Hi Clyde,
This method is generally considered appropriate, but there's a lot of detail left out. What kind of filler, what kind of resin? What happened to produce the damage? How bad is it? It sounds like mostly surface-level issues, is there any structural damage to think about?
If the damage is just shallow pocks on the lead keel ballast itself, this isn't a high stress area. My preferred method would be to fill with epoxy resin and high density or low-density filler depending on how deep the gouges are (Google search West System fillers), or read my blog post about epoxy resins and fillers to familiarize yourself with these materials, here:
American Mainsail Sailing Co.
Here's a picture of an in-process keel fairing job I snapped in my yard last spring. They are using epoxy and low-density filler (West System 407 - the brown areas) to fill fairly shallow imperfections in their keel. The general process is to sand down to bare fiberglass or lead, overfill the imperfections with filler, sand the filled areas until fair once the epoxy has cured enough to take to sanding, barrier coat, and lastly, apply bottom paint.
View attachment 227056
In my experience, yards will normally reach for more traditional, polyester resin mostly for cost reasons (they're already charging you for labor, cost control is important). This is appropriate too. Many good repairs have been effected with polyester resin.
Epoxy is stronger and doesn't stink like polyester does, so I normally reach for epoxy for small repairs and DIY.
Something to think more about is how these gouges got there. Did you run aground? If the keel took a major hit, consider performing more inspection on the keel/hull attachment. Sound around the keel joint for separation of the hull and structural hull liner. Look for and stress cracks, dislodged/out of place furniture, and vertical and/or side to side movement in the keel when the boat is hanging from the slings and transitioned to sitting on the ground. If in doubt, hire a professional surveyor. Sometimes, insurance companies will pick up the tab for a post grounding survey to make sure that everything is sound.