Let's breathe some life back into this sadly neglected upper right hand fairing panel from a BMW R100RT. It is representative of a common type of damage that should illustrate how these "three sorta easy steps" are applied.
* Step one is to grind out the damage into surrounding solid/stable material.
* Step two is to use fiberglass cloth and resin to patch and fill in the area cut or ground out in step number one.
* Step three is to grind off everything that doesn't look like how the fairing is supposed to be shaped!
The BMW panels appear to be molded from some sort of fiberglass reinforced plastic and can be repaired in the same manner as a more traditional laid up fiberglass panel to which these techniques can be applied as well. This bike appeared to have been laid over, maybe tipped over, on it's right hand side and the fairing panel was cracked and moderately damaged where the mirror mount attaches. There were also several other cracks in the panel that may or may not have been related to this particular incident. Click on the following thumbnails for details of the damage.
The first thing to do is to grind out the damaged material on both front and rear sides. The following picture only shows the "top side" of one of the areas but the opposite side is ground out in like manner. The Crack was ground out with a 36 grit disc on a 4" grinder/sander. I apologize for only one picture of the crack prep. I actually ground out more than is shown in this picture but you get the idea. All of the damaged/weakened material must come out. Make the edges of the area that you grind out angle or slope in toward the center of the repair so the fiberglass cloth has something to stick to.
Once all of the damaged material has been removed it is time to "lay in" new glass to patch and fill the area. I haven't seen much difference in fiberglass repair kits (unlike plastic body filler) so I would guess there is no problem using whatever is carried by your local auto parts store. The kit will come with a couple of square feet of fiberglass cloth, a can of resin, and a little tube of hardener. Cut out strips of the fiberglass cloth sized and shaped to fill the repair and extend into the undamaged area. Once the area is prepared, cloth strips are cut, and the resin and hardener properly mixed, the fun begins. I highly recommend using disposable latex gloves (unless allergic) as this stuff is nasty-sticky. Using an acid brush (or other small brush) paint a coat of resin onto all surfaces that will have fiberglass cloth overlaid. Then, dip one of your strips in the prepared resin and lay it onto the crack, working out bubbles and voids with your resin saturated brush. Continue this process until all of your fiberglass patches have been applied.
On edges that are damages, I cut up cloth into resin and make kind of a putty that I use to build up the edge. The first picture is an edge where I have "filled" in the major gap in the edge with my home brew fiberglass putty and then laid a layer of fiberglass cloth over the top. There was a pretty good chunk ground out of this edge so duct tape was used as a "form" to keep the gooey mess in place while it cured. The other picture is of the inside repair of the main crack after the fiberglass has cured and been rough-sanded to general shape. There is a corresponding patch on the opposite side that was applied in like fashion. Once all of your repairs are in place find something else to do while the resin cures as it knows when you are watching it and will take it's own sweet time! ;-)
The following picture is a close-up of the edge that was reconstructed with the fiberglass putty and cloth after the duct tape was removed and it was rough ground to shape.
Once you have laid in enough fiberglass cloth and resin to build the repaired area back up to it's proper thickness a light coat of plastic filler, generically known as Bondo, can be applied. Only use a quality, professional grade body filler. I have taken a liking to the 3M lightweight filler, but I have also had good results from the Martin Senour brand product sold by NAPA, and a brand called Rage. This shot is after rough sanding the coat of filler and the mirror was attached to make sure the contour was correct. On panels where you have to sculpt a body line, or other contour, you can often use a contour guage to take a profile of the same area from the opposite side. I used the undamaged left hand panel to check the contour that I had to build when I was roughing this out.
Once things are contoured and shaped properly you can then move on and prime with a high-build primer surfacer. This is DuPont URO Prime which is a two part, urethane primer that can be either mixed as a primer-surfacer or a primer-sealer. This is one coat used as a sealer and I will then do the final shaping and any light glazing that needs to be done over this sealer coat. Once this is finished then it will be primed and block sanded in preparation for paint. Someday, when I get really bored I will finish the panel and post the finished pics, but I think from these you will get the idea.
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