How To Fix Rust Spots On A Car

HOW TO REPAIR RUST SPOTS ON A CAR Without a doubt, one of the most commonly misused terms when it comes to rust is “surface rust”. As the owner of a bodyshop specializing in custom and classic cars, I see this far too often. Many people firmly, but wrongly believe that if the rust hasn’t formed a hole yet, it’s surface rust. Probably the easiest way to determine this is to check if the paint is still over the rust. If the rust hasn’t broken through the surface of the paint, you know that rust couldn’t have come from the surface of the metal. It came through from the inside, and is therefore not surface rust. True surface rust is when the paint has been removed, usually by a scratch or chip, and the exposed bare metal begins to rust. It is vital to know and understand the difference before attempting any rust repairs. If you have true surface rust, simply sanding or grinding out all the rust from the area is sufficient to remove it. It will be necessary to also roughen up the surrounding area so a thin layer of filler can be applied to smooth out the repair. The area under the filler needs to be rough (course sandpaper will take care of this), in order for the filler to adhere properly. I keep a section of a body panel at my shop to demonstrate how rust is commonly misunderstood, and therefore misdiagnosed. From the outside, the paint still looks good other than some bubbles that are concentrated in one area. The customer that brought me this particular car told me it just had surface rust. He was convinced that I could simply grind it down and the rust would be gone. Decades of experience dealing with rust provided me with the knowledge that it was more than surface rust, and when I cut out that section of the car’s body, I saved it to show the customer. Although the outside visible damage consisted of bubbles in an area about 2 inches by 6 inches, the inside of the panel was coated in rust, about 6 inches by about 18 inches. In a matter of months, if not less, that rust would have come through if I had repaired it the way the customer believed was sufficient. This is a very common mistake and I have even seen professional body shops misdiagnose surface rust. This is why shops don’t guarantee rust repairs. This is also the reason you see rust on a car that was just painted a couple of months earlier. The person who fixed it likely thought it was just surface rust, simply ground it down (if that), possibly applied some filler over the rust and painted it. A month later they can’t figure out why it’s rusty again. Most shops these days don’t bother to repair rust. They simply replace the entire panel. It’s more cost-effective for the shop. It’s also a good way to ensure that you are indeed removing all the rust. Although the labor is usually less than it would be to repair the rust, you pay more for the replacement panel. Often this puts the job out of many people’s budgets. If you want to save some money, you may have to make the repairs yourself. Do it right though, or it will need to be done again, and possibly again. The lip at the bottom of a car door is one of the most common places for this type of rust, and also one of the most commonly misdiagnosed. The outer skin of a door is wrapped around the inner skin of the door along the edges and bottom. The rust you see and can get at with the grinder is on the outside of the outer skin. You can grind that until the bare metal shines, but you won’t get the rust out. That rust, in almost all cases started inside the door and penetrated through the metal until it got to a point where you could see it. That rust was there long before anyone noticed it. The only way you are going to eliminate that rust is to either replace the entire panel or cut out the rusted section. This rule applies to all rust. You have to get rid of it completely or it will come back before you know it. If the problem area is rust along the bottom of the door, the following steps will guide you through the procedure, and variations on the same basic procedure can be applied to any rust repair. For most vehicles you can buy after market lower outer door skins. These patch panels cover approximately the lower 10 inches or so of the door and wrap around like the factory skin. If they don’t make one for your vehicle, buy a universal fit version of the same panel, and with a little work, you can make it fit. Lay the new panel up against the old rusty door and use it to mark where you have to cut. A metal cut off wheel (also known as a zip cut disc or zip cut wheel) on a grinder is a handy tool for this job. You have to use the new panel to mark where you need to cut though. If you draw a line along the edge of the panel, you will know approximately where the new panel will come to. The line will be a little off because it is over top of the old panel. That old panel will be removed before you weld on the new one. You will want to draw another line about half an inch below that. The lower line is the one you cut on. Safety equipment is very important. Don’t worry if you think you look goofy wearing safety goggles. Try to imagine how you’d look missing an eye. Those goggles are looking pretty stylish right about now huh? Don’t wear loose clothing or anything dangly that may get in the path of the grinder. If you’re lucky, the grinder will simply chop through whatever got in it’s path, but quite often it will just get the item wrapped around the disc