Nice pens Sharpy. Oak has quite open grain, and if you use 'cheap' (read poor quality) sandpaper the grit comes off the backing paper and gets into the grain. Used to happen a lot to me ... If you have access to a compressor give the wood a good blow once you've finished sanding, or else as Brian says use some white spirit to wipe the wood clean then leave to dry for a bit before finishing. He's a tart, BTW, because the last time we had this debate he insisted he would never touch a wood pen barrel with anything wet ... Once a bike always a bike, Brian! (No I haven't forgotten that you asked Derek to marry you just days after you begged me to leave my wife for you!Hell hath no fury and all that ...)
