Plastic Repairs (Gas Tank Example)





Introduction: Plastic Repairs (Gas Tank Example)

About: Clinical Engineer. PhD, MBA, CET, BMET, MCSE Works with electronic, mechanical, medical, and automotive stuff. Systems Design, Repair, Modification, Repair.

My lawnmower had an issue where my gas tank was rubbed enough to produce a hole on the side. Faced with the aspect of replacing it for $200+, I decided to repair it instead for a few pennies.

I do repairs on plastic items for medial devices and other things and have done so for many years. I was told that this might be a good idea to put out there as this same process can be used for anything that needs to be repaired that is plastic like this. I did the same thing for a washer fluid bottle on my old Fiero as well, so I am putting it out there so you too can follow and fix all your stuff as well.

Repairing plastic is actually very easy to do, but requires patience and attention to detail. Being careful with the materials is key to the process as they must be clean and organized. Focus on the task must be done because time is an issue in fixing plastic stuff like this.

This repair is for the slimy plastic that can stand up to gasoline like a gas tank or stuff on cars for example.. The process is the same for all things like this, so this example can be applied to nearly any situation to fix plastic damages.

Check it out and see if you can do it..

Step 1: Preparation and Organization

Time is of essence for repairing plastic items because heating things and cooling them is a timed process and you need to be able to move and act immediately. Organizing your tools and materials is important to facilitate this as it is in many tasks you do everyday.

Repairing plastics does not require many tools at all for basic tasks, so here is a list of tools you should or could use:

  1. Heat Gun - You can find these at nearly any store such as Harbor Freight or Lowes. It can not be a hair dryer because those do not get hot enough. They are cheap and should be from $9 to $30 or so..
  2. Box knife - This is used to clean the damaged area and remove any straggling pieces of plastic and clean edges to give you a smooth surface to connect to.
  3. Painter Spatula (Metal) - This is used to flatten the materials and squeeze the materials together. It should be just larger than the area you are working on so you only need to make one pass. Do not use a plastic one because it will get hot and might melt.
  4. Screw Driver (Flathead side) - This is optional in my opinion as I use it as a miniature spatula to push little areas of plastic around as well as applying pressure to specific spots as needed.

That is about all you need in terms of tools, but you need a place to work and power to drive the heat gun. Bring your skills and a little courage and you will be good to go!

***DO NOT use a soldering iron or anything that is hot where you must touch it to the plastic. No no no.

Step 2: Clean Clean Clean Clean... Did I Mention Clean the Parts.

You want your plastic parts to stick to each other, so obviously you do not want any dirt, grease, wax, etc to come between your parts, right? In my case here, I have a broken gasoline tank so I removed it and cleaned it inside and out with dish soap and water until I did not smell gasoline any more on it. This is not only to keep foreign material off the plastic that I am trying to fix, but for safety as well because I do not want my gasoline tank to explode when I apply heat to it. LOL... It might be a good idea to use a rag, scrub brush, brillo pad, etc.

Once it is all cleaned off you nee to really go after it with a solvent like alcohol to absolutely get everything off squeaky clean and ready to process for application of plastic. I am using alcohol because I am going to use the bottle as my patch material since it is the same material as my gasoline tank (how convenient!). I could have used a gallon milk jug as well, but I already have this material wrapped around my alcohol already... LOL.

Note that in the pictures the gasoline tank looks brown. This is a chemical change in the plastic and not a stain or contamination. Many times plastic will change its chemical structure because of heat, chemicals, contact with other items, etc.. once you get to the point where you are finally cleaning with alcohol, it is clean. Take your time and make sure it is really clean. I can not emphasize this enough to clean it off really well.. both the part being fixed and the new part to fix it with. They need to go together with nothing between them.

Step 3: Prepare Work Area and Patch Materials

Now that your work area is cleaned off really well, you must prepare it by removing any plastic fingers or rough spots around the damaged area. Use your knife to cut any loose plastic strings and smooth out any rough edges so that they will not promote splitting or cracking. Take your time and do a good job thinking about stability and taking into account any stress that could occur to your piece. In my case, it is a hole on the corner of my gasoline tank, so I realize the patch will have to span around two sides of the hole.

Step 4: Make It Fit the Area and Shape

Look at your repair area closely and observe its lines and then cut your patch material to match the area as closely as possible. take your time and trim it out so that the material overlaps the repair site by about 1.5 times the damage area. In my case, the patch material will take the place of the missing material of the hole, so I will stretch it a little longer over the area of the repair, and must take this into account when making the cuts of the material to fit. Another idea is to make a template on paper and cut with scissors to use to cut the plastic out from your repair material. I just cut straight from the material though.. (Note that I look for curves in the surface that closely match the curves of my repair site on the gasoline tank).

