Plastics repair with ABS cement

From Ninja250Wiki
Jump to: navigation, search


To state the obvious, the Ninja 250 has a good amount of bodywork. It is ABS plastic, which can be heat formed or heat-welded. Most plastic repair can be done on the cheap and without many exotic tools. So, if you are poor, cheap, or have made a terrible life decision to become a grad student making $600 a year, this article is for you.

This process repairs the plastic but does not address paint or making it look good. This page will just cover the structural repair.

Make sure that you have a well-ventilated area for the noxious fumes to escape from.

Materials and Tools

  • A soldering iron with a medium tip.
Consider this tip useless for electronics work afterwards. The tip will be contaminated with plastic.
  • ABS Cement
Use ABS plumbing cement with low VOCs (volatile organic compounds) . This is used as a sort of "bondo" to fill in cracks and deformations in the bodywork. Unlike bondo, the ABS cement will melt and fuse with the ABS bodywork, making a uniform repair material. Home Depot or anywhere that sells plumbing supplies usually carries something like Oatey Low VOC ABS Cement.
  • Sandpaper and a sanding block
200, 400, 600 grit, and then 1100 grit wet/dry sandpaper.

Total investment: around 20 bucks.

For a little more money, some people like to use a plastic welding kit.


The theory behind the plastic repair is simple. A crack or a break in the ABS bodywork can be bonded back together by welding the plastic together. This can be done chemically or by heat. Heat is used to make the primary bond, and then the surface imperfections are filled in with the ABS cement. This takes place in several steps.

The first figure shows the cross section of a break in the plastic. The top surface has a paint layer and a relatively thick plastic layer.


Start by sanding the paint off, to clear out the surface. This makes repainting REQUIRED. You can skip this step and only weld the break from the back. This makes a nice-looking, but weaker, repair. If the paint gets into the ABS, it contaminates the bond, and you'll have to sand anyway.


A hot soldering iron melts together the two sides. Run the hot iron down the length of the fracture from the rear (unpainted side), with your finger on the painted side, moving gingerly up and down the surface. The idea is to keep the feeler fingertip from getting too hot. This is especially important if you are only welding one side. If your finger can't take the heat, neither can the paint. In this picture, both sides are welded.


A run of 220 grit reduces the surface deformation. This doesn't have to be perfect, just neat enough to fill.


Cleaning off the sanded plastic, the final process is putting ABS cement into the new repair, filling the grooves or any deformation. From here, the plastic can be put through 400, 600, then 1100 grit to polish the plastic to its natural black color (for the cheapskate) or be repainted properly.



This is the front inner fairing. It's the part that isn't too visible unless you look at your bike from the front. There are two spots to repair. First off is the clean break down the middle of the piece. The painted surface has already been sanded.

Inner break.01.jpg

This shows the sanded area; only 2 or 3 mm on each side of the break has been sanded.

Inner break.02.jpg

The backside is welded together. You can mold the plastic from the surrounding area to reinforce the repair a bit. This is the side no one will see. It is important to keep the parts immobilized and in the proper position when welding together. If not, the plastic will be joined warped, and you'll be placing stress on the repair when you reinstall the plastic.

Inner break.03.jpg

The front side, however, is kept much nicer. The center groove is shiny, unlike the surrounding sanded plastic. The shiny part is the welded plastic, as the heat has glazed the groove.

Inner break.04.jpg

Next is a crack on the same piece.

The lower end of this piece shattered and left a fracture running up it. The crack is a good candidate for repair.

Inner crack.01.jpg

With some sanding, the surface is exposed. Thankfully, this isn't a very visible part of the bodywork.

Inner crack.02.jpg

Showing the inside. The crack is held back to its original shape and welded together.

Inner crack.03.jpg

A very messy repair, but the crack is completely bonded together.

Inner crack.04.jpg

A quick sanding shows the crack is all but gone. The marks can be filled with ABS cement, then sanded again to remove all traces of the crack.

Inner crack.05.jpg

Highlighting the original crack just for reference. The tiny fracture in the paint that continues down the left of the picture was ignored. That will disappear in a future repaint.

Inner crack.06.jpg

Other larger repairs can be made using the technique.

This is the upper fairing, on the left side. A crash had run a long, jagged break into this fairing. This was repaired using the above repair process, and the pieces put back together. The cracks are almost invisible, if it wasn't for the paint contamination into the weld. Sometimes you have to sand off a good amount of the paint to fill in the deep scratches in the plastic.


This shows the location of the crack. The orange highlighted area was ground away on asphalt. That's all ABS filler, rebuilding that corner of the bodywork.


This is an image of the backside. It's really not necessary to hide the repairs. The weld marks and the ABS fill are quite visible. However, the extensive repairs made are far less apparent from the other side.

Leftside back.01.jpg

The mirror mount in the upper right was entirely broken off. Here it's welded back into one piece, like some 3D puzzle.

Leftside back.02.jpg

This image shows a rider position view of the fairing. Greatly preferable to looking down and seeing a broken plastic mess.

Leftside top.01.jpg

The lines show the fracture ran along the edge, which was reconstituted using ABS cement. As a result, the rounded edge of that corner is gone. It's now a sharp edge, which can easily go unnoticed. The orange is also significant. That section was gone. Entirely gone. It was rebuilt by making a thin ABS layer, then rebuilding the surface. The rebuilt section was less than perfect, and cracked while back on the bike. This was because the shape it solidified into was not the same as the original shape. After welding the new crack to relieve the stress, it has been doing fine.

