and took the test I was welding like I never had before. I now prepare cars the same way I did those coupons, SPOTLESSLY clean. Now I weld the same way on my repairs.
The thing is, there is a point where you can simply pull the trigger and you will weld. No burning holes (to speak of) no dripping molten metal, no popping and flying fireballs, you just weld. I have gotten so that I can weld "blind" with only the "buzz" as a guide. I trigger the gun with my thumb, index finger, left hand right hand, it really has changed things with me.
Without the thorough understanding of how the MIG works one can not properly "tune" it. Creating the balance between voltage and wire speed is the key. With the proper balance, you literally point the wire where you want it and pull the trigger. It WILL weld perfectly (providing the metal is clean). With the proper balance you can hold the gun up against the metal without even moving it at all, pull the trigger and just...hold it welding away. The wire will feed into the molten metal at just the correct rate. The weld will not blow through because the wire melting into it will "keep up" with the rate of the melting and cooling (solidifying) metal.
However, the first thing is GET THE METAL CLEAN and the pieces fitting tight. Nothing but nothing will aid more in welding inverted than clean metal and a tight fit. Of course it should be no cleaner than all your other weld areas, but if you are going to need it, its' here. When I say clean, I mean bare metal. Not even traces of paint or primer should remain. This includes the BACK of the metal that you will be welding. That's right, the back of the metal. Remember, the metal is going to be melting when you weld, right? Well, when the metal melts as it is welded it will bring into the molten area the impurities from the other side of the metal. THIS CLEANING IS VERY IMPORTANT. Don't use a grinder to strip the metal. It takes off too much metal. Use something like a 3M "Roloc" disc (#07485) or a "Clean and strip" disc (#07466) In hard to reach areas I do use a cut off wheel or carbide bit. But this is a last resort and only used because there is no other way. Just don't cut too much metal off. On a long "pinch weld" for instance, you should have the entire length BARE METAL (with only weld thru if it is going to he between two sheets). Not just where you will be welding, but the whole area where metal will mate. I say this because if you start "cheating" and only striping right were you are welding, you will soon get into a situation where you are welding up to that dirty metal and WHAM you have a contamination problem. So, if you clean it ALL you will never have a problem.
Yes there are some times when you leave primer there, but only when you KNOW what it is. What I am referring here is "weld thru" primer. This could be an existing primer found on the new parts you bought in the form of an "E" coat. Or one that is specifically made for this purpose. "Weld thru" primers come in aerosol cans (3M # 05913) and brush on. They are made by a number of manufactures. They should be lightly applied over the metal after it has been THOROUGHLY cleaned as described previously. Put a light coat, some are even transparent so don't think it has to cover like regular primer you would paint over. It is simply a zinc coating similar to galvanizing. So look at it like an at home galvanizing job. Look at a piece of galvanized metal like chain link fence, you can see right through the coating to the metal. It should be totally dried by the time you weld. If it has any unflashed solvent it will burn and your welding will be negatively effected.
The "E" coat primer (the "E" stands for "electrically" applied and obviously NOT something you would do at home) is also zinc rich and is a "weld through" primer. When I install new parts, I don't remove ANY of this primer. I punch holes through it for plug welding. I weld to it, through it, whatever. It is a perfect weld thru primer. And please, when welding the weld thru primers wear a welding respirator (3M #07187). They are zinc rich remember and the fumes are VERY harmful. The factory "E" coat may be in black, gray, green, and other colors. To be sure you have a real "E" coat rub it with a lacquer thinner soaked rag. If it comes off, it is NOT an "E" coat. If it does rub off on the rag, strip it like you would any paint or primer around a welded area and coat it with "weld thru" primer.
Weld thru primer does not have good adhesion properties (E coat has excellent adhesion properties). It is only to be used where metal will be over lapping and hidden. All other areas just leave bare metal until after welding and then treat them the same as you would any bare metal on your car. By the way, after welding the "ROLOC" or "Clean and strip" discs also work very well in cleaning the welded areas of discoloration and any welding residue. This is very important, that welding residue is a poor substrate for your undercoats. The "ROLOC" discs will actually cut metal, so you can use them to "grind" down any slightly high weld too. They work nice and leave a gentle, rounded surface. If you have a particularly large amount of metal to grind, go ahead and do so with a grinder and then before you get close to the base metal change to a ROLOC and finish it off. They really leave a nice look.
