Trowel size for laying Hardy Cement Board

[2020Tiling]

Hey all, Would it be ok to use a bigger trowel when over boarding slightly uneven floor boards with hardy? Don't feel confident using SPF with a 6mm trowel, not sure if it will fill the voids :smilewinkgrin:

 

[DHTiling]

6mm .. pull the trowel along the length of the boards... and if they are that bad then it needs sorting first.