I don t want or expect an accept that is followed with negative feedback.
Can you seal marble with polyurethane.
If not you take the chance of sealing in dirt and debris.
I m sorry there isn t a cheap easy fix for your problem but i not only answered your question i told you what it would take to fix it.
When a drop of water soaks into the marble and doesn t bead up it s time to seal.
Ok this is my question i bought marble tiles that has this polyurethane clear coat over that makes it look really shiny.
I also told you the only viable fix for your problem.
So in my opinion i would not use polyurethane on your marble top.
The installer says i can t apply a sealer because it wont penetrate in the stone because of the polyurethane clear coat that has on it but the previous estimater that came to me house says that they will goin to seal the marble i call floor and decor where i bought the.
It will penetrate the surface and protect it.
If residual sealer is not completely removed from the surface of stone it may cause problems including a haze on the stone s surface that may develop.
Use a sealer made to seal marble.
Your question was can i put polyurethane on marble i answered this question.
Although it is possible that your table has a polyurethane or varnish finish it s much more likely that the marble was polished to a high gloss that has become scratched and dull.
The shine most likely will not last and and when it begins to break down it will look very bad.