Is this any use for GRP seats, or will it just be worn away in a matter of days?

i think it would be worn off too quickly

It depends how abrasive your buttocks are

It's not very robust stuff, I think it would wear very quickly especially if you wore jeans....

If you get flocked plastic its quite easy to rub off the flocking fairly quickly...

Cheers... confirmed what I suspected!

Can you remember what your Action Mans head looked like after a few months?

What... ???

That was never flock! It was real hair!