After hours of dry time, why are there air bubbles on top of the Sikaflex Crack Flex Sealant?

If the joint is deep or very narrow (past the 1/2 inch depth limitation), the sealant will travel down pushing air out. Most bubbles will pop but if they reach the surface when the sealant is "skinning" over, they will not burst. It is highly recommended to keep the cartridge nozzle in the sealant while dispensing the product to avoid occurrence of these air bubbles. To improve the aesthetics, you can apply another layer of Sikaflex Crack Flex Sealant on top to hide the voids.