How to Fill Holes and Gaps with a MIG welder- [FULL GUIDE]

Last Updated on February 2021


Metal Inert Gas (MIG) welding, also known as gas metal arc welding (GMAW) or wire welding, melts and joins two metals together. It uses a continuous solid wire electrode that is heated and fed through a welding gun into the weld pool. The gun also sends through a shielding gas together with the electrode, which helps protect the molten weld pool from contamination that may occur due to reactions with atmospheric elements.

Key benefits of MIG welding include but are not limited to:

  • Speed – it is faster because of the continuously fed electrode wire
  • Can be used with a wide variety of metals and In all positions
  • High-quality welds and cleaner finish – do not use flux hence avoids messy sputter
  • Waste reduction – mainly because the gas shield guards the arc hence the loss of alloying elements is significantly reduced.
  • It is effortless to learn because starting the arc only requires pulling a trigger and the fact that the wire automatically feeds into the weld puddle.

Despite its many benefits, users still experience some challenges and defects, especially with cracks, holes and gaps. Not to worry, as this happens to the best of them all, from beginner to expert welders. Some of the issues that can lead to you developing weld defects and gaps are:

  • When you hold the welding gun in one place for too long or operate it too slow.
  • When you have any form of contaminants on your working area
  • When you use an anti-splatter spray or gel excessively.
  • When you run out of welding gas, have contaminated gas or its flow is too high
  • When you have an open weld joint at the base.
  • When you hold the weld nozzle at a wrong angle or too far from the object.
  • When your welding gun’s nozzle is blocked, or hose gets contaminated.

The ideal state is to create smooth and clean work through and through. However, even when you apply utmost care in your welding work, sometimes you stumble upon gaps or holes on your finished product. While this is an annoying experience, it is encouraging to know that the problem is not without solutions.

Types of Welding Gaps and MIG Solutions to fill them up

First, you need to appreciate that these gaps are all different, therefore require different techniques to fill correctly. Below we shall identify common welding gaps and discuss how to fill them up effectively using a metal inert gas (MIG) welder:

Corner Gaps

Corner welding joints are a popular weld in the sheet metal industry. When you have corner pieces that do not align together evenly, they are brought about, meaning you will need extra metal to add on and close the gap.

As a rule of thumb, stick to welding all joints from the top as much as possible and welding inside a corner to create stronger, nice-looking and durable welds.

Different welders have sworn by different (yet very similar) techniques to deal with corner gaps. Depending on the exact problem you are dealing with, guaranteed you will find a useful method to the madness in the solutions below:

  • For better penetration, you are better off dragging the weld as opposed to pushing. If you need to get deeper, try running a triangle pattern with Hobart MIG. Use it to burn into the lowermost part of the seam, and then rise towards the radius edge. If need be, let your puddle build up a little on that spot and cross over to the butt edge. It is advisable to come out of the seam from the radius edge instead of the butt side.
  • Depending on your circumstance, decide whether you need to weld to fill the gap, or grind the open tube end so that they align with or match the curved corners.
  • Instead of attempting to fill large gaps in a corner, it is better to recut the pieces at a 45 degrees angle that will form a 90-degree angle. This will help you achieve a perfect finish.
  • Instead of concentrating your heat on the cut edge, focus it on the radius edge, then adjust the position so that the cut edge on the bottom and radius is on top. This enables you to favour gravity, which is instrumental in helping the puddle flow down onto the cut edge easily without concentrating your arc fully on it. Ensure that you build the puddle up on the side that will absorb the most heat then cross it over to the other side in rapid whips.
  • When you concentrate your heat onto the radiused piece and let your puddle flow into the butted piece while making large and slow circles to fill the gap, you will get excellent penetration into both tubes.
  • Lay down tacks every few inches while stringing some along until you generate a bridge in the gap. After this, you can form weld beads and stacks above and through it. Always stay ahead of your molten puddle with the right speed to ensure that you do not burn through your metal.
  • When dealing with larger gaps, ensure that you are getting the proper penetration. Also, be keen to clean up any leftover slag then grind it down properly to a smooth weld.
  • To prevent burn out, build up a few passes on the radius edge, and run a quick pass down the cut edge.
  • Push the puddle, then whip the electrode. When you notice that it is beginning to burn through, quickly jump ahead and allow that specific spot to cool down. Sometimes skipping ahead and returning to weld back into your weld in quick short bursts does the magic.
  • If you use flatter beads to cover your gap, always employ the push method instead of dragging or pulling your pool towards yourself.
  • Consider lowering your settings to a thickness slightly below what you are using. Place the rod in the puddle and hold back for a little while to give room to the “fast freeze” then bring it back into the puddle.
  • Another option is to lay the bead on the radius as opposed to concentrating heat on thin walls. While it requires constant practice to master the skill, concentrating your weld on heavier wall material then swiftly whipping it on lighter material works well.
  • If you are doing body works, try running tacks. Do this by putting a decent tack in one spot and then jump over 50% to put another.
  • The secret to welding corners lies in moving with precise speed, heat control and ensuring proper settings on your Hobart.
  • It is advisable to use short bursts when welding as this ensures that you form a bridge between both pieces of metal. Additionally, be keen to space your welds in a way that gives time for the metal to cool down.
  • Lastly, ensure that you do a dry run on unimportant metal. Testing is key to ensuring that you do not entirely ruin your weld by burning through when you lose control of the heat.
  • In the rare case where all fails, you can always employ the good old “STIG” technique. This is where you smash a different rod’s slag and input it into the puddle with your other free hand.