DON'T use a liquid "metal conditioner" on these welded areas, use an etch primer and or epoxy instead. The liquid "metal conditioner" is not recommended by any auto body or welding organizations because it can get trapped in between the layers of metal. This however is a whole different topic, I just wanted to mention it.
Okay, you have nice clean metal to weld, now be sure it is fit tight. The metal should be laying nice and flat on the lower piece. If it is not, you have to make it fit. This can be done with clamping and or tapping on the edges and reforming them with your hammers or other tools. Sometimes this is a problem because the metal springs back up after you have struck it down (or up in the case of inverted welds). If this is the case one way you can do it is to put a little tack weld and then tap on the surrounding area. The fact that it is tacked will hold the metal where you want it. It will then bend instead of bouncing back. After you have the metal laying nice and tight and clean, you can then put all your thoughts in welding. Instead of fighting contamination or poor gaps, you can just simply weld. That is one step in getting nice welds and being able to do those elusive inverted ones.
I have to reiterate (from basics of MIG welding) trimming off the wire to the proper length before every weld, I mean every weld. I just today finished up the insulation of a rocker panel on a full size van. There was about 50 inverted welds. I paid particular attention to the wire cutting trick. I just went about my welding, sometimes trimming the wire, sometimes not. There was a very noticeable difference when the wire was cut or not. The cut wire provided a faster initial arc, every time. It didn't matter whether it was on bare metal, weld thru primer I had applied or the "E" coat. This is very important, you want the arc to start as soon as possible. Especially on a plug weld (where you are welding through a hole, usually 5/16"), you need that arc to start so you get good penetration. If you hit the metal with the wire and it doesn't arc fast enough you will then pile up weld in the hole before the weld can heat up and melt into the base metal. This of course can happen with any weld that is small. If you are laying a six inch long bead, it is not so critical. However, if your weld is like a plug weld or a short tack and you don't get that initial arc to start melting the metal. This is very important if you are doing a series of welds down a seam for instance. You may be welding little half-inch welds to keep the heat (warpage) down. Well, if you don't get a good, fast arc you may not have very good welds. They may look decent but there will actually be only melted wire sitting on top of the metal. Their penetration may not be sufficient and you can't tell because the weld is laying on top of the metal. You may grind it down flat and never see that the seam is still there.
Okay, we have established that you should weld to get a fast arc. One way to help is with more voltage. If you hit that metal with a hot sharp wire, it will start welding right NOW. One of the reasons inverted welding can be a bare is if you are popping and cracking and not getting the darn weld going. Plus if it is hot and fast you get the weld done, and get out of there. This means less time to get "extra" molten metal falling on your chest. The hot, fast weld also lays flatter. If you are welding hot and fast, the wire is running into the molten metal instead of laying on top ready to fall because you have poor penetration.
Always set the welders voltage and wire speed at the setting for the thickest metal you are welding. In other words, if you are welding a 22 gauge piece of metal to an 18 gauge piece you would set the welder up to weld the thicker 18 gauge. As pointed out in the first "Basics of Basics- MIG welding" you want to direct the weld on the thicker metal when you are welding too different thickness. You start the weld on the thicker piece and then when you have a good arc started and the weld is molten on the thicker piece you move the weld over to "grab" the thinner metal. This is particularly important when you are making a small weld like a "plug" weld. If you are plug welding a 22 gauge metal to an 18 gauge (a hole is drilled or punched in the 22 gauge and you are welding through it) you REALLY need to get that weld started hot and fast. You need to start the weld right in the middle of the hole on the underlying 18 gauge and get a good arc. Once you are melting that 18 gauge you simply move the weld over to the edge of the hole and it will grab the 22 gauge and you can weld a nice flat weld.
Now, If you are butt welding tow different thickness of metals you need to target the thicker one for the first arc. However if you are lap welding two different thickness metals you may still want to strike the first arc on the underlying "base" metal even though it is the thinner of the two. The reason being striking the arc on the center of the thin metal STILL may take more time to get the established than the edge of the thicker one. That is an example of how much harder it is to establish the arc on the center of metal as opposed to the edge. The edge will start melting long before the center will.
