How to Fill Trim Holes
Customizing your car often means filling unwanted holes in your exterior sheet metal. This is most often the case as you will need to fill trim holes once the trim is removed. Even newer vehicles sometimes use alignment holes to secure trim or badges to the body, but every older car (pre-90s) is going to have some holes that require closing up if you want to remove any of the trim. That means welding.
Body work is often the last thing that the average hot rodder wants to get into, even less sheet metal welding. Due to the fact that sheet metal is thin, it warps easily when welded and burning through is very common. If you want to eliminate some holes, the only option is welding, as everything else will just become a bigger problem later.
Some might say “hey, just slap some body filler or fiberglass over the hole and be done with it.” This is all too often the cheapo way out, but in the end, those little body filler patches pop out, leaving you having to paint the car all over again. It is a poor fix that should not be used. Welding sheet metal to fill trim holes is not that difficult, and will result in a better final product. There are some tips and tricks we can give you to help you along in your quest to eliminate some unwanted holes.
Aside from the welder itself, there are a few tools that will make welding easier. Some of these you need and some you can live without, but why try?
Clamps – There are a thousand different styles of clamps, and you really should have a good assortment. If you do sheet metal work blind clamps, and Cleco pins are excellent. VISE-GRIP® C-clamps of various sizes are the welder’s best friend, you can never have too many clamps. NAPA offers just about every clamp you could ever need for any welding project.
Copper Bars – Steel MIG wire doesn’t stick to copper, which makes copper the perfect material for filling holes. With a piece of copper on the backside of a hole, the filler material sticks to the steel but not the copper, leaving a clean backside. There are copper spoons, plates, even magnetic copper bars. Every welding cart should have a few pieces of copper.
Magnets – Sometimes you can’t get a clamp where you need it. That is where the magnets come in, for ferrous metals at least. Magnets are good for holding two parts together while you are welding them. The typical triangle welder’s magnets in various sizes provide the help you need. Small magnets are especially handy in sheet metal rust repair, such as holding a small piece of metal inside a hole.
Weld-thru Coating – Many welding operation leave a section of the weld hidden. Fresh welds are more susceptible to rust and oxidation than just plain steel, they need to be protected. Weld-thru coatings contain high levels of zinc, which when heated bonds with the metal and leaves a protective film that keeps the fresh weld from rusting. Sheet metal repairs should always be made with weld-thru coating on the seams before the welds are made.
How To Fill Trim Holes With A Welder
The process of welding to fill trim holes is done with stitch welding. A long bead does two things on thin metal- puts a lot of heat into the metal, and tends to burn through the metal. Both of these create more work for you. Stitch welding alleviates those problems by allowing the metal to cool between welds. Stitching is basically a series of small spot welds along a seam. To start, the panel is positioned in place and spot welded about every 3-4 inches. Once the entire panel has been tacked in place, you return to the first weld and repeat with a series of spot welds (using the C pattern, connecting each weld ), no more than 1/2-inch long, then you move to the next section, this is done till the entire panel is fully welded. You can aid cooling by blowing compressed air over the fresh welds. A properly done stitch weld will look like a stack of dimes laid over, much like a TIG weld. For trim holes, the process is the same, but if the hole is small enough, you can use this same method without a filler piece. Anything larger than 1/4” really needs some sort of backup filler material.
Filling larger holes, like this door handle hole, requires a filler piece. We cut a round disc from some scrap metal (see, it was worth keeping!) to match the size of the hole. A magnet was used to hold the filler patch in place.
Then the hole was welded using the stitch method. You need to give a little extra time for the weld to cool before moving on to the next one as the weld area is so small.
Smaller holes, like these trim holes on a 63 Buick LeSabre, can be filled without filler pieces. Start by filling one side with a tack, then move to the other side. Create a bridge between the two, and then fill in the rest of the hole.
Once the welding is done, you need to dress the welds. A few light taps with a body hammer will knock the high spots down a bit, allowing you to leave more weld on the metal and reduce the potential for weakening the weld.
Use a grinder with a flap-wheel to knock down the weld. You want it to be flush with the surrounding metal. A flap wheel disc will cut down on the heat put into the metal and leave a smoother finish for the body work.
Mix up a little body filler (click here for our story on how to use Bondo correctly), and wipe it on the clean metal. You must clean the metal before wiping the filler. Paint thinner works well for this.
You can then finish out the filler with your method of choice, and you are all done.
Welding to fill trim holes is not very difficult, but it a good idea to practice on a piece of scrap metal that is the same gauge as the body panel you are working with. This will help you get your welder setup correct and give you some opportunity to figure out how the metal is going to react. Have fun pluggin’ holes!
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Jefferson Bryant View All
A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.
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