To All:
I have never tried this on hydrocal cracks but have used this method for other jobs. To fill thin gaps and cracks on plastic walls I sometimes apply a superglue (thinnest liquid possible) then apply baking soda to the glue. the baking soda cures the glue instantly and forms a good joint compound that can be sanded out.
Large holes divots etc I use Squadron White Putty.
Joe