Thank you so much for the answers!! I´ve used the code written by chuff and it worked. However, this is was just part of a bigger code, which has to do the same thing with 7 sheets in many ordered woorkbooks (1 to n). So, I have repeated the code for each woorkbook but it doesn´t change nothing. One thing important is that "True" is the result of a OR function in excel.
 Sub Teste2Sigma()
 Windows("datacao_v2.xls").Activate
        Sheets("SamList").Activate
        Range("f2").Select
        varPasta = ActiveCell
        varDatacao = varPasta & "datacao_v2"
        Sheets("SamList").Select
        Columns(1).Find(What:="fim").Activate
        fim = ActiveCell.Row 'linha correspondente ao "fim"
        first = Cells(4, 3) 
        aux = Range(Cells(fim - 3, 1), Cells(fim - 3 - 9, 1)).Find(What:="GJ", SearchDirection:=xlPrevious).Row
        last = Cells(aux + 1, 3) 
        nInt = (fim - (fim - aux) - 3) / (first + 2) 
        nVal = first * nInt + last 'número total de amostras lidas
        nPlan = Ceiling(nVal / 4)
        media = Range(Cells(2, 1), Cells(nPlan + 1, 1))
    For K = 1 To nPlan
        Windows(K & ".xls").Activate
    Dim c As Range
    Dim ClearAddress As String
    Dim ClearRow As Long
  With Worksheets("Standard 1").Range("AI3:AJ42")
      Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
      If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 1).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
  With Worksheets("Standard 2").Range("AI3:AJ42")
      Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
      If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 1).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
With Worksheets("Sample 1").Range("AI3:AJ42")
      Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
      If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 1).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
With Worksheets("Sample 2").Range("AI3:AJ42")
      Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
      If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 1).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
With Worksheets("Sample 3").Range("AI3:AJ42")
      Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
      If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 1).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
With Worksheets("Sample 4").Range("AI3:AJ42")
      Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
      If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 1).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
        With Worksheets("Blank").Range("Z7:Z46")
            Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
             If Not c Is Nothing Then
                 ClearAddress = c.Address
                 Do
                  ClearRow = c.Row
                    Cells(ClearRow, 1).Value = ""
                        Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
             End If
  End With
        With Worksheets("Blank").Range("AA7:AA46")
            Set c = .Find(What:="True", LookIn:=xlValues, lookat:=xlPart)
            If Not c Is Nothing Then
          ClearAddress = c.Address
          Do
             ClearRow = c.Row
             Cells(ClearRow, 23).Value = ""
             Set c = .FindNext(c)
          Loop While c.Address <> ClearAddress
      End If
  End With
Next K
End Sub