In the picture, I already have the patch applied and welded in place, but I made another one that you can see where I matched it to fit the repair area above it..The other picture is me holding the patch material in place to test fit it so I know what kind of action I will have to take to make it stick down.

It is super important that you get everything ready to apply and prepared before heating anything: Preparation is 99% of the process, so the better the patch fits and the least physical manipulation is needed, the better the repair is going to come out in the end. Ideally, the patch should simply lay down on the repair area with little or no effort on your part. when you feel confident in the fit of the patch and status of your repair item location, you are ready to heat things up.

Step 5: Heating Things Up!

Now for the key to getting it done the right way.

Start up by heating the repair area with the heat gun and holding the gun a distance from the surface so that it heats evenly in a broad area, and slowly. There are a couple of things to watch for while you are heating this area:

  • Nothing should produce smoke or singe or burn... never.
  • The goal aimed for is to cause the plastic to become pliable but not melt or lose shape.

keep the heat applied in a broad area and not focus on a spot but keep it away from the material and let it heat up slowly and evenly over a larger area. When you notice the plastic being pliable and beginning to look "wet", move the patch material into the air stream so that heat now is being applied to the patch area to be applied to the repair area while it is being diverted to the patch area, deflecting off the patch material. Make the patch warm broadly and slowly until it seems "wet" as well. Once you reach wetness on both surfaces, drop the patch on the repair area, aiming to be where you plan on it staying. Pull your heat gun away from the patched area now slowly and set aside. Pickup the spatula and lightly apply on the patch in an effort to remove any air bubble that might have been captured and slowly knead the patch into the wet repair area surface.. Do not apply pressure here, but try to help the two surfaces mingle. Cleanup the joints and materials while the plastics are cooling (this will take some time to cool off, so do not try to force them to cool by putting water or other stuff on the new patch... Let it sit and cool as this will allow the two materials to coalesce together slowly... Everything about patching is slow temp up and slow temp down...

Step 6: Cooling Off and Inspect Your Work

With luck, your patch is complete. If the patch material is too thin, it might be a good idea to lay a second or third layer down on top of the last one to build up the thickness using the same process as the last slide. The thinner the patch material will make the repair more controllable. so do not be afraid and be ready to lay down multiple layers as needed for this. Note that this is a molecular bond of the patch and the repair area, so once the patch is made, it is there permanently and is part of the repair area, not glued or any kind of physical bond that will go bad.

These two pictures of of my repair to my Gasoline tank hole as it is cooling down. I only needed one layer I felt so I stopped right there. It has been in use holding my gasoline for nearly three years now, so I am happy with it.

Step 7: Testing Afterward and Do Your Victory Dance..

When my gasoline tank cooled off and sat for a while, I tested it by filling it with water to check for leaks, followed by pressurizing it to test for fatigue issues. The patch was solid and permanent. the patch was actually stronger than the original material.

Remember, this is a molecular repair and not a glue or physical bonding situation, so you should never have to visit this repair again (unless you bust it up again).

Keep these things in mind:

  • NEVER EVER physically touch hot tools to your plastic becaue this contaminates the plastic and simply melts it in a tiny area and ruins the plasticity of the materials... If you do this, you had better just toss it in the trash can.
  • Clean everything before applying any heat. Clean clean clean clean clean...
  • Preparation work is essential to effective patch repairs. 99% of work should be prep work and cleaning.
  • If you see smoke, you are doing it wrong. Heat the materials slowly and do not melt them!
  • Patch material needs to be same as the repair area materials.
  • Every kind of plastic melts at different temperature and at different rates.. Test a piece before really doing it.

Hope that helps someone out!




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    52 Discussions


    1 year ago

    Thanks for a helpful demonstration and explanation. However, one important step seemed unclear. How do you hold the patch material while it's being heated (and then move it into the repair location) without burning your fingers off? You advise against using tools or gloves, but what else can you use?

    6 replies

    might hold it with some pliars or something while it is heating up. You just need to be able to "drop" it on the spot where you want it to be permanently. You can use anything you think feasible to your task to hold the patch while it is heating up. I apologize.. I left out that point because I thought it would be self-evident. I probably should have said something to that point though. I do not wear gloves because I like to have more control and can "feel" where I am dropping the patch.. You can wear gloves if you wish though. I think someone else brought up that point as well. It would be a good idea for safety sake though in my opinion.

