Looking to caulk the vertical joints for HardiePanel Cement Board Siding. Will this be a good choice?

Thank you for posting your inquiry. DAP CONCRETE & MORTAR FILLER & SEALANT is not rated/recommended for use on fiber cement. As per James Hardie’s recommendation: “Manufacturers' Caulk and Sealant Recommendation Sheet S-100B, Issued July, 2000: APPLICATION: General: James Hardie believes it is good building practice to seal the joints to prevent moisture, such as wind-driven rain and snow, from penetrating the wall cavity. Caulking around windows, doors, eaves and trim edges gives added insurance that leaks will not occur. When James Hardie siding products butt into wood-based materials, a 1/8" gap will allow for expansion and contraction of the wood-based product. Caulks and Sealants: James Hardie recommends the use of caulks and sealants that remain permanently flexible. Look for the words 'permanent flexibility' or 'permanently flexible' written on the label or in the accompanying literature. Allowing caulks to dry for the recommended time will help to prevent paint from flashing and/or cracking over the caulked joint. Products such as DAP DYNAFLEX 230 or DYNAFLEX ULTRA (among others) would align with these recommendations. These types of more specialized elastomeric joint sealants offer several times the amount of flexibility offered by DAP’s general purposes sealants. Should you have additional questions or require further assistance, please feel free to contact DAP Technical Customer Service at 888-327-8477.