How to thicken Paint
Diisocyanates are used in some coatings and polymerize in contact with moisture. Flexithane contains a non-reactive polyurethane (already reacted diisocyanate), and so is simply a solution of rubber in a mixture of solvents (SDS: http://www.hmgcoatings.co.uk/msdspdfs/Flexithane.pdf).
The literature claims dry to touch in 5 minutes, ready for a second coat in 20 minutes at 20C. One way to thicken the paint without adding anything that would destroy its performance would be to evaporate some of the solvent. In a can about half-filled with the paint, stir well, with plenty of ventilation (as advised by the SDS) for a few minutes. At £49.66 to £75 per 500 mL, it could be be an expensive experiment if you don't stir well enough and you get lumps or skin in the paint when you try to apply it.