If your raw material is zirconia, no polishing in the world will make it a diamond.Isn't that great?
I’ve heard of writers who have been reworking their first novels for eight or ten years in an attempt to get them published. They don’t seem able to move on to their next: instead they keep niggling at their first novel hoping to make it perfect, when they’ve got little or no hope of doing so.
The truth is that very few first novels get published, and that most published writers have at least one unpublished novel behind them. A writer learns so much with that first extended piece that subsequent works are almost always better.
Katie Fforde wrote eight novels then nearly gave up writing before getting a contract for her ninth, Living Dangerously. Her books are now everywhere.
Patricia Wood (who sometimes comments here: hello, Pat!) wrote three unpublished novels before Lottery was picked up: she ended up with the sort of advance that we all dream of, and a shortlisting for the Orange Prize for fiction.
Nicola Slade (who also comments here: I hope you’re waving, Nicky!) wrote six novels before her seventh, Scuba Dancing, was published: she's gone on to sell another, Murder Most Welcome, and has another deal looming.
By all means polish your work, but remember to keep writing new stuff too. Your writing will change and improve over the years, and there’s only so much you can do to improve an early, weak attempt.