Introduction: How to Fix Rust Holes on a Budget Using Fiberglass - NO WELDING

By 4DIYers
About: Car enthusiast YouTuber creating helpful tutorial videos on both repairs and customizations. Be sure to hit that SUBSCRIBE or FOLLOW button!

Video tutorial on how to repair rust holes on your vehicle’s body panels using fiberglass. Now before I get into it, as a disclaimer, this is a budget repair and when done correctly can last for a very long time. However with that being said, if you are wanting to restore a vehicle properly, then this isn’t the repair for you. This is a repair where it can be done at home, doesn’t cost a load of money or require any specialty tools, and can easily clean up your vehicle. 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. And it may have bonding issues with steel, so both these areas will be covered in this video. 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.

Supplies

  • grinder with cutting disc
  • surface sander with 80 grit discs
  • 80 grit sandpaper
  • 180 grit sandpaper
  • 220 grit sandpaper
  • 320 grit sandpaper
  • 400 grit sandpaper
  • 600 grit sandpaper
  • 1000 grit wet/dry sandpaper
  • 1500 grit wet/dry sandpaper
  • 2000 grit wet/dry sandpaper
  • sandpaper backing pad/sanding block
  • chemical rust treatment
  • wax and grease remover
  • denatured alcohol
  • clean lint free clothes
  • plastic container
  • paint brush
  • stir stick
  • fiberglass matting cloth
  • 2 part spot/glazing/finishing putty
  • short strand fibreglass filler with hardener
  • polyester laminating resin with hardener
  • acrylic enamel single stage color matched paint
  • black undercoating
  • etching primer
  • filler primer
  • safety glasses
  • latex/nitrile gloves
  • respirator
  • water
  • bucket
  • carwash soap
  • polishing compound
  • microfiber clothes
  • painter’s tape
  • paper

Step 1: Cutting Out the Rust

So as you can see we have quite a few soft spots along the fender. Unfortunately with this fender design, the inner fender folds along the inside, creating a prime spot for dirt to get trapped, holding moisture and eventually rotting out the steel. Luckily the lip is still salvageable so it’ll be easier to maintain the shape and body lines.

Removing the paint and rust, this can be done by hand or machine done. You can use anything from a surface sander such as what I have here, media blasting, orbital sander, etc. When removing material, it’s also important to wear a mask so exposing yourself to any contaminants is minimal. An 80 grit sanding diss will clean up the surface quite quickly. While the surface sander can create heat, if you’re careful it can be minimized. We want to avoid heat as it can warp the steel metal. 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. From the rust damage, there should be about 2” to 3” around that area to ensure we have a solid base and didn’t miss any rust which may jeopardize the repair. Try to clean the area up fairly well, you can go over the area again just before the fiberglass is applied.

Using compressed air, blow out any dirt or debris which may be stuck in behind the fender. Next using a die grinder, clean up the rust hold, cutting out some of that thin rusted metal.

Step 2: Treating the Leftover Rust

Once that rusted metal has been cut out, unfortunately, there will most likely be rust in behind the fender and this can be tough to tackle. Using a media blaster would be the best option, however, that can be messy and if you don’t own that equipment, it can be a bit costly to purchase and I’m trying to keep this as a budget repair. A chemical rust converter would also be another option which is what I’m using. There are various products on the market, some do work better than others. 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.

Inside the hole and in behind that fender, use 80 grit sandpaper to scuff the surface. A rough surface allows that fiberglass to have a strong bond to the bare metal. 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.

Using a hammer, lightly tap in the edge of the hole. The fiberglass cloth will go behind and over top of the hole, this will help keep the repair level with the original body shape.

Clean the surface using a wax and grease remover and a clean rag.

Step 3: Applying Fiberglass

Gloves are highly recommended when handling or working with fiberglass. The fibers can imbed in your skin causing irritation. You’ll need to cut up various length and sizes, this matting cloth will need to be placed across the surface in multiple layers. It’s best to have plenty of pieces cut up first as this can be messy if you need additional pieces halfway through patching the holes.

Fiberglass work is messy as the resin will most likely start dripping when it’s applied. Cover up any surrounding areas to help contain the mess. Resin can be tough to clean up when it’s dried on a surface or object. 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.

Using a plastic container, dump a desired amount of resin. If you do run out of resin partway through, that isn’t an issue as you can always mix more. Don’t mix too much as you may go outside of the resin’s work time. Then apply the hardener. The ratio between hardener and resin can vary between products and also your weather conditions. More hardener tends to be needed in cooler weather and less in warmer weather as it cures faster. But if you add too much hardener, the resin produces heat when curing, this can cause too much heat warping the fiberglass. Too little and the resin may not cure. When the hardener is added, the resin will change color which is normal. The resin must be well mixed and try not to cause bubbling either. Also when working with fiberglass, work in a well-ventilated area.

With a cheap disposable brush, apply the resin to the surface. This will help stick the fiberglass cloth on the surface.

