To repair plastic with the welding process you need have the parts clean, oil and decal free. I use the Castrol degreaser to clean any parts that may have oil on them from the crash, it is cheap and can be had at Walmart for around $6.00
Next, off come the decals, the most effective way to remove the decals is with a heat gun and a plastic body paddle and a can of Preston brake cleaner and a dull razor blade. Heat up the decal, just so it is hot to touch, don't make is so hot that you bubble the panel. once the decal is warm you can use the dull razor blade to lift the decal and peel it back with your fingers.
Once the decal is off spray on the brake cleaner and let it soak in. it will require a couple of coats of spray to keep wet, let is soak in for about a minute. Then use the plastic puddy knife to remove all of the glue residue that you can. Once this is off re apply the brake cleaner and let soak a few seconds and then use a coarse rag to take off the remaining glue.
For the repair of the plastic you will need a welding iron, I use a hot foot style welding iron made by the Urethane Supply company, you will need a dremel tool and a bit for v ing out the crack in the plastic. You will also need some stainless steel mesh to re-enforce the repair and 80 grit paper
Clean the damaged area with 80 grit paper, take your dremel tool and take off all of the paint down to the black plastic, on both sides of the crack, use the tool to then v out the crack on the back side, clean with a wire brush, weld in one pass with the welding rod, you will want to practice on a bunch of broken plastic before doing this, there is a fine line between to much heat which will make the weld brittle and one that is to cold to stick.
After making the pass, you will cut a piece of steel mesh to melt into the plastic on top of the weld, take the time to melt in the mesh and make it flush with the rest of the part. After this has cooled you will flip over the part and v out the other side. you have meet the weld you have just laid in, clean with a wire brush and weld in a pass of welding rod.
You should be able to sand down the area with 36 grit and then 80. To fill in any voids and imperfections or spots of road rash you will use a material made by Ever coat,called Poly-Coat it is very expensive, $38 for a small bottle of the stuff but it is at this time the only product that works. a 1/8 layer will take a 70 degree bend when it dries, with out breaking. Follow the directions on the can, for mixing, being very careful not to add to much hardner. Block sand your body filler down and then sand down the rest of the body panel for the next step.
In all of the areas were you have broke through the factory paint you will have to add a adhesive promoter. one good one is a product called Bull Dog the other is one made by Sherwin Williams, spray the damaged area with the adhesive promoter before it dries completely you will spray on the primer
The primer that have had the only luck with is a product called Rapid Prime, #8445 it is for rusty metal and a very fast high build primer, it is the only primer I have found that will flex with the plastic and is easy to work with, it is also on the expensive side but have found no short cuts for it.
Spray on a light coat of primer and let flash over, it is very important not to put it on to heavy the same properties that let it bond to the plastic also will dissolve the factory finish causing it to bubble if put on to heavy.. making a real mess.
Apply two to three coats of primer to the entire panel. I have played with doing partial repairs and it has always come back to bite me in the painting process. Let it dry a couple of hours and then block sand with 120 grit any areas that appear un-even. then re apply another coat of primer.
To get ready to paint you will sand with 220 grit wet and then 320 wet and then 400. the primer has to be painted within 12 hours of doing this.
Visit our web site EmpireGpP.com to get some ideas of the type of repairs that can be made, with some patience and time you can repair almost any part on your bike, the above info is for ABS plastic, many bike use a PP plastic "different method," For the side panels Honda and Yamaha come to mind. the rest of the parts are ABS and can be repaired with this method.
If you get stuck e-mail me.. If you want your parts repaired by us it is the cheapest part of the painting and repair process, send me a note and I can give you and idea of the cost of repair.