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How to Fix Rust on a Car

202304 how to repair rust on car scaled jpg How to repair rust on a car | illustration by Paul Dolan

Rust repair is an art that often requires a lot of knowledge, time, equipment, and supplies to pull off, at least if you want it fixed anywhere near as good as new. As such, anything more than a minor repair is probably not something you want to tackle yourself. Going beyond that can quickly require a major expenditure of time and money — and you still might not get it to look right — so it’s probably only worth the investment if you’re planning to do a lot of it. Otherwise, it’s best to leave it to a professional.

A general rule of rust: It only gets worse. Stopping it from spreading further can be a worthwhile goal, as any future repair to a larger area is going to be more difficult and obvious than to a smaller one.

Related: How to Repair a Cracked Windshield

Rust Repair With Minimal Investment — of Both Time and Money

Rust often gets its start from a rock chip or door ding. It can also be caused by snow, water, salt or a combination getting in behind the metal. In the latter case, you may not see any indication of rust until it bubbles the paint or leaves a hole in the metal. While you may chip off the bubbled paint and find fairly solid metal underneath — in which case you can try some of the suggestions below — holes or “squishy” metal indicate a larger job that’s beyond the scope of this article (and possibly your time, skill and budget). About the best you can easily do in this case is apply some rust converter to slow the spread of rust, which is covered below.

But first, it’s important to take some safety precautions when performing any of the following tasks. That includes wearing safety glasses (for spray, dust and chipping rust), gloves (chemicals), a long-sleeve shirt (ditto) and a face mask (for airborne particles from sanding and spraying). A respirator is best when spray painting, and you should always work in a well-ventilated area, particularly when using chemicals and paint.

Sometimes you’ll see rust on top of the paint, sort of like a stain, that may be running down from a rusted area above it. While you can get store-bought rust removers (some of which can be very caustic, so follow the directions), you might first want to try some household items to remove the stain. Recommendations include WD-40, which you can spray on, let sit, then rub off; soda, such as Diet Coke; lemon juice; distilled white vinegar; and baking soda rubbed on with a potato with an end cut off. Many of these may also be useful for more serious rust removal, as well, which we’ll get to in a bit.

If chips or dings have left a small break in the paint, allowing you to see gray primer or underlying metal (or, more likely, rusted metal), it’s worth attacking the rust as soon as possible before it gets worse. A simple bottle of color-matched touch-up paint (which often includes its own little brush) can be useful here. If you see gray primer, just dab on a little paint. If you see bare, rust-free metal — which is unusual, as bare metal can rust quickly — dab on some primer first. If you see rusted metal, use rust converter first. Both can be applied with a small artist’s brush. What you want to do is stop any rust that’s started and seal the chip or crack.

What’s rust converter, you may ask? Rust converter transforms rust into something like primer via a chemical reaction. Not only can you end up stopping the rust from spreading, but you can also end up with a primed surface ready to be painted. Rust converter comes in both liquid and spray-can form. For smaller areas, the liquid version (which is applied with a paint brush that’s usually not included) allows for more control, as the spray may end up on a lot of areas you don’t want it. Pour out as much of the rust converter liquid as you think you’ll need into a separate container, as you shouldn’t double-dip your brush into the bottle nor pour any unused portion back into the can.

Note that there’s a difference between a rust converter, a rust inhibitor and a rust remover. A rust converter uses a chemical reaction to essentially convert rust into primer, which is what’s best in the above scenario. (Paint generally doesn’t stick well to bare metal; it needs to have primer under it, which sticks well to both metal and the paint, thus “joining” the two.) A rust inhibitor, on the other hand, may be incorporated into a primer, but it’s aimed more at stopping future rust than converting existing rust. Rust remover is what it implies, and it just leaves a bare metal surface that won’t hold paint well; it can be very caustic.

Repairing Larger Rust Areas

If the rust area you’re seeing is somewhat bigger — say, the size of a golf ball or tennis ball — and not yet a hole or bubbled paint covering a hole, you can use much of the same procedure. However, you might need to start by chipping off any loose paint or rust with a scraper or razor blade. It’s also important to keep surfaces clean. There are lots of dedicated cleaners for this (an auto parts store may have recommendations, including bug and tar remover, grease and wax remover, and tack cloths), but some recommendations for items you may already have include alcohol, WD-40 or grease-cutting dish soap (rinse and dry well afterward) applied with a microfiber cloth.

If the resulting surface is fairly smooth, you can try removing any rust with the rust removers noted above or with store-bought rust remover. If the surface is left somewhat bumpy, you can try sanding it with sandpaper; something around 80-100 grit, which is fairly rough, should work, though it may end up smoother if you use 180-400 grit either instead or after. (The lower the grit number, the rougher the sandpaper.) Once you’ve gotten the surface to an acceptable level, apply the rust converter.

If there are small imperfections remaining, you might be able to apply some spot putty to smooth them out, then let it dry and sand if necessary. Spot putty comes in a small tube like toothpaste, doesn’t require mixing and is easy to apply. You can also spray on a sandable primer, which is thicker than regular primer and is designed to fill in small scratches and then be lightly sanded; be sure not to spray on too much, though, or it will run.

For repairs of this size, a bottle of touch-up paint may not work as well as a can of color-matched spray paint, which we’ll cover next.

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Getting the painting part right can be trickier than getting the underlaying bodywork right — and you likely won’t be able to get the paint exactly right, especially on cars with metallic paint. Not only does the color of the paint have to match (remember, the original paint might have faded a bit), but so do the size and orientation of the metallic flakes, which you can’t easily control. Even professionals often have to paint the entire side of a car to get the paint to look consistent, and you can often see when they don’t. Non-metallic colors tend to be easier to match, but they’re somewhat rare nowadays.

It’s also important to cover areas you don’t want painted with newspaper or something similar held in place at the edges with painter’s tape, which is like a masking tape that peels off more cleanly. Spray the paint across the area working from bottom to top, extending beyond the bare area, which helps keep the paint from forming runs (globs of paint that run down due to gravity). When painting, multiple light coats of paint are better than heavy coats, as heavy coats of paint are much more likely to form runs. Also, you should wait until the gloss dries off the paint before applying another coat. Also, if the layer you’re painting on top of is rust converter, paint it with an oil-based or epoxy paint, as most manufacturers don’t recommend using water-based or latex paint.

If the painted area ends up looking good — or if there’s an obvious transition from new paint to old — you may want to go the extra mile to make it shinier and blend in better with the surrounding paint. But doing so may require another trip to the parts store. Depending on how smooth the paint is, you might be able to just polish it with a polishing compound. (Don’t use a rubbing compound, as that’s for heavier-duty applications.) If it’s a little rougher, you might want to start with fine, wet sandpaper of 400-1500 grit, which is intended to be used with water to help create a smoother finish. You may also want to polish it afterward; either way, finish it with a clear-coat spray, which comes in a can like spray paint, to give it a nice gloss.

The bottom line here is to try and keep your expectations low. Your primary goal is to stop that rust spot from getting worse, with any improvement in appearance being a bonus. After all, professional body shops are around for a reason.

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