Bondo and Fiber Cement Board

[DependaCoat]

Gentlemen,
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.
Thanks, DependaCoat

 

[joecaption]

First off the siding was installed wrong in the first place if there's that big a gap. I'd also bet no one installed a strip of tar paper behind that joint or sealed the ends of it before installing. (look on the Hardee web site for proper install directions.
All I've ever used is this and never had a problum.
DAP Products - Caulks and Sealants - DAP® ALEX Ultra® 230 Premium Indoor/Outdoor Sealant with MICROBAN® Antimicrobial Product Protection

 

[DependaCoat]

I appreciate your help. Any thoughts on the Bondo?

 

[mabloodhound]

Yep, siding was installed wrong AND Hardi says DO NOT caulk the joints between boards, only where it meets corner boards or windows.
Definitely needs flashing behind every joint; I use aluminum but tar paper will work.
With that big a gap, try applying the Bondo to only one end of a board and use a piece of aluminum flash or tar paper between the other end. Hardi does not recommend a gap at the butt joints.
HardiePlank Lap Siding
The boards must be allowed to expand and contract so bridging the joint with Bondo is not good.

Do a couple of boards and see what happens before doing the entire house.