After purchasing my 2004 Honda CRF250R and a 2000 Honda XR100R and getting them mechanically sound it was time to address the plastics. The XR had severe oxidation on the gas tank and fenders, but I wanted to tackle the CRFs fenders also.
It should be noted there are thousands of opinions and ways to do this and the method I picked seemed to work the best for me but may not work the best for everyone else. There is a lot of info out there, but the one similarity that keep coming up was that there is no magic chemical to remove oxidation (or if there is they don't work well). To do it properly it is going to take time and effort. So here we go!
Tools/Equipment Needed:
-Wet Sandpaper in grits 400/800/1000/1500/2000 (at your discretion, I used them all)
-Utility Razor blade (optional)
-Heat gun/Hair dryer/Blow torch (I used this 1800w heat gun)
-Drill of your choice (I have a cordless 14v snap-on drill, but wired may work better here)
-Degreaser and Sponge
Optional:
First thing to do is remove all the plastics from the bike and clean them. I used simple green degreaser and a sponge. If you are doing the gas tank then remove all the gas from it and all bolts/caps/bushings, then clean it completely. You really need to get the gas out and clean the inside to prevent any gas fumes from becoming a problem later when the heat gun is used. I highly recommend thoroughly cleaning the inside and letting it sit and vent for a few days before working on it.
Once the parts are clean the first step to remove oxidation. There are a few methods to do this.
1. Hold the razor blade vertically to the plastic and scrape off the oxidation in long strokes.
or
2. Used 350 grit wet sandpaper to remove the oxidation.
I personally used the razor blade on the large sections then followed up with the 350 grit on curved sections. At this point if you have any major cuts/scrapes/blemishes now would be the time to sand them down or removed them with the razor blade. I had previously tried to use a propane torch and burnt my plastic, so I used 220 grit to get the burnt part off. It does not look great, and I don't recommend going to that low of a grit.
Once all blemishes and oxidation are removed. Wash clean with water and move up to 400 grit wet sandpaper. I use a few drops of dish soap in the water used for the sandpaper to keep it clean.
Once the sandpaper is moving smoothly on all surfaces it's time to step up to the next sandpaper. I dried it out in between to show what the surface looks like, but this is not necessary unless you need to inspect for missed areas or additional blemishes.
I went to 800 grit next and keep going higher and higher once the sandpaper moved freely on the surface and did not catch. I went from 800 to 1000 to 1500 to 2000.
After 2000 grit and left to dry:
Now I rinsed and dried the fender prior to beginning the heating phase. For the heating phase I used a 1800-watt heat gun and slowly moved back and forth on the surface to heat up the plastic till it slightly melts. You really do not want to stay on one spot because it will burn. Just get it till it turns glossy then move on. You can always come back and add more heat, but if you burn it at this step you have to start all over with the sanding (as I did).
Additional Methods: Using a hair drier (may not get hot enough) or propane torch (not recommended, too easy to burn the plastic).
After going back and forth it should look like this:
Continue until the entire fender is semi-glossy.
A lot of people stop here, and you can. It looks pretty good at this point. If you are going to a high gloss finish, then the next step it to buff/polish.
In order to do this, I used both a hand pad and a wool buffing wheel attachment for my drill. Combine that with a rubbing/polishing compound (I used Meguiars M100 Mirror Glaze Compound) and apply to the fender. Again, keep in mind that you need to keep it moving, even the buffer wheel as the ability to burn and ruin the plastic.
If you still see white spots (not the bent plastic parts, but the sanded parts) you can apply the heat gun again or use the rubbing compound. This is as far as I went with mine, but a lot of people apply a sealer/shine to the plastics. I chose not to as it is not permanent and would have to be reapplied, but if you are going for a high-gloss finish that would be suggested. Keep in mind that if you plan to apply a decal set, I would not suggest adding any sealer/shine before applying them as the adhesive will not stick as well to the plastic if there is a gloss coat and after all you would want the decal set to be high gloss also.
If you do end up wanted to add a high gloss coating, I would recommend Maxima SC1 coating. Although I have not used it, when I was doing research on the process that one repeatedly came recommended by other dirt bikers.
Before and After next to a spare white fender:
TIPS:
-Rinse the plastic after every different grit
-Sand in circular motions
-Keep the surface wet when sanding
-Buff outside! The wool can get everywhere if it's a new wheel.