Shellac repairs will always show up after the stock is finished. If it is a dent without broken fibers, you can make an invisible repair with regular acraglas. Just plain acraglas-no sawdust or floc added. Build it up in layers until it is above the level of the wood. After drying, sand back even with the surface. I have made repairs like that and could not find them after the stock was finished. Shellac will look like a birthmark on the stock. The acraglas will be clear and you look right through it.
The first time I read about repairing that way I did not believe it would work. But it does.
