Has been some discussion of putting hard finishes over oil. I have always read and it is somewhere in Flexner's writing too that it is not a good idea to put had finish directly over oil finish. This article is by David Marks and expresses what I have practiced in reference to combining known or unknown finishes.

Applying a Durable Lacquer Finish Over an Oil Finish

Thanks for the link, Gerald. What he says makes sense with using the shellac. Kind of seals one type of finish from the other (oil from lacquer)  

I would second the notion of not putting on too much sanding sealer.   Years ago, one of my retail customers had a nightstand or end table.  While in the showroom it had a lot of damage to the top, each dent was highlighted by white marks.  I concluded it was too thick of coats of sanding sealer.   I tried to convince the owner to just let me strip and refinish the top but he wouldn't go for it.  So I did a lot of spot sanding and once got the white out, applied more lacquer to level out.  That worked, for a few more weeks, then more damage showed.  Repeated the process.   Thankfully it sold before it needed a third round.

 

And following Flexner's advice the only "sanding sealer" I use is SealCoat  (that is simply dewaxed shellac promoted as something else) when I'm refinishing and expecting silicone contamination.