A 10% gelatin solution, made with Knox gelatin, should be plenty thick enough. I use a similar gelatin and coat with an even thinner mixture, about 8%, and it coats very well at room temperature of around 70 degrees F, with the solution at about 95-100 degrees. Assuming the Knox is same as the one I have used (regular Knox food gelatin) a 10% solution should coat very well at 70 F.

It is possible that you broke the gelatin down by soaking it for too long in hot water. From time to time I have done this and in those circumstances the gelatin loses it ability to set.