Terry had his own brand of blue masking fluid made to make it easier to see on the paper as well as a thin consistency to enable a more fluid covering.
Terry used masking fluid regularly in his paintings and used either a ruling pen or masking fluid brush to apply it.

Maksing Fluid is a liquid latex which when applied to watercolour paper ans allowed to dry, protects the paper enabling layers of watercolour to be applied over the top.
Once the paint has dried it is removed with clean fingers or a cotton cloth. (Fiona uses a cotton cloth), revealing the white paper underneath.

Always test on your paper before applying to your painting. 

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