Populate gridview only on button click - asp.net

Question

My page consists of 4 text boxes (query filters), a submit button, and a gridview. The gridview data source is configured to take text from the 4 text boxes and filter the query results. This part works great.
This issue comes about in that for the query to function, I can't have empty text boxes or the query won't function. Something must be entered, or % for wildcard.
To be friendly for my users, I tried to set a default value of % for each of the 4 control sources(text boxes) in the gridview Data source configuration. However, since the gridview automatically populates on page load, this doesn't work since all 4 controls are set to wildcard. And when you have a DB that has hundreds of thousands of rows......
TL/DR: I want my gridview to only populate on button click, and I want blank text boxes to be accepted as wildcard (%). Ive tried a couple different code snippits concerning IsPostBack, but they didn't seem to do anything. Language = VB. Any tips you good folks can provide would be greatly appreciated. asp below:
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:TaxRollConnectionString %>" SelectCommand="SELECT [OWNER], [ADDRESS1], [ADDRESS2], [ADDRESS3], [CITY], [STATE], [ZIP], [WELL], [YEARBEGAN], [OPERATOR], [SURVEY], [ACRES], [TYPE], [INTEREST], [VALUE], [YEAR], [COUNTY] FROM [Owner] WHERE (([OWNER] LIKE '%' + #OWNER + '%') AND ([WELL] LIKE '%' + #WELL + '%') AND ([OPERATOR] LIKE '%' + #OPERATOR + '%') AND ([COUNTY] LIKE '%' + #COUNTY + '%'))">
<SelectParameters>
<asp:ControlParameter ControlID="textOwner" Name="OWNER" PropertyName="Text" Type="String" />
<asp:ControlParameter ControlID="textWell" Name="WELL" PropertyName="Text" Type="String" />
<asp:ControlParameter ControlID="textOp" Name="OPERATOR" PropertyName="Text" Type="String" />
<asp:ControlParameter ControlID="textCo" Name="COUNTY" PropertyName="Text" Type="String" />
</SelectParameters>
</asp:SqlDataSource>
<asp:GridView ID="GridView1" runat="server" AllowSorting="True" AutoGenerateColumns="False" CellPadding="4" DataSourceID="SqlDataSource1" ForeColor="#333333" GridLines="None">
<AlternatingRowStyle BackColor="White" ForeColor="#284775" />
<Columns>
<asp:BoundField DataField="OWNER" HeaderText="OWNER" SortExpression="OWNER" />
<asp:BoundField DataField="ADDRESS1" HeaderText="ADDRESS1" SortExpression="ADDRESS1" />
<asp:BoundField DataField="ADDRESS2" HeaderText="ADDRESS2" SortExpression="ADDRESS2" />
<asp:BoundField DataField="ADDRESS3" HeaderText="ADDRESS3" SortExpression="ADDRESS3" />
<asp:BoundField DataField="CITY" HeaderText="CITY" SortExpression="CITY" />
<asp:BoundField DataField="STATE" HeaderText="STATE" SortExpression="STATE" />
<asp:BoundField DataField="ZIP" HeaderText="ZIP" SortExpression="ZIP" />
<asp:BoundField DataField="WELL" HeaderText="WELL" SortExpression="WELL" />
<asp:BoundField DataField="YEARBEGAN" HeaderText="YEARBEGAN" SortExpression="YEARBEGAN" />
<asp:BoundField DataField="OPERATOR" HeaderText="OPERATOR" SortExpression="OPERATOR" />
<asp:BoundField DataField="SURVEY" HeaderText="SURVEY" SortExpression="SURVEY" />
<asp:BoundField DataField="ACRES" HeaderText="ACRES" SortExpression="ACRES" />
<asp:BoundField DataField="TYPE" HeaderText="TYPE" SortExpression="TYPE" />
<asp:BoundField DataField="INTEREST" HeaderText="INTEREST" SortExpression="INTEREST" />
<asp:BoundField DataField="VALUE" HeaderText="VALUE" SortExpression="VALUE" />
<asp:BoundField DataField="YEAR" HeaderText="YEAR" SortExpression="YEAR" />
<asp:BoundField DataField="COUNTY" HeaderText="COUNTY" SortExpression="COUNTY" />
</Columns>
CodeBehind:
Partial Class _Default
Inherits System.Web.UI.Page
Protected Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
textOwner.Text = ""
textWell.Text = ""
textCo.Text = ""
textOp.Text = ""
End Sub
End Class
EDIT
I managed to fix my two main issues by removing the datasourceID from the gridview, and the following executed on click:
Protected Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Call BindData()
End Sub
Private Sub BindData()
With GridView1
.DataSource = SqlDataSource1
.DataBind()
End With
End Sub
However, in doing so I've managed to break the table sorting capability of the gridview. I suppose I'll find a way to do the sorting client side.