Welcome to the Weekend Workshop, a place where you can save some cash by getting your hands dirty
These tech guides aim to arm you with the necessary info and knowledge to get out there and give it a go yourself, with no professionals needed, and at a price that won’t break the bank.
This time we delve into rust repairs, something any lover of vintage tin will be well aware of when trying to restore a shell from the ground up, or simply pass that next dreaded WOF. We spent the afternoon at Auckland-based GT Refinishers, and followed owner Grant as he rebuilt the lower rear quarter on this MkI VW Golf the team is currently bringing back from the dead.
While this job is somewhat specific to the Golf, the tips and skills shown are transferable to any rust repair you might have.
All too often people ignore rust bubbles that appear in their paint, but the nature of the chemical reaction taking place means rust never sleeps. The longer you leave it the worse it gets, slowly eating away at your panels until there is nothing left. So get out into the shed armed with this magazine, and give it a go.
Tools of the trade
- Grinder with 1mm cutting discs and 80-grit sanding disc
- Mig welder and associated safety equipment
- Tin snips
- Panel hammer
- Spot weld drill bit
- Small amount of .8 -1mm electro galv panel steel
- Pre-formed patch panel
- Copper zinc primer
- 2K epoxy primer
- Hearing, eye and hand protection
This single patch took Grant, a seasoned pro, between four and five hours to complete, so expect a first-timer to lose the best part of a day completing the job.
The biggest expense for this job was the patch panel imported from the UK. You can pick up these ones for around $30. Patch panels are very common for most makes and models, and a little searching on the internet will usually find what you’re after.
First of all you want to take a close look at how bad the affected area is. If you’re lucky, there will be an off-the-shelf patch panel available for minimal cost. It is very likely the spots where your car is rusting are common for that model, and patch panels will be available either locally or on the internet. Get on Google and see what you can find.
Grant found the panel for this Golf in the UK, at around $30. It’s extremely cheap compared to what it would cost in labour to recreate it. If possible use new old stock (NOS) OEM ones, as they are more likely to fit better. If you’re dealing with something a little rarer, cutting the patch from a donor car could also be an option.
Once you have sourced a patch panel, place it against the car and mark some guidelines onto the body. The affected area in question is made up of a total of four pieces, the lower rear panel, rear quarter panel and two inner guard pieces. The first thing to do is drill out all the spot welds. Grant was lucky with the Golf as you could see the welds clearly through the paint, if you can’t, run a wire wheel along the seams and it should reveal where to drill.
The drill used is a special spot weld drill available from any autobody or specialist tool outlet. These types of drills have a shallow pitch, so you only drill through the single skin. If you don’t have one, use around a 10mm standard bit, but take care not to drill into both pieces.
Here you can see the rear panel has been unstitched and the affected area cut out. This piece will be repaired and refitted as it’s not too badly affected, and a patch panel was not available.
When cutting out the area where your new patch panel will sit, ensure to leave yourself 10mm grace with the first cut. Once this piece is removed, you can fit up the patch and make your final cut to ensure a tight fit, something that will make your life 100 times easier come welding time.
With the outer skin removed you can now see the extent of the damage to the inner skins. These will be the first pieces to replace. They’re simple in shape, Grant reproduced his own pieces using some .8 or 1.0mm electro galv sheet metal. Leave the rusted steel in place until you have the replacement formed, this will give you a guideline to work to.
An invaluable tool in Grant’s arsenal is a shrinker/stretcher. You can pick up a new one for under $300, and it can save you plenty of time when fabricating patches. This tool is ideal when building right-angle pieces with radii, e.g. windscreen gutters or the the lip of this patch panel. Alternatively you could cut out two pieces of sheet metal in a radius shape and weld them together at right angles. If you have a lot of panels to do, spend the cash and save hours of work.
Once you have both inner panels fabricated, hold them in place and then cut the rusted steel away. Now work at getting both panels fitting nicely. In the case of this panel there were two spot welds that join the two pieces, pre drill these holes on the piece that will sit on top. You can then plug weld them later on. With the pieces fitting, ensure all welding contact places are cleaned to bare steel on both sides. Don’t worry about the electro galv, as you can weld directly on this. Make a few tacks to hold each piece in place.
With the inner pieces tacked, take the opportunity to double-check the outer patch panels still fit. If you’re happy with the fitment you can begin fully welding. You will want your welder on a low amperage setting, and be careful not to have your wire feed up too high, as it will add unnecessary material to your weld. Make sure to do a few practice/set-up welds on some scrap before hitting the real job.
You will want to see the weld penetrate through both pieces of steel and have a very flat bead. Take the time to set the welder up correctly, as it will save time further down the track and increase the quality of your finished product. With the small welds on the inner skins you can fully weld the piece in one go without fear of warping, but it’s a different story on bigger outer skin welds.
With the welds completed, use an 80-grit belt sander, 80-grit flap disc, or a cutting disc on its edge to take back the welds until they’re flush. This isn’t necessary as the panel is hidden, but it’s nice to give the work a professional finish.
Before attaching the outer skins all bare steel must be treated with copper zinc primer. This primer is able to be welded through, but will seal off the bare steel to give a lasting repair. Failure to properly seal these under-skins will see your old mate rust back within days of the repair, and a few years down the track you will be repeating this repair, only this time to a bigger area.
Clamp the first of the two outer panels in places and cut the remainder of the steel away from the body to give that tight fit. Here you can see where the factory spot welds joined the two skins, again pre-drill these before attaching the panel. Once you’re happy with the fitment, go ahead and make two tacks. Let the panel cool completely and plug weld your pre-drilled holes, allowing time for the panel to cool in between welds.
When it comes to the butt weld, this is the crucial part to a good finish. Rush it and you will have a substandard repair. Don’t attempt to weld in a single pass, instead build up spot welds along the panel. It’s important you don’t overheat the panel, as it will warp and ruin the repair. Due to the thin nature of the material the heat will spread very fast. If you can get to both sides of the weld use a block of alloy or similar metal, pressing it against the backside of your weld to absorb the heat. This was not an option with this job. Take your time by doing a few welds and going away and continuing with something else while it cools. This weld was completed over the course of an hour.
With both outer patch panels fully welded, go ahead and sand back the butt weld again using some 80 grit on either a sanding disc or belt sander. You don’t want to get too much heat into the panel so take your time, just like with the welding. Before you go skim bogging the repair, you will want to seal the bare steel with 2K epoxy, a couple of coats will ensure good coverage. You can bog directly to this. If you have taken your time to get the patch fitting right, and have not overheated the panel when welding or sanding, you should require very little filler before prime and paint. You can see Grant’s repair above — making it look easy.