I recently finished building a Honda XR100 with CR80 suspension, XR200 shock, 120cc BBK, Pipe, Carb, custom frame cradle and believe it or not, folks want to know more about the shiny plastics than the rest of the bike.
So I thought a little how to was in order:
Along with the mid 80s CR80 suspension, I also used CR fenders on the humble XR.
A pic of the front fender as found on the donor bike:
And the rear:
Restoring plastics is all about sanding, or how much of it you can tolerate. This bike is a rider, so perfection is not a necessity: but I just can't help myself.
I bought all of my materials at Canadian Tire (similar to Advance or Pep Boys), 3M has 1/3 sheet variety packs with 320, 400, 600 & 800 grit. I also purchased some 1000, 2000 grit; Turtle wax Hand Polish and a 3M plastic Headlight restoration kit. You'll need some rags, a spray bottle, water and dish detergent (1 cap of detergent for every .5L of water.
Practice on a scrap piece if possible.
1. Examine your piece, locate the deepest scratch that you want to eliminate (there will be cases where if you sand down to the lowest scratch the piece will be so weak it will be prone to stress marks when flexed or cracking) and start there. Never dry sand plastic; it will only clog up your sand paper and create even bigger scratches in your work piece. Where possible, mount the piece to the bike; this will give a solid mount that allows you to use both hands and identifies areas that don't need serious attention.
2. Starting with the 320, work on the areas noted in step one. Keep the piece wet with the spray bottle like a medium rain on your windshield, not so much that it runs but provides good coverage. I had good success using a palm sander on the flat areas, just be sure to keep the piece good and wet. If the plastic starts to 'ball up', your either too dry or too aggressive with the paper and creating heat. When you're in the 'sweet spot' the water will discolour and haze. Go ahead and sand the entire piece to an even finish. Make sure any deep scratches you want to remove are at least 90% removed before moving to the next step.
3. Don't skip on the grits. Always work the entire piece to an even finish before moving to the next grit. Now you should be using the 400 grit and really be focusing on an even finish than trouble spots.
4. Now we're ready for the 3M Headlight kit. I purchased it for one reason only, the 3" hook and loop drill adapter and sanding sheets. These are available from 3M / your local auto body refinishing supplier but only in mass quantities. This adapter makes life much easier in the curved portions of your piece. Just like working metal with a grinder, let the disc do the work. The kit starts at 500 grit and goes all the way to 3000, however the discs don't last very long. Reserve the discs for tight spots and curved areas, use the 1/3 sheets on the remaining portions of the piece. If your undesirable scratches are still noticeable after this step, now is the time to address them as the finer grits remove very little material.
5. Continue through 600 to 2000 grit. On an average size fender you should be sanding at least for 30-45 minutes for each step in grit. You'll notice that once the piece dries that it seems to continuously have a haze to it and zero shine. Don't get discouraged, you'll also note that the haze lessens with each step in grit.
A pic of my XR80R tank @ 1000 grit
6. Now your finished sanding, and not a moment too soon. You can either apply the Turtle wax Hand Polish with the 3M headlight kit pad or by hand. I used the pad and saved some time, the results are well worth the effort especially for a relatively unique project like my bike (all OE Honda XR100 suspension conversions are pretty rare). Repeated application of the Hand Polish should get you to a near mirror finish. If you want to take it all the way to perfection, drop them off at an auto detail shop for the final polish and buff.
The finished product:
Not a fan of sanding...