When we first began formulating our liquid latex compounds, one of the first requests of mold makers was to make the latex thicker so they can fill in deep undercuts.  The product we developed is known the world over as Ceco Powder.  Adding the Ceco Powder to your liquid latex will thicken it to a point where it can be applied with a trowel.  You should apply your first 4-5 coats of rubber using straight liquid latex.  Once you've captured the detail of your model, which is generally after your fourth coat, you can then start building up your mold using the latex and Ceco Powder mixture.