Skins disappear when overriding Render method

[John]

I'm trying to override the page's render method, and capture the result for
processing, eg:

protected override void Render(HtmlTextWriter writer)
{
//base.Render(writer);
StringBuilder renderedOutput = new StringBuilder();
using (StringWriter strWriter = new StringWriter(renderedOutput))
{
using (HtmlTextWriter tWriter = new HtmlTextWriter(strWriter))
{
base.Render(tWriter);
}
}

// save renderedOutput.ToString() to a file
}

This works great, but my page theme (specifically gridview skins) arn't
applied! If I replace the call to base.Render() they work fine - does this
mean skins / themes arn't rendered in Render()???

Thanks,

John