Sheet Metal Gaps

  • When dealing with sheet metal, you must appreciate that strength and penetration are non-issues while heat and warping are a bigger headache.
  • Designers of sheet metal pay close attention to the specificity of hole sizes, locations, and alignment. Bigger hole diameters than the sheet’s thickness are preferable. Proper spacing and distance between holes guarantee a strong metal and prevent the deformation of holes in bending or forming processes.
  • Nevertheless, even the best-designed metal sheet can get undesirable gaps or holes during work. Here are some tips and tricks to deal with sheet metal gaps/holes:
  • Placing sheet metal flush to each other enhances flawless fusion in short bursts while allowing them to cool in between. Continue in short circular motions until it is well done. Remember always to control your heat to avoid burning up and destroying the base metal. Once complete, clean and grind it to a smooth finish.
  • Do not apply a lot of heat at once as it will burn the hole, making it bigger. The trick is to use a welder to build up an edge on the hole, then grow that across it.
  • It is advisable to work on more than one hole ( 2-3) at once to avoid the application of too much heat on any one spot.
  • For best results, use the right amount of wire tip, hold the Hobart MIG welder gun tip at the right angle, and distance from your work surface.
  • The flatter the sheet metal, the higher the chance of warping so look out for those curves. But if you must, just weld slowly with a few at a time. If the holes are many, there is no worry about your sheet metal cooling down as there is ample time before returning to fill them.
  • If you have access to the back of the sheet metal hole, you can drop in a piece of copper then use magnets to hold it in place. Then use your Hobart MIG welder to fill the hole from the front side. Complete the process by grinding, finish and paint.
  • You can also back with a piece of copper then perform a tack weld to fill it in from the top.
  • When dealing with small in length gaps mainly due to uneven cutting, MIG weld dingle berries on the opposite ends, grind and flush. Continue this process until they meet in the middle hence filling the hole. Test that the area is not too thin by gently knocking that newly welded spot. When the front is well done, repeat the process for the back area. Always remember to control the heat to avoid overheating the panel and warping it.
  • Through experience, anything more than a 1/16″ is harder to weld. It requires you to hold the trigger for more prolonged periods which get overheated and warp the metal more.
  • Smaller gaps are easier to penetrate, especially when you use a fast tack weld to regulate the heat then perform the criss cross technique until they get to criss-crossly 1/4″ apart. For a full guide on the top rated sheet metal snips [Read Our Full Guide]

Gaps and Holes Caused by Burn through or Drilling

Burn through refers to the hole that is created when the weld metal goes through the base metal. This usually occurs at the root pass.

You can prevent this by turning down your amperage and making smaller welds to keep the base metal from overheating.

Some of the causes of burn through include

  • A large gap at the base of your joint
  • Incorrect setting of your machine
  • When the metal is less than ¼” which is too thin
  • Too much heat
  • Improper penetration

Let us examine some of the techniques for dealing with these holes:

  • When weld-repairing holes caused by burn through, always begin with the edges on the outside of the hole and work towards the centre.
  • You can also start by grinding off the backside to remove any beads and clingers. Then clamp it down, cover the hole and weld it shut while letting the brass stick to the hole.
  • Another option is to weld from side to side on both edges of the gap until it narrows down due to the buildup of weld on the sides, which paves the way for you to close the weld
  • Move your rod from left to right or vice versa in small increments. Move your arc on one side and hold on briefly, quickly cross over the centre and then hold on again on the other side. It would be best if you aimed for horizontal, circular motions until you close the hole, and then grind it flush.
  • On either side of the hole, run stringers next to each other until they meet at the centre. This movement should be mad speedily to avoid meltdowns on either side.
  • Take full control of the heat and regularly moving from side to side while building the puddle on both sides. Then using the electrode, bring in the metal to cover the hole.
  • When dealing with tubing that is too thin, run a quick pass on the butt rear tubing side to gather some material on the edge before fully welding. You can then burn a second pass focusing more heat on the radiused side while moving the puddle across to the piece quickly heats up.

