Hiding All Panels On A Web Content Form Within A Master Page

I'm trying to hide all panels on a page, when a button click occurs.

This is on a web content form, within a master page.

The contentplageholder is named: MainContent

So I have:

foreach (Control c in Page.Form.FindControl("MainContent").Controls) {
if (c is Panel) {
c.Visible = false;
}
}

This never find any panels. The panels are within an Update Panel, and I tried

foreach(Control c in updatePanel.Controls) { }

and this didn't work either. I also tried :

foreach(Control c in Page.Controls) { }

and that didn't work either.