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If the boards are in good nick do NOT get a drum sander...get a finishing sander.
The drum sander is literally a big drum that rotates with a sheet of sand paper on it. It actually cuts away the top layer of wood to a notable depth. it is quite hard to use to be honest and you will 99.99% end up with groves where you managed to let the sander cut deeper on onw pass than the other. You can spend your whole day goign round and round tryign to flatten it out.
make sure you get a finishing sander, the ones I have used were square. A vibrating plate rests on a pad that has a sheet of paer under it. This is literally like a mechanical way of hand sanding..takes off far less of the wood, but in my case, which sounds like your case thats all you need. I did 3 passes on every floor. Grade 60, 80, 100. You also need to do the last inch near the skirting boards by hand....no way roundn that. i myself used a small hand held electric sander. You can also hire edging sanders if you like.
Hiring a sander to do only one room will work out quite expensive. I hired mine on a Fri evening, it was Oct bank holiday weekened...brought it back on the Tues and only had to pay i days hire.
Also, the sheets and the pads will double the cost. 4 quid a sheet ,4 quid a pad. Both wear out so you will need multiples fo each.
You also then need to sand very lightly between coats with around 250+ grade paper....and the biggest mistake you can make is to varnish with dust in the air, because the dust will settle, stick and your floor will feel rough rough!!!
I would also suggest using water based varnish...wasn't a believer in it myself....always thought it wasn't as good as the oil ones but it is.
We used 3 coats of water based Ronseal Diamond Coat of the Light Oak colour and a fourth coat of clear bvarnish over it. The advantage, (very important at this damp time of year) is the drying time, and hance recoating time. Waiting an hour to recoat is a lot lot better than waiting 12 hours.
Also, the guy next to me stained his boards first by hand, with the cloth and dye method. He told me he was gutted he went that way. Found it really tough to get consistent colour and foudn it incredibly tiring. Bear in mind that with varnish you can use a roller...
Any specific questions about my experiences...just ask!