It should be remembered that welding over weld metal is actually a very common occurrence. Multi-pass welds after all are manufactured by welding over weld metal! There are also many accepted procedures in which welds overlap.
With the 3xx series stainless steels, two re-welds may be tolerated, but in the case of the duplex and super duplex stainless steels, only one re-weld operation is advisable, and then with extreme care and control over the heat input.
Running over the top of an existing weld will add little, if any strength to the joint as you're having to penetrate the new weld material, not the parent metal. If they need rewelding then I'd suggest grind them off as much as possible so you're near as dammit back to the parent metal.
Repair welding is a commonly used technique in which the cracked material is removed by arc gouging and the element is welded to re-join the material on either side of the crack.
Regarding your first question, “Why can't I stick weld with my MIG machine”, the answer is because your MIG machine only has CV output, which is not intended or recommended for stick welding.
Defects which are welded over, and not melted out, can suffer locally intensified strain age embrittlement by static or dynamic strain ageing at the region of concentrated strain at the flaw tip, leaving a planar defect with its tip in a region of low toughness (Dawes M G).
When done correctly, stick welding produces stronger joins than MIG welding.
If you do grind a perfect weld, yes it will make it weaker. If you just want to smooth it a little to make it look nicer, it will weaken it a little bit but should still have structural integrity.
Don't make a weld over the same spot twice to try to cover up for a bad weld. To do so effectively, the work must cool and then be hit with a much higher current.
There's a common misconception that welding is stronger than bolting, but that simply isn't true. Bolting is just as strong as welding—and can be stronger in some situations. The strength of a weld is largely determined by the expertise of the welder.
TIG welding is the hardest form of welding to learn for a variety of reasons. The process of TIG welding is slow and takes time to get used to as a beginner. A TIG welder requires a foot pedal to feed the electrode and control the variable amperage while maintaining a steady hand at the welding torch.
Properly made, this welded joint is at least as strong as the metals joined. However, there are disadvantages to this linear-joint-welding approach. The joints are made at high temperatures – high enough to melt both base metals and filler metal.
American Welding Society (AWS) D1. 1: The minimum distance between welds should be at least four times the thickness of the thinner part joined, but not less than 1 inch (25 mm).
Signs of a bad weld include: Underside of weld has insufficient penetration, excessive globules of metal, over-sized weld, undersized welds, undercut, overlap, incomplete fusion, porosity and/or cracking. Excessive grain growth or the presence of hard spots cannot be determined visually.
While it is true that some jobs that could only be performed by a human are now overtaken by automation and robots, welding will not become obsolete by any means. Welding is a complex craft that has unique challenges that technology can't solve on its own.
The major cause of a crack is when internal stresses exceed the strength of the weld metal, the base metal, or both. And once a focal point for these stresses—that is, a stress riser—develops and accumulates, a crack can propagate.
The answer can seem a little complicated, in a sentence though, the reason you blow holes when spot welding, is because you have high resistance in your weld joint, (stack) and not enough pressure to control the excess heat produced.
Welding creates a high-strength joint
This is because the welder melts the two metal surfaces and interlinks the two substrates. Sometimes, the weld joint is so strong it becomes stronger than the metal itself.
Using too much electric current causes an overlap. Too much current leads to over-melting of the weld material and a consequential overflow above the metal.
While it is true that welded pipe, in theory, is weaker when it is welded. Many advances in the welding process and quality checks have closed the gap. However, technically speaking, seamless pipe is still stronger on paper and in extreme applications.
Some of the best metals for welding include: Low Carbon Mild Steel. Aluminum. Stainless Steel.
Unsuitable for Thick Metals
While MIG welding is suitable for thin metals, it does not deliver proper penetration for thicker steel that requires a solid weld.
Static strength
Butt welds in carbon-manganese steels, made by arc welding with consumables giving weld metal matching the parent metal strength, are as strong as or stronger than the steel itself.