Sanding Your Motorcycle Paint Project
Once you have let the filler cure completely according to the instructions for you filler, you are good to start sanding. Before you start, get a dust mask. Filler is made for filling dents and such, not your lungs.
Sanding is pretty straight forward - the big thing is that you want to avoid using a sand paper that leaves big scratches. 80 grit is a pretty good option to use, and using an aluminum oxide paper helps with avoiding clogging which can just get annoying. You can always blow the paper off if you have a compressor, but not having to deal with it in the first place is just a better option. You can always pick up a package of assorted grit paper and work with different grits to get a good finish. Sanding blocks can also be a great way to work with the curves involved in sanding at this point. Softer blocks will follow the curves better, so favor a softer block if you go this route.
If you are all squared away with your supplies, you are good to start sanding of the excess filler. The best way to look at this part of the process is to try and work your way back down to the tank with your paper, leaving only the filler in the low spots. After you have worked your way down to a finished looking tank, run your hand over the tank with your palm down and feel for low spots. The goal is to not find any, but if you do feel and small dips just mix a little more filler and apply it to the area the same way you did originally, let it cure, and sand it back down. Keep doing this until you feel your shape is perfect, and then work through progressively finer grits (120, 240, and 400). You will notice that if you skip the finer grits you will have issues with sanding scratches coming through in the priming and painting stages if you slack at this stage. Take a look at this post for a thorough overview of the sanding process.
The big thing to keep in mind here is that this part of the process is seriously the foundation of your paint job. Filling and sanding will be the most involved, time consuming part of the process, but not putting in the time here will affect the way your paint turns out more than just about any other step - in other words, it won’t matter how great your paint job turns out if you put it on a tank that has grooves, scratches, dings, and other defects that you can fix easily at this stage. Yes, this can take a few hours - but these hours turn into years on your bike.
Now that you have a good idea of what this process will look like, check out this great video on Filling, Blocking, and Priming a Motorcycle Tank for a Paint Job: