Plastic Repair Guidelines

by Mark Roberts


This article was written by Mark Roberts and posted on the VFR List. He has graciously allowed me to post it here. I think it is an excellent article and offers some guidance about plastic repair in general.
For all those who asked about my plastic repair procedure, here it is for your review.

Getting Started

First of all you need a liquid, solvent-type glue for plastic. These kind of glues are almost like chemical welding which they dissolve or liquefy the plastic wherever applied. When you put some on two (2) pieces of plastic and press them together, the liquefied plastic on the surface of both pieces mixes and when the solvent evaporates and everything hardens up and it is all welded together into one piece. It is not quite as strong as the original but I'll address that later.

You can buy the right kind of glue at any well-stocked hobby shop. It usually comes in 1 or 2 ounce bottles. Just make sure the stuff you get lists ABS as one of the plastics for which it is suitable. You will also need some disposable ABS plastic scrap. If you do not have any old broken fairing parts lying around (and if not, what kind of biker are you anyway!), try asking a local dealer if they have some. If they try to charge you for broken fairing fragments leave immediately and take your business elsewhere, like around back to their dumpster!

Now to Begin the Work

Clean the piece(s) to be repaired thoroughly. All the liquid plastic glues I've used have an applicator brush built into the bottle cap. Run the brush along the edge of the crack(s) and capillary action will draw the liquid in all the way along the break. Hold the pieces together until set (usually 30-60 seconds). In the case of a really large break, you may want to temporarily stitch the parts together with safety wire before gluing.

If you have (A) really large cracks, (B) significant holes to fill or (C) a repair in a highly-stressed area, you'll want to reinforce the repair with fiberglass. This is so inexpensive and easy I do it with every repair I make just to be safe. Use a fiberglass repair kit for car body work.

Now back to the front side of the part you're fixing. File or sand any rough areas or raised sections if you didn't get the pieces lined up perfectly (not too uncommon).

Time to Prepare the Filler Putty

Get that scrap ABS fairing section and just start drilling holes in it. COLLECT THE SHAVINGS: these are what you use to make your putty. I use a 3/4-inch wood boring bit that generates lots of plastic shavings quickly. Collect some of these shavings in a shallow non-plastic container. I use an empty cat food tin. Pour in a little of the plastic glue and you'll soon have a gooey mess that's basically liquid ABS. Use this goo to fill holes, scrapes and gouges. For large holes, build up several thin layers rather than trying to do it all in one go. When it's set, you can file, sand and paint it just like ABS because that's what it is. I have several round and flat files that I use to shape the final surface. Then I prime and wet sand it and get it painted.

This DIY plastic filler putty has several advantages over Bondo and other products of that type. It has the same flexibility properties as the rest of your fairing. It has the same thermal expansion and contraction properties as the rest of your fairing. It has the same paintability characteristics as the rest of your fairing. It adheres more tenaciously to your fairing if you've done everything right it virtually becomes part of the plastic.

Final Notes

I hope this is helpful to you and good luck with your repair work.

Addendum by Brian D. Jones

This addendum was written by Brian and he has graciously allowed me to post it here. His experience was helpful to me and substantially adds to this discussion on plastic repair.

I am sending this to you in regards to an article posted on the yft site in the technical section. While I found the article by Mark Roberts a useful guide, I did some experimenting on my own.

The ABS "glue" sold at Lowes and other home improvement centers is a combination of MEK (methyl ethyl ketone) and ABS resin. The solvent is MEK. I first experimented with a piece of ABS pipe, cutting a 4" length into 3 pieces. I made one butt joint using the ABS compound. I made a 2nd butt joint by holding the two (2) ands together and using a small brush to apply MEK to the joint. Capillary action pulled the MEK into the joint. After an hour or so of cure time, I hit the pipe with a 2# dead blow hammer. The pipe is no longer round, but neither of the butt joints (which are the weakest form of joint) gave way.

Next I started on the broken faring (from a Kawasaki - sorry about that). The fairng was apparently broken when the bike leaned against something in the garage. A 3" x 3" x 3" triangular shaped piece that encompassed one of the screw mounts had broken completely off and there was another large (4" long) crack in the faring below the broken wing. I held both of the areas to be repaired in place with tape and then applied the MEK from the back. The nice thing about pure MEK is that it is very thin, and it capillaries into the joint very well. The repair seems to be as strong as the original plastic. I can bend it back and forth and there is no sign of any flex in the joint itself.

I did go ahead and sand the back side and "painted" the back side with the ABS glue from the home center. This added several layers of ABS plastic to the back of the repair. I also threw in a couple of pieces of plastic across the crack that I cut from the pipe I was experimenting with and painted the ABS glue over them too. This was probably overkill.

Cosmetically, it is not perfect, but it is not too bad either. The crack does try to swell apart a little, but if the pieces fit back together well, the repair looks OK. The nice part is that the repair itself is cheap. A small container of the ABS glue is about $1.00. A quart of MEK is about $3.00. For spare plastic, a 10 ft. length of ABS pipe is less than $3.00. I did get a couple of camel hair brushes - be careful - the MEK will dissolve the glue that holds the bristles in (that is why I wound up getting a couple of them :-) ). I "improved" the mechanical clamp on the 2nd brush with a pair of channel locks and those bristles stayed in place.

I have not tested the repair for durability on the bike, but given the experience with the pipe, I do not expect any problems. In summary, the pure MEK is a great substance to work with (use ventilation), and is the "glue" of choice for ABS plastic. I did try a fiberglass repair kit first, but it pealed of the ABS with no problem at all and the joint was obviously weak. It is a waste of money.
Brian D. Jones
Broomfield, CO

Technical Corner | Main Garage