Hi there, first of all you need to make your button invisible by default as like
<asp:LinkButton ID="LinkButtonExport" Text="Export to Excel" runat="server" OnClick="LinkButtonExport_Click" Visible="false"/>
and then wherever you need to make it visible by using following code you can do that
Page.Master.FindControl("LinkButtonExport").Visible = true;
Better to add this line of code in the method or block wherever you are binding your grids.