As far as brush strokes being seen if your 30cm away yes you can see them but stand a metre away and no you cant see them but like I said your first coat is the important one if this is not even in colour and you get build up in spots its to hard to keep it consistent with the next coats thats why I found using a flat brush helped this alot ,as far as width for the large areas I use a 10mm wide one and for smaller jobs 2mm one
Havent really worked out hours on this one as I use it for a stress relief when other projects piss me off
Another question if I may? If you make a mistake or are not happy with a section/brush stroke etc, is simply wiping off an option? If so is turps the best bet or just a dry rag?