Fill blank cells,select from the last cell

Hi guys, i have this code to fill the blank cells, the problem is that it doesnt work if a blank cells appears in my column, so
i im trying to find the last cell and then copy all wich is abovethis cell, but my code doesnt work , please help me with this thank you

Sub Rellenar()

'Declaración de variables
Dim Rango As Range
Dim Producto As Range

Dim FinalRow As Long
FinalRow = Range("B1048576").End(xlUp).Row
'Definir el rango a cambiar.

Set Rango = Range("B1:B" & FinalRow).Select

'Empieza el bucle
For Each Producto In Rango

If Len(Producto.Value) = 0 Then
Producto = "No disponible"
End If

Next Producto
End Sub

thank you for your reply, but it doesnt work or i dont know after wich sentence do i have to write that code
the code i need is to fill blank cells but just from the last used cell to the bottom, because my code select all the cells and fills with "no disponible" all cells till the end of the rows

Thank you!!!! , now it works!!!!