and pull you in. This often causes personal injury, like losing a finger. Hold on tight to the grinder and let it do the work. Don’t force it to cut faster, as this will overheat the grinder and the metal. Your grinder won’t last as long and your door will become warped from the heat. This will make a lot more work for you, or for the guy you have to pay to fix your mess. Carefully cut across the line. Using a grinding disc (not the cutting disc), grind through the outer edges of the area that is going to be cut off. This includes the lower sides and bottom of the door. As you grind through, the layers of metal will separate and the rusted part will then fall off. Grind off the rest of the lip and look at all the rust on the inside of the door that you couldn’t see with the outer skin on. That is where the rust originated, and must be cleaned up completely before you continue. Carefully grind that metal until all traces of rust are gone. If you were cutting out a rust spot in the middle of a panel, you would simply cut out (using the cutting disc on the grinder) the entire rusted area and the area surrounding it by a few inches. Then carefully feel the inside of the remaining metal to check for more rust scale. If it’s still rusty, cut a larger section out, because if you don’t, the rust will return. Now that you have the rust removed you are ready to attach the new panel, right? Wrong! You are almost ready, but not quite. That bare metal will rust in no time if you don’t cover it, and that new rust will attack your new panel and all your work will be for nothing, except for the experience you got. You can’t paint that area and then weld or the new paint will catch on fire. There is a product called Weld-thru primer which comes in a spray can. Apply according to the directions, and let dry thoroughly. Once this is dry, your metal is protected and you can safely weld on the new panel. If you have no welding skills, or friends with welding skills and equipment, most auto body supply stores (the place you buy the body panels and paint) will also have a product called 3M panel bond adhesive. If they don’t carry 3M brand, they may have a similar product. This will bond the panel strongly to the door, and no special skills are required. To ensure a proper fit, and to avoid warpage, I recommend dry-fitting the panel before welding or using panel bond adhesive. It might be handy to have a friend help you here, so one of you can hold the panel in place and the other can drill small holes and screw the new panel to the half inch or so overlap on the old panel. A crimping tool is ideal for this situation because it allows the new panel to fit flush over the old panel, but if you are trying to keep the repair cheap, you won’t want to be spending extra money on tools. Just overlap the panels slightly and that can be easily hidden. I like to start the screws in the middle and work to the outsides, thus eliminating warping of the metal and creating a nice tight fit. If you’re welding the panel on from this point, tack it leaving gaps about 6 inches between welds. You can come back and weld between the first welds later. This keeps the metal from getting warped by the heat. A solid weld is not necessary but some people prefer it. Generally a weld every inch or so apart is sufficient. If you want a continuous weld, spread your welds out as previously said and come back and make another tack between each weld until you have achieved the desired result. If you are using the panel bond adhesive instead of welding, unscrew the panel that you have dry-fitted, apply the adhesive as per the directions and screw it back on. I like to allow it to sit that way overnight before removing the screws. This allows the adhesive to fully set up. Unscrew the screws and your panel is attached. Whether you chose the welding method or the adhesive method, you are now at the finishing step. You may have noticed I didn’t include the pop riveting method because that is not a way to achieve a lasting, quality repair. Grind down any welds, or sand smooth the adhesive that oozed out from between the panels (depending on the method you used). Apply a layer of fiberglass filler. I prefer the short strand fiberglass filler. It is considerably stronger than plastic fillers and will last a lot longer. Work out any air bubbles in the filler as you apply it. Air pockets in filler is the number one reason for bubbling later. It’s not the filler’s fault, it’s the way it’s applied. Your repair should be fairly smooth at this point, but you can now apply a thin layer of plastic filler and sand that smooth. Use a long board sander for this to ensure a straight repair. Long board sanders are available in air powered or manual versions. A good coat of high solids filler primer should take care of any other minor imperfections in the repair and once sanded (reapply and re-sand if necessary until smooth) you should be ready for paint. In general rust repair can be done following the steps outlined here, with professional looking results. The most important thing to remember is to be sure to remove 100% of the rust or you are wasting your time and money. If a replacement patch panel is not available, you have to cut your own out of steel, conform it to the shape of the repair and weld that in. The basic repair is the same after that. Now that you know what is involved, are you ready to tackle that rust? 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