One of the first rules of painting, even when it's exterior that folks don't generally look at up close...is to use good equipment. The paint goes on better, smoother, more evenly. And in the scope of things, that "good" brush, even if you inadvertently ruin it for some reason, is a small price compared to the high quality paint you (bravo!) chose to use. I don't even bother to buy "cheap" brushes anymore...the good ones are actually less expensive in the long run.
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The most expensive tool is the one you buy "cheaply" and often...