Then apply the cloth. First, you’ll need to apply it to the backside. You can pre-soak the cloth, but that can get very messy so I’m taking the slightly cleaner route. This cloth must be fully coated with resin, otherwise, it may not stick on the surface or its structure will be jeopardized. Using the stir stick, I’m able to maneuver the cloth behind the fender to stick it in place. The exposed metal edge basically needs to be sandwiched between the fiberglass cloth layers. The more layers of fiberglass, the stronger the repair will be. If there are high spots, that isn’t an issue as it can be ground down.

Once the area is filled up, using the painter’s tape, layer it over the repaired area. This will help keep everything together, reduce the change of edges pulling away before it cures, keeps any bubbling down, and when it’s cured you’ll have a smoother surface.

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. The hole should be overlapped by at least an inch so there is a good strong bond. If you go past the cloth with the resin, that isn’t an issue as it can be sanded afterward.

Remove the tape and as you can see the repair has hardened.

You can, of course, hand sand the surface, but this can take a bit of time. 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.

Or you can use a surface sander which will cut down those high spots quite fast. You’ll need to rough in the area, a filler will take care of the fine work after. Considering there fender’s shape was gone due to rusting, this will need to be made again. If you do find any thin areas or holes in the fiberglass, more cloth and resin can be applied.

Step 4: Applying Filler & Sanding

Clean the area using a wax and grease remover.

Using a short strand fiberglass filler, this will be used to get that shape back. This is a strong waterproof filler. The mixing ratio for the hardener and filler will be on the can and the filler should be dragged across the surface when mixing so you don’t introduce air bubbles in the filler. The filler should be mixed on something which doesn’t soak in the resins, I’m using wax paper over a cardboard backer.

Then apply the filler to the surface. If you don’t get all the spots on the first try, more filler can be applied. You’ll have a certain amount of work time when applying the filler, once it goes past that window the filer will be hard to work with and won’t have as good of a bond.

Allow that filler to dry and then block the surface using 80 grit sandpaper. Getting the shape back does take some time and if you’re new to bodywork, it can be tough. It takes patience, time, and always looking at the panel from different angles. Being that the panel isn’t one color, this can make spotting those angles tough as well.

Using coarse sandpaper makes roughing in the shape quicker and the sandpaper doesn’t tend to plug up as easily. But when it does plug up, use a wire brush to clean the paper. I would recommend wearing gloves, however, at the moment it’s quite hot outside so it’s a bit more comfortable without them.

On the curved edge, use a flexible rubber backing pad to help achieve a smooth contour. I can already see I do have some low areas which will need to be addressed in a moment.

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. Once it dries, bend the applicator and the filler will break off in large chunks. When the filler is thin, it can flex with the applicator and can be hard to remove.

Apply more filler using the same process as previously, mainly focusing on those low areas or any flaws that are on the surface.

Sand the area again using 80 grit sandpaper. I did end up applying more filler in a couple of missed areas. 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 3mm in total. Applications of the product should be 1/16 of an inch, 1.5mm, or less so the filler can cure efficiently.

Considering it was getting late, I want to finish up the fender and have it somewhat sealed from the elements. Sand the fender down first using 180 and then finish up with 200 grit. There is bare metal, however, there is also filler and paint so an etch primer can be used as this can sometimes cause a chemical reaction. So a filler primer is used instead to help seal up the surface somewhat. Feather in the paint’s edge so we have a smoother transition line.

Clean the area using a wax and grease remover.

Then mask off the surrounding area. 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. This will reduce the chance of a hard tape edge which can be hard to sand afterward. Overspray can be removed, this can be done using a wax and grease remover or clay bar.

Give the area another wipe with the wax and grease remover, then apply the filler primer. First is a light coat, then 2 medium to fully wet coats. You’re looking at about 10-15min in between coats, this can vary depending on your temperature. Remove the paper within 5mins after the final coat.

Step 5: Treating the Inside of the Fender

Now working on the backside, you may need to seal it up depending on your fender design. 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. You’ll most likely need to remove the wheel for this and the vehicle needs to be safety elevated.

Using a wax and grease remover, clean the surface.

Using an etching primer, apply it to the surface. 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. 2 are needed, wait 5-10 minutes in between each of those coats.

Next, I applied a regular grey primer which will be a barrier between the etch primer and undercoating. Etch primer can sometimes cause a chemical reaction depending on what products are used. 2 coats are all that’s needed, again 5-10 minutes in between coats.

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. If you did scuff the surface, all give it a wipe down with denatured alcohol.

Mix the undercoating accordingly and spray it on the surface, starting out with a light coat first. Considering it’s warmer outside, you’ll be looking at about 15min in between each coat. Applying more coating to the surface will leave a smoother surface, while light coats tends to put more texture.

