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Don, I find mixing in some wood glue helps the bond and also gives the dried layer a bit of flexibility. Is it tile grout you are using as I know you have a few different names for things out there, I tend to use drywall filler or spackle will also work and both are softer than tiled grou so easier to work.J
I tend to paint mine with an off white acrylic paint which seals the paster. Then apply washes over the top of that to get a bit of colour variation and it also sits in the joints and makes them more visible. For the was I put a touch of oil paint into some turpentine.Jo should hopefully be showing us her bricklaying and flag laying skills shortly