Note: I refer to the "base" metal as the metal that is usually under another piece. I am not sure if this would be a common term, it just makes sense to me to call it this. If for instance you were welding a patch over the floor in your car. The old floor that the patch was laying on would be the "base". While the patch would be the "overlaying" metal in my descriptions. The "base" is metal that you are NOT welding at the edge. I needed to make some way of describing the metal that is NOT being welded on the edge and came up with the "base" as a name.
Whenever you are welding a lap weld you have to use the same procedure. The plug weld would be a lap weld. You are welding on the flat of a piece of metal up to the edge of another piece. The edge is going to melt much faster than a weld you are starting in the center of a piece right? Well, you have to get that arc started hot and fast on the "base" piece and then move over to the edge of the other piece. If you start right at the edge where you "really" want the weld to be you will likely burn the edge and blow it away before the bottom "base" metal ever gets molten.
If after you hit the trigger and the arc is established you start to blow a hole or the surrounding metal starts to melt too fast, let off the trigger. Then, just as the metal starts to cool and solidify hit the trigger again. This is sometimes called "popping", you "pop" with the trigger over and over as you fill a hole. The molten steel solidifies in the second that the trigger is off and then you hit the trigger again and it arcs real fast because the metal is so hot. But it has cooled and solidified enough that it forms a stable base to continue the weld.
Learn to work with gravity.
I am sure you have learned that molten metal will run down. It will not run up or sideways, so you are one up on it already. If you know where it the molten metal "wants" to go, use it to your advantage. It is like using a long wrench over a short one, you can take advantage of the science of leverage or you can fight it, it is up to you. When welding a vertical weld you start at the top and go down. This way you keep the weld arc ahead of the molten metal. The molten metal will start to solidify staying in place behind the weld arc as you go. If you were to start at the bottom and go up, the molten metal would hang off the bottom of the weld arc. It will at the very least make for an ugly, lumpy weld, or at the worse cause the molten metal to fall right off the weld arc. You can "fool" gravity and go up that vertical weld however you will find that you will work with the gravity most of the time.
This fact that molten metal will want to flow down is the reason you need your arc to start hot and fast. You need to "push" the molten metal into the weld when you are welding inverted. The arc has to be established fast on the base metal with the MIG tuned well. At that point you have the luxury of making a nice molten puddle that will not fall. Only then do you move "it" over to the edge of the overlying metal. This will produce proper penetration, which of course is the main objective. The weld needs to be well established in the base metal first, then only a little on the edge of the overlying metal is needed.
The rule on penetration is you need the weld to melt into twice the thickness of the metal. So if you were butt welding to .035 thick pieces of metal by this rule if you melted into each piece .070 you would have proper penetration. It makes sense doesn't it, it does no good to pile on weld when the metal is only .035 thick anyway. If you had a piece of metal that was not cut, it is doing its job at .035 thick, you do not need to pile on a weld to make it strong. All you need is that .070 penetration and you will at the very least have it as strong as if there was no weld at all right? If it were perfectly flat and only had that .070 penetration it would be at the very least as strong as with no seam or weld at all. The thing is, you must get the penetration. Without the penetration a pile a inch high of weld is NOT going to hold the two together.
Starting the weld hot and fast is going to provide you with the needed penetration on the base metal. Once you have that going all you need is that little on the edge of the overlying metal and you are done. You don't need to pile on a bunch of weld. This weld I am describing is a very flat weld. You want to melt your metal together not make a pile of melted wire. If a pile of weld is started, it will only grow and fall. If you are welding inverted and the weld is not flat and melting INTO the metal, I guaranty you will have a pile of weld. Many times that pile of weld will drop off onto your chest, YEOW!
Should a weld start piling up, STOP. You can not save it. If the base doesn't have good penetration it NEVER will. The piling on of weld is not going to do a darn thing about your penetration. As my dad would say "The boat has left for Rangoon". You see, you can use the gravity to your favor but that is another "basics". What is happening when the weld piles up is it is forming a "barrier" to the metal under it. That weld is solidifying as it cools. You can pile on all the weld you want, the metal under it is NOT going to melt. I don't have how red, how molten that pile is, the weld underneath has cooled too much.
If your welds need a lot of grinding, you are piling on too much weld and possibly they are lacking in penetration.
I can tell you right now, there is not a bit of difference in a horizontal, vertical, or inverted weld. The only difference happens between your ears.
Good luck and weld safely.