Hi

1. You can add RedirectToURL attribute to

<uc1:SignOutButton runat="server" RedirectToURL="your_login_page" ID="SignOutButton" />

You can find this line in <your_project>/CMSTemplates/your_site_aspx/Root.master

2. If you prefer portal engine, open Properties of Sign Out Button web part and insert path to your login page into Redirect to URL field.

After set up this attribute, you will be redirected to Log In page, like you wanted.

Best regards,
Ivana Tomanickova