One problem which most of us have faced from time to time is how to repair cracks and broken lugs on plastic parts. I was faced with this task again recently, on a 400/4 right hand side panel. There was a crack starting at the cutaway near to the seat lock and finishing near to the middle of the panel. Another was about halfway down the rear facing edge, finishing again on the outer panel surface. In addition to this, the front slot, which hangs over the tongue on the frame, was missing altogether. Common sense should really have told me to bin it and buy another, but what about when these become obsolete items, as many others already have?
I have tried, like many of us, the various lotions and potions with wonderful claims for glueing ABS, but I have yet to find one, which does the job for an extended period of time. The main problem seems to be that they rely on mechanical bonding, rather than specific bonding which solvent based adhesives employ. I asked a chemist friend of mine if he could find a solvent for a piece of ABS, which I had given him. He came up with two - Diethyl Ether and Dimethyl Ketone (acetone). I chose acetone on the grounds that I already had some, for cleaning glass-fibre resin from lay up brushes.
I cleaned all of the paint from the panel and went down the cracks with an Abrafile (a fine blade with teeth all the way around, great for cutting out shapes). Next, I chopped some broken parts from an old Gold Wing fairing lower (well, only the best will do!), into very small pieces, and gradually dissolved these in the acetone. It took quite a few hours of revisiting for occasional stirring before I had a nice syrupy concoction. I then brushed neat acetone into the cracks to soften the surfaces to be bonded, this being followed by the dissolved ABS, dripping it from the end of a small screwdriver blade and kind of ‘puddling’ it in with the same implement. If you are patient and repeat this several times, you will be amazed at the final result, but you will need to leave it to dry thoroughly for a week or so, until no further chemical action takes place, otherwise it might react with the painting processes.
When making the lug piece, you must consider how to get the largest surface area of contact for your repair. I did mine by making it rather larger than the damaged part. Any finishing to size can be done when it is all cured and saves messing with small fiddly bits. As a belt and braces exercise, I filled up at the back of all repairs with the ‘glue’ to strengthen the whole thing and left the extra thickness untouched, wherever it would not cause any problems.
From here on it becomes a normal filling, rubbing down and painting exercise, with perhaps a little extra caution in paint thickness when wet. A heavy build up might cause further chemical reaction beneath, depending upon which type of paint you use. One important point about the rubbing down process, is that you cannot hope to get good shape to a surface by holding the abrasive paper in your hand – you will just follow the original contours and leave grooves where your finger ends have been; the best way is to use a block, which in the old days would have been cork. I find that dense foam is a good substitute and can be cut to any shape with a craft knife. Occasionally I resort to applying a film of spray adhesive to the paper and block to hold everything in place. Finger ends are still the best though, for the little fiddly bits in corners and also for blend radii.