title: replace all strings to a default number in DataFrame
body: <p>I have a pandas DataFrame of numbers (int and floats) which results in a datatype of float for all columns.  or so I thought.</p>
  
  <p>These tables are the result of OCR scanning to EXCEL.  in some case there's ascii or word values because of a bad scan.</p>
  
  <p>How do I perform a blanket str value to default -999999 for all columns, all values in the DataFrame?  </p>
  
  <p>The problem I have is the pd.replace() function appears to be string specific.</p>
  
  <p>Thanks!</p>