Just 3", that's your problem right there -
no jokes, please. I found a brush like this one but much longer ...
Need a paint brush that's adapted to deal with timber, exterior paints and other treatments? This wide brush is specially designed to suit all your outside jobs, so it’s ideal for the painting of sheds, fences and other garden woodwork.Because the filaments are a combination of pure and...
www.toolstation.com
... and did ten panels in 3 hrs. That's 18 mins each AND I've got a dodgy shoulder at the moment so had to share the work with my slower left hand.
I said, no jokes.
It really was a "where have you been all my life" moment.