Can I apply a coat of polyurethane for protection?

Can I apply a coat of polyurethane for protection? Images

Can I apply a coat of polyurethane for protection?

Can I apply a coat of polyurethane for protection? Description

I have slate in the kitchen and (http://maplered.com) breakfast room which were installed 4 years ago when we built the house.   It has come to the point where they really never look clean.   Also, they look wonderful when wet.  Can I apply a coat of polyurethane for protection and (http://maplered.com) shine?   Would this require a lot of upkeep?  Your site is wonderful and (http://maplered.com) I look forward to your answer.  Thanks Gwen
 
 
Gwen Augustine.