There's quite a few different "Hard as Nails"... which one do people use?
I use the clear with nylon most, but they have one called "No Chip" too which is good, and one called "Top Coat" for a nice shiny final coat.
Depending on what types of flies you tie, there could be uses for several of the Sally Hansen products, as some are thinner than others.
To the OP, Wapsi used to have a thinner for their head cements, which is what you should use with flexament. Not all nail polish thinners are compatible with all head cements. So be sure to use the correct thinner. I ruined a couple of bottles of head cement learning this!
Nail polish is not always cheaper than head cement if you compare the prices per ounce, but shipping & postage regulations have gotten so strict that it's much more economical in the long run to buy the Sally Hansen's, which is widely available, unless you have a fly shop near by that sells an inexpensive head cement. (Cheap at a fly shop, fat chance of that!
When I tied commercially, I used to buy one that Wapsi sold by the quart. Had to buy the thinner, because I had the same issue with it thickening, because I just couldn't tie fast enough to use it before it got thick. But postal shipping regs changed because it's flammable & the shipping for it & the thinner got to be ridiculously expensive, so I just went with the Sally Hansens & never looked back!
As others have said, it's as good as any head cement you will find too!