To apply spackle to the gable edges, use a Q-tip or a stiff paintbrush, and then touch up the paint. Since the siding is never perfectly flush with the gable joints, spackle will fill in any small gaps. This step is essential before installing your gable gingerbread trim. Avoid using your finger, as it can create clumps of spackle on the siding and blur the edges near the gable. A brush will allow you to get the spackle into the gaps without it accumulating under the siding.