Judging by the pic the problem with the Future was caused by the brush and the model surface that weren't too clean. The white deposit and grains are dust pieces soaked with the varnish thinner affected by the moisture.
Regarding your question .. the Future is for home using so it should be thinned with the clear water. But I don't recommend that. Instead of the water I would suggest the alcohol ( vodka ) also you may use the Tamiya or Gunze thinners for acylic paints.
Additionally, it is quite useful to add some of retarder liquid used for the acrylic colours usually.