[ubuntu] Gimp: how to avoid/fix dodge/burn overlap

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http://ubuntuforums.org – I'm working on a photo: the sky's a tad pale and needs burning. But there are fiddly hills and stuff in the background. So I can burn in most of the sky okay; but when I switch to another brush size to burn in the sky round the tops of the hills, I get that "double burn" effect where the 2 instances of burning overlap. How can I avoid this problem? (I really do want to burn in the sky, so please don't suggest I avoid the problem by not doing any burning in the first place. Thank you.) (Hardware)