I would like to hear some advice on a project that is haunting me. It's a fiber cement board home that we coated. Some of the vertical seams in the clap board siding were almost 1/4" wide but not deep enough to need backer rod. We caulked them using top quality caulking. The wall gets full afternoon sun and is a medium yellow color (Rookwood Antique Gold SW2814) and gets very hot up about 170. The caulk failed by pulling apart at the seams in several places. We dug out the old caulk and recaulked using Sashco Big Strecth and this time it expanded out of the cracks by as much as 1/4 inch. We have coated several cement sided home and never had any issues with expansion joints. Can we use Bondo in the seams with the high heat or does anyone have any experience with anything better. This will be the third time I'm redoing this wall and want it to be the last.