Methods of Welding Gaps and Holes using MIG Welder


As its name suggests, this is the process of laying down welds on top of each other until the beads create a mound big enough to cover the whole piece.

For gaps that are too big, you can tack weld a backing strip and weave beads in a way that thrusts the slag into a location that creates a barrier between the strips. At the end of the weave, remove the backing strip, grind it, and then finish with a back weld.

Another option is by concentrating full heat on the beam in the opposite direction of the weld. This enables expansion of the beam, which reduces the gap that you are dealing with to a manageable size.

A quick fix involves tacking on the same spot as many times as needed to generate a line across the hole/gap. This is, however, very risky and should only be applied if there are no alternatives.


Padding involves overlapping weld beads intending to generate even surfaces.

It is an appropriate method for bridging large gaps by forming many rows of padded beads until the gap is filled up. Place multiple beads very closely together, tightly stacked with an image that resembles a rope.

A padding weld can either be single or multi-layer type. The surface must always be thoroughly cleaned before depositing a padding weld. Deposit the first run narrowly at the surface edge and the second run deposited in an overlapping manner. All subsequent layers of padding beads are deposited across the ones below them.

Padding involves welding pieces of metal together that can be broken again. An indicator of a poorly done padding is the ease of breakage.

This method is excellent for beginners because it is easy, repetitive and does not consume a lot of metal.

Filler Rod

A filler rod helps to fill weld gaps by tacking in a bar stock in the weld.

When using this method, it is recommended that you leave a shim uncovered while welding. Do not just melt the shim to form a bridge, instead, fillet the plate to the shim and the shim to the column and this will be effective as long as it is appropriately handled.

Proper planning, cleaning off any lingering welding flux or contaminants from the surface before work for the way for good filling of any large gaps.

Ensure that as you use the filler rod, it is not fed directly into the arc column instead input it into the leading edge of the weld pool slowly at an angle of 10–20 degrees. This helps to avoid spatter and any other accidental contaminants to the electrode. Also, to prevent oxidation, hold the filler rod’s tip inside the gas shield while it is hot.

…Three Step Summary

Bridging the Gap

Start with making the initial bridge with your Hobart MIG welder across the narrowest section of the gap, (to build up more metal with each pass). Then weave continuously with each pass, as you slowly move the weld forward.

The overall process is slow-paced, due to frequent breaks or pauses that give room for the metal to cool after completing each bridge.

Getting Deeper in the Gap

As you approach larger gaps, you will need to build up more metal along the sidewalls by creating your weld in a horseshoe pattern. This ensures that you properly close the gap with each new pass of your horseshoe weld.

As you dig deeper into the gap, you will need to take more breaks to avoid burning the built-up sidewalls that contain a lot of filler welds that actually heats up faster. This also gives adequate time for cooling the filler weld and base metal.

Making the Weld Look Pretty

Because gobs of filler is not aesthetically appealing, you can use a handheld grinder to clean up the metal and shape the weld to create a smooth, seamless and beautiful look.


To avoid common MIG welding defects, Robert White, the Managing Director of R-Tech, once said: “From experience, most problems you will encounter with MIG welding will come down to one of four things. They are incomplete penetration, excessive penetration, incomplete fusion and whiskers. In each case, the solution is pretty simple. To solve incomplete penetration, increase wire feed and reduce the amount the wire protrudes from the MIG gun. To correct excessive penetration, reduce the size of the gap you’re trying to fill. You can usually fix incomplete fusion by not allowing your electrode wire to get in front of the weld pool. As for those whiskers? Keep your wire at the front of the weld, make the weld pool smaller, and adjust the wire, so it sticks out of the MIG gun less”.

If you are dealing with customers, ensure that you remain factual and practical about the gap range you can tolerate. This is because there are gap differences that are big enough and those that are almost invisible and this means that the kind of power, gap filler and technique used would vary. Being realistic with your customer goes a long way in avoiding future disappointments.

When you work with a decent MIG welder with the right settings and wire diameter, filling those holes will be a walk in the park. If the ‘right‘ way to do things matter to you, we highly recommend Hobart – 500553 Handler 210 MVP MIG Welder, Small.

Find out more about it here.

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