The Detroit Post
Saturday, 16 October, 2021

How To Fix Rust Holes On A Car With Fiberglass

Paul Gonzalez
• Thursday, 05 November, 2020
• 7 min read

Video tutorial on how to repair rust holes on your vehicle’s body panels using fiberglass. Fiberglass is strong, so it won’t fall out like a body filler even when using the reinforcement mesh.



The two main concerns with fiberglass are that it doesn’t expand or contract like metal, so a repair may be visible. I have used this method in the past for low budget daily drivers without any issues and it held up great despite living in an area where vehicles are exposed to harsh winters.

Luckily the lip is still salvageable so it’ll be easier to maintain the shape and body lines. When removing material, it’s also important to wear a mask so exposing yourself to any contaminants is minimal.

Getting the panel down to bare steel can give up a base point of what areas need to be cut out and where the rust ends. This same product I also used to clean up the rear differential on my Toyota Tacoma and will be posting a video on that in the future.

Application processes will vary, with this product I dumped the desired amount into a plastic container and then applied it to the surface. Keep the surface wet, the slower it dries the longer it can attack the rust.

Once it dries, the next day I went over the area again with 80 grit discs and abrasive pads to ensure all the paint is removed and bare metal is exposed. A rough surface allows that fiberglass to have a strong bond to the bare metal.

rust holes fix step wasnt visible

If paint or primer is in between the fiberglass and metal, this may give an area for the bond to fail. I’m also wearing gloves that helps prevent the oil from my skin contaminating the surface, this can also cause adhesion issues.

The fiberglass cloth will go behind and over top of the hole, this will help keep the repair level with the original body shape. For this project, I am using a polyester laminating resin which will remain tacky when it dries and can easily have additional layers applied if desired.

The ratio between hardener and resin can vary between products and also your weather conditions. This cloth must be fully coated with resin, otherwise, it may not stick on the surface or its structure will be jeopardized.

The exposed metal edge basically needs to be sandwiched between the fiberglass cloth layers. The work time of the resin will depend on how much hardener is mixed in, along with weather conditions.

Dry times also vary, this should be ready to sand in a few hours, but I’ll leave this for the next day. If you go past the cloth with the resin, that isn’t an issue as it can be sanded afterward.

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This should be done with 80 grit sandpaper and I’m using a block of wood, this was the same setup that was I used in my cab corner replacement video. You’ll need to rough in the area, a filler will take care of the fine work after.

If you do find any thin areas or holes in the fiberglass, more cloth and resin can be applied. The filler should be mixed on something which doesn’t soak in the resins, I’m using wax paper over a cardboard backer.

Being that the panel isn’t one color, this can make spotting those angles tough as well. On the curved edge, use a flexible rubber backing pad to help achieve a smooth contour.

Clean the area using a wax and grease remover, then allow it to evaporate before applying more filler. A fast and efficient way of cleaning up the applicator is by scooping up the remaining filler from your mixing board.

Apply more filler using the same process as previously, mainly focusing on those low areas or any flaws that are on the surface. There isn’t a limit to the number of coats you can apply and I usually try to keep the thickness around 1/8” or 3 mm in total.

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For masking, I’m using packaging paper which is cheap and will soak up any paint so there is no risk of it flaking off onto the work surface. Place the paper over the area which will be painted, apply the tape, and then fold it back over.

Give the area another wipe with the wax and grease remover, then apply the filler primer. I do have a couple of holes in the inner which won’t be getting patched but it does expose the backside of the repair and it may allow for moisture penetration.

First using the die grinder with a wire wheel, clean up any dirt or rust on the backside. An etch primer is needed on bare metal as it increases the bond for additional products and can help reduce future rusting to some extent.

Etch primer can sometimes cause a chemical reaction depending on what products are used. Being that I left it for the next day, the primed surface should be lightly scuffed with a 400 grit abrasive pad by hand.

If you apply the undercoating 20 minute after your final coat, there is no need for scuffing the surface. I know this is fairly close and can see the high and low spots, therefore I won’t bother with a guide coat.

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Considering fiberglass filler is thicker and has a higher chance of causing pinholes, these will need to be filled. This can also be used to fill any mildly low areas or other light surface imperfections.

This is a thinner product that allows you to achieve a smoother finish and it can be feathered in much easier. Again you’ll have a limited work time window and this will depend on the weather, along with how much hardener was mixed in.

Using 220 grit sandpaper, block the surface using the same process before, working across the panel avoiding any waves. A guide coat can even be used if you wish to help highlight those low spots.

You can go over with 320 grit sandpaper over the main area and 400 at the blend point where the transition will be between the primer and original paint. Once dry, to smoothen out the transition between the primer and paint or remove any orange peel to level out the overall surface, block the area with 600 grit sandpaper.

When sanding, make sure not filler or metal shows through, otherwise, this spot will need to be touched up. If you don’t fully cover those 1000 grit sanding marks with paint, that isn’t an issue as they’re fine enough they can be polished out.

filler step sanding paint

You’ll need to paint at least a couple of inches past the repair area where the primer’s edge finishes. Tape the area off just like previously, laying the paper over the repaired area and then fold it back, so we have a softer tape edge which makes it easier to blend after, making the repair nearly invisible.

It’s a cooler day, so I warmed up the paint cans in a bucket of hot water. If you have part cans, I would recommend using those first as they don’t always spray as nicely due to the lower pressure, or they make only work at certain angles.

For paint, this is a single-stage acrylic enamel which I had mixed at a local auto parts supplier. Ensure that can of paint is thoroughly mixed and apply a light coat to the surface first.

Typically, I like to get the harder or awkward areas first, then finish up a coat on the larger more noticeable sections. A two-stage paint requires a base coat applied first, then a clear coat is applied to provide a layer of protection and that final gloss finish, but it can be more expensive and slightly more work-intensive.

Around those taped areas, allow the paint to taper off with over spray there was we don’t want a hard each which would be harder to blend after. I’m not making a huge deal about the over spray on the rest of the truck as did get a full paint correction and I have already released a video on this, so be sure to check it out.

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The final paint layer should be a fully wet finish, while runs can be sanding out, try to avoid them. Ensure the area is clean and free of any contaminants that can cause damage during the polishing process.

Starting with 1000 grit sandpaper, this will remove a majority of orange peel and over spray. As you can see the backing pad isn’t flexible enough to bend with the contours, so it’s leaving marks in certain areas.

With 1000 grit sandpaper, the surface should be fairly smooth and only light orange peel should be left behind. Finish up with 2000 grit sandpaper, you can, of course, go higher but I find it isn’t needed if you’re willing to spend a couple of extra minutes with a polishing compound.

Using a soft cloth, apply the polish compound, and then work the product into the surface. I did apply a light mist of water to make the polish a little easier to work with.

There is a variety of products on the market, workability, and quality of finishes can vary. After a couple of minutes, you can see the shine has been restored and the transition line is almost invisible.

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Being that this is a blend, it will show imperfections much easier than compared to other colors, so it can be a bit harder to work with at times. Wax should only be applied once the paint has fully cured, I would wait about a month to be safe.

When polishes correctly, this surface will remain shiny just like the rest of the vehicle’s paint. If you want a proper repair, especially if you’re restoring a vehicle, cutting out the rust and welding in a patch or replacing the whole panel at the factory body seams is the best route.

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