You can only Stain over unfinished wood, not over previously Stained and Varnished (or Sealed) surfaces… that being said, it is best to work in the mornings, during spring, summer or fall weather… before the temp gets too hot (above 90F) and with lower rel humidity (<50%)… always use an Oil Based Stain…

I have a web page explaining more: <a href="//davmagic.com/paint/PAGES25.html" rel="nofollow" target="_blank">STAINING</a>