Step 6: Finishing Filler and Sanding

You can use a guide coat on the outside, just like I did on my cab corner video. I know this is fairly close and can see the high and low spots, therefore I won’t bother with a guide coat. Sand the surface using 220 grit sandpaper and a backing pad. The edges between the old paint and repair will need to be feathered, along with any surface imperfections. If you sand down through the primer to bare metal, more primer will need to be applied.

Considering fiberglass filler is thicker and has a higher chance of causing pinholes, these will need to be filled.

A spot putty also referred to as glazing or finishing putty will be used here. This can also be used to fill any mildly low areas or other light surface imperfections. A two-part product is best as it’s a much better quality product than compared to a single part product. This includes both a filler and hardener. Again it’s mixed by dragging it across the surface until it’s a uniform color and the mixing ratio will usually be printed on the side of the product.

Apply it to the surface using a plastic applicator, the same way as the fiberglass filler. 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. The thickness of this filler should be no thicker than 1/16 of an inch or 1.5mm in total.

Dry times will vary again, however, as a general time it should be ready to sand in 30 minutes. 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. If the sandpaper becomes plugged, use the wire brush to clean it.

Considering a filler was used again, this can leave outlines in the paint so a primer is needed. 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.

Step 7: Applying More Primer Before Paint

Tape the area off just like before, the primer needs to be applied over those 220 and 320 sanded areas, otherwise, the sanding marks can show in the final paint finish. The area will need to be cleaned using denatured alcohol to remove any contaminants.

Apply the filler primer. Primer colors can vary depending on the color of your vehicle, I’ll cover that for another video. This black does cover well so the white filler primer isn’t an issue. You’ll be looking at about 3 coats. One light coats and the other 2 will be medium to wet coats. You’ll be looking at 5-10min in between each coat and try to avoid runs as that will need to be sanding out.

Remove the paper within a few minutes of that final coat and then allow it to dry.

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. Typically with thicker paints, 400 grit can be used before that final coat of paint, but with thinner or metallic paints, 600 grit should be used.

Between the new and old paint transition, where your tape edges will be, use 1000 grit here. 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. You’ll need to paint at least a couple of inches past the repair area where the primer’s edge finishes.

Clean the area again with denatured alcohol and then let the product evaporate.

Step 8: Finally Onto Painting

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. This helps the paint flow better on the surface. 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. They were able to mix the color based on the color code I provided.

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. This paint does not need a clear coat as it will dry with a gloss finish. Single-stage doesn’t tend to be as durable, however, it is less work to apply and is cheaper. 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.

Apply the second coat of paint. Considering it is a little cooler today, I’m waiting for about 10 minutes in between coats. Around those taped areas, allow the paint to taper off with overspray 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 overspray 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. Overspray can travel quite a ways, so try to keep your paint away from other areas where this can cause damage.

I am applying 3 coats so I have enough material for wet sanding and polishing. If the paint layer is too thin, then there is a greater chance of sanding through that layer. The final paint layer should be a fully wet finish, while runs can be sanding out, try to avoid them. Wait about 5 minutes after that final coat and remove the paper. If you wait too long, the paint can harden and you may risk the chance of pulling paint off the surface. At a tape edge, while the paint is still fresh it has a chance to smoothen out the edge too.

Step 9: Wet Sanding & Polishing

Wait at least a week before wet sanding so the paint has cured enough. Cure times vary depending on the weather condition, with cooler weather the paint will take longer to cure.

Ensure the area is clean and free of any contaminants that can cause damage during the polishing process. Using a bucket with a water and soap mixture, pre-soak the sandpaper. Starting with 1000 grit sandpaper, this will remove a majority of orange peel and overspray. Apply light to medium pressure, working evenly across the surface. A backer pad is recommended for even pressure bend the sandpaper, this can be some like a flexible rubber backer pad or a sponge.

Here is a close up after a couple of minutes of wet sanding. With the odd contours of the fender, your hand can be used but be extremely careful. As you can see the backing pad isn’t flexible enough to bend with the contours, so it’s leaving marks in certain areas. This is something that needs to be avoided. With 1000 grit sandpaper, the surface should be fairly smooth and only light orange peel should be left behind.

Move up to 1500 grit sandpaper. Rinse the area and sandpaper of sanding debris from time to time and the area always needs to be well lubricated with the water and soap mixture. The 1500 grit will remove the remaining orange peel.

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. You can go on the original paint with the 2000 grit sandpaper, this will remove some light orange peel and can help blend the repair.

As a final stage, getting that paint’s shine back, using a polishing compound. This can be done by hand or a machine polisher. This is fresh paint, so polishing by hand would be the safer route. 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.

Remove the polish and buff the surface using a microfiber cloth.

After a couple of minutes, you can see the shine has been restored and the transition line is almost invisible. I can lightly see an outline, but after a few more minutes of polishing it should blend better.

Finish up with the rest of the fender. 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.

Step 10: All Done!

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. As mentioned earlier, this is a budget repair that will still be durable and last for a very long time when done right. 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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