Leftside top.02.jpg

Rebuilding plastic parts from molds.

This is a new experiment, and still being worked on. In this case, one of the lower mounting tabs cracked, and needed to be rebuilt. It didn't break off from the main fairing, which is a weld repair. This one needed to have a missing section refilled. PLASTEX, the commercial kit, can do this as well. This is an attempt to do it on the cheap.

Clay is used to take a mold of a good tab. This good tab mold is fit to the broken tab, then filled with ABS cement.

Tab mould.jpg

The tab is rough, and not exactly perfect. However, it is a full plastic repair of the missing section, and a great improvement over what was there.

Tab fixed.01.jpg

The highlights show the originally missing quarter of the tab. It may not be pretty, but no one is going to see this. For a $3 can of ABS cement, the results come close enough to the $25 PLASTEX kit for most people.

Tab fixed.02.jpg

As you can see, plastics repair doesn't take a great deal of money, just a bit of patience.

Addendum and explanation of terms


Before you are scared off by the word chemistry, be assured that it isn't a difficult topic, at least not concerning what we're covering here. ABS, or AcryloNitrileButadineStyrene, plastic is what makes up the Ninja 250 bodywork. What is notable about the plastic is that it is flexible and able to be chemically and thermally formed. What does that mean?

You can melt the bodywork using heat (like a soldering iron) or chemicals. Like salt melting in water, nail polish remover (acetone) and MEK (methyl ethyl ketone) will melt the plastic. This can be used to repair the plastics. This repair guide covers the primary method of using thermal forming, or a hot soldering iron, to weld the plastics together. The chemical bonding method is also worth noting, as reforming missing pieces and filling voids are done using this.

Advantages of chemical bonding

Using ABS cement yields two significant advantages. One is that you are creating a uniform plastic-on-plastic repair. For surface gouging or deep scratches, ABS cement will not only fill those imperfections, but will melt and solidify with the plastic base. When it dries it is real ABS, not just filler material. This will theoretically allow a more uniform paint adhesion, and thus a better paint job. In reality, modern primers and paints are very good, and bond to different surfaces with very little drama. It does, however, make sense to use actual ABS for repairs, instead of bondo.

The second advantage is the lack of delamination. Fiberglass or other repairs may be stronger than simple plastic. This is a problem if you shock or twist the bodywork. The stronger (stiffer) parts won't flex as much, and will delaminate, or peel off. This is especially a problem if there is less than ideal adhesion between the fiberglass epoxy and the plastics. A hybrid approach has been suggested, using ABS cement as an epoxy on a fiberglass backing. It wasn't tried for this article, but it might work. It's certainly worth investigating.

ABS cement

ABS cement is the primary agent used here. It can be used for two main things:

  • Filling in surface imperfections, like bondo (but better).
  • Recreating missing parts, or molding new parts.

The cement is nothing more than actual ABS plastic melted into some acetone. What is important is that you can make your own to modify the thickness and properties of the ABS cement. The ABS plastic inside store-bought cement is brittle, and cracks long before bodywork will. This is no problem for filling in surface imperfections, like deep scratches. It does become a problem when you are trying to rebuild missing sections.

Adjusting the thickness is important for molding new pieces. ABS cement is very much a liquid when first bought. It is easier to form and to use if it is a little bit thicker. This adjustment is decently easy. What is very neat is having some ABS cement that is thick enough to be formed like clay. Plastic in this form can be cut and shaped with a knife and other tools, and will harden to a solid piece of plastic.

1) Thickening The Cement

To thicken the cement, there are two different methods. One is to let the can stay open, and let it evaporate off the acetone. This will make the cement thicker and easier to use. Make sure to do this in an open space with plenty of ventilation. Even the low VOC formulas are quite noxious, and will damage your body. Don't kill yourself to save a few bucks. Seriously. Don't. Brain damage isn't funny. Go outside, get some fresh air. Keep a window open. Wear a breathing mask.

Alternately, get some ABS plastic and add it into the cement to thicken it. A good source is an industrial plastics shop. Ask for scrap pieces, as you are likely to be able to cut a deal. Even better, ask if you can get the milling dust of ABS. If you are lucky, you might be able to get the dust for free. However, it has to be pure ABS. If there are other plastics mixed in, forget it.

Take the piece of plastic and grind it down. This is probably the most productive method. Alternatively, get a drill and drill holes into the plastic. This will yield little bits of the plastic. Using a knife to shave parts or ruining a cheese grater might be fun. Take the plastic bits and mix them into the cement until completely dissolved. This has the added benefit of making the ABS slightly less brittle, assuming you mix in less brittle plastics.

2) Thinning the cement.

The consistency of the plastic can be anywhere from a runny fluid (straight out of the can) all the way to something almost like a thick dough. A half empty can will solidify if left unused. Add some acetone to the plastic to thin out the cement. This can be done to completely solid plastic to make it a fluid again. This also means you have to keep the acetone and stuff away from the bodywork. Spill it, and it shouldn't do much damage right away. It takes a bit of time, and a good amount of solvent before your bodywork does a Salvador Dali impression. Still, don't take the chance.


To learn about ABS, its uses, and how to work with it: