I had this same question during the rebuild of my boat. I found little to nothing about it.
The first questions are, why does it have bumps and where are the bumps. If you have a bolt on lead keel, the most likely answers are: up high near the keel stub, and water intrusion into the filler that was used to fair the keel. If this is correct, I can help you out.
The first step is to remove the old fairing compound. Many boat manufacturers used automotive body filler, which is why you have a problem now. Body filler is easy to remove with a propane torch. Just heat it and scrape it off. The heat won't hurt the keel, but be careful above the lead. I would grind the filler off the keel stub. If it's not body filler, you have to grind it off.
Now grind out any nicks or dings in the keel to prepare for filling. On many boats the keel stub is about 1/4 inch bigger than the keel, and the difference has to be filled in. On my boat I used west epoxy and phenolic microballon filler. I mixed the epoxy and filler to a consistency of peanut butter and filled in the gap using a body putty spreader. Since I don't do auto body work, it took a few tries (or 6) to get it right. Once you start sanding, the low spots will be apparent and you'll need to fill again.
The fairing part is pretty straight forward once you have the proper tools. You have to make the most important tool as it can't be bought anywhere I know of. A longboard is simply a very long, flexible sanding block. I bought a roll of self adhesive 80 grit sandpaper at the auto parts store. 3M Stikit Gold Longboard Rolls 2-3/4 inches wide The sandpaper is 2 3/4 inch wide. I went to Home Depot and bought a small piece of 3/16 plywood (36x36 inches), 2 closet door handles (the kind that don't actually have any guts), and some self adhesive cork drawer liner. I had the plywood cut on HD's panel saw to a 2 3/4 strip. I cut and attached 2 small blocks of scrap 3/4 plywood to the thin plywood strip about 6 or 8 inches from the ends, then I attached the door knobs to the blocks. Then I cut a strip of cork the same width as the longboard and attached. The sandpaper then attaches to the cork and you're ready to sand.
Once the filling and fairing is complete, I coated the whole thing with unthickened epoxy to seal it.
You have to figure out what is causing the bumps before you decide how to fix them. On an older boat, they might be caused by many layers of old antifouling paint, that are splitting and blistering. If your keel is lead, with paint over it, then all you need to do is strip the old paint off the keel with a chemical paint stripper (preferably one that is suitable for use on fiberglass). If your keel is lead encased in fiberglass, then you still need to strip the old paint off the keel, but, when you're done with that, you'll see whether the fiberglass casing is damaged, and whether it needs to be repaired. First identify the cause of the problem, and that will dictate how you fix it.