    Thanks for the reply. If you don't mind a follow-up: For this Instructable, how did *you* hold the material as you heated it and how did you hold it as you transferred it to the patch area?

    I used a pair of needle noses pliars pointing down dangling the patch piece with the tank in its back so the hole was pointing up and at an angle so the corner broken was up. This allowed me to "drop" the patch shaped like the corner in place. Since the corner was pointing up, the patch simply dropped vertically into place so all I had to do was smooth it and massage it so the two plastics would melt into one. Gotta be careful not to "melt" them fully though or get too hot or plastic will thin out.

    Thanks for the further clarification.

    In the meantime, I stumbled across something in a fabric store that could also work. As long as you cleaned any manufacturing residue off of these, I would think they wouldn't interfere with the bonding process. (Fellow readers, feel free to correct me if I'm wrong.)


    That would probably work good.. interesting..

    How or what you use to hold the patch with is not very important, and I could have used an array of items actually.. the needle nose pliars was there at that time though. Also, the patch was a little hotter than the tank corner, with the intent on keeping the rigidity of the corner intact so the patch was more mailable than the base material.. that might be something useful to know...

    ABS isn't is never used for gas tanks. It would always be a Poly (PTFE 'Teflon' or PET-G less common chemically inert plastic because ABS doesn't do well with long term UV exposure and dries out cracking and breaking down over time) based plastic not styrene.

    We make petrol tanks out of PE i havent heard of PTFE or PET-G can you please tell me what they are?

    If the tank is ABS, you can use acetone and bits of ABS to make a slurry of liquid ABS in an airtight container. Then you can simply lay a piece of ABS over the hole and use the slurry to "glue" the patch in place. The slurry will dry as the acetone evaporates to form solid ABS that is as strong as the original ABS. I have done this to make boxes of ABS made by cutting and heating ABS sheets to form the box, then "gluing" the corners together.

    Great 'ible. The biggest problem I have is matching the new material to the old one. I wish where was a easy and quick way to identify plastic types. (I know about the codes on the bottoms, but older plastic items may not have them. Maybe someone can do an 'ible on what plastics will work with other plastics.

    Thanks again!

    8 replies

    There are ways to find out what plastic is what. You can get plastic welding rods from welding places that sell plastic weld guns or bunnings ebay etc. You need to take the top layer of plastic off of what you are welding aswell as the rod you can use scaper or die grunder with deburring tool. You then heat both rod and surface as you start laying a line with the rod weld a couple of cm and let cool. If it pulls off uts not compatable if it doesnt then ut is. I think gas tanks are usually made from PE polyethylene. And i would be careful what plastics you are welding as some are extremely toxic. Nylon for example.

    Yeah.. I did not really get into the identifiers of the kinds of plastics. I started to, but thought it would cast confusion for most people. Sorry if I confused you with that. I was trying to use terms and examples that would not be too technical..

    Information is never too much it's good to point it out but overall this was a clever idea to repair something by using a method that assuming the plastic you used as the patch is resistant as I believe it's probably made from PT and should be safe and chemically inert enough to hold gas after all the isopropyl alcohol is probably more abrasive we work with 99% pure polypropylene and that stuff will melt through paint, the bottles that it's in even at that purity are no different so hopefully your thermal layered patch lets call it should last you a few good years of service.

    patch has been there almost 4 years now with no issues. Thanks!

    emphatically no! This repair scenario is used to repair thermoplastic material. The material can be remelted. Your fender is most likely a thermal (chemical) set material. It is created in a process quite similar to the well known epoxy process. Here, two components are mixed together and a chemical "curing" process occurs. This procedure is used where operating temperatures are "seen" that would melt a thermoplastic material. "Bakelite" is an excellent example.

    As someone who works with plastics for a living a 3d print shop I can tell you (as we have done it) with nothing more then heat print a gas tank, the key is the type of plastic used such as PET-G is a chemically resistant high temp plastic that will withstand gasoline and while technically requires around 240c to melt is still just heat no curing. This looks like a two part thermal formed gas tank probably made of PTFE (and can be bought in a sheet and then melted together hell it can even be printed if it was spooled if you had a printer that could hit the 320+C or just put that heat gun on it for a while it would soften around half that). But you are right that a curing 2 part plastic resin might be a better idea.

    You are correct and accurate for sure. I was just trying to put a way there someone can do this at home with very minimal knowledge or equipment needs. I did not get into the kinds of plastic or anything like melting points. Its a gas tank for an old lawn mower, so I just did it to be done... nothing really professional there. I just hope it can incite someone to do something new to them.

    It could save a lot of time if we can identify the original material...