I use the round sanding discs for smoothing, grinding, etc.
One of the problems I've found is that the resin seems to loose its adhesion quality when I change discs (to use a heavier or lighter grade of abrasion).
But I found that...
If you take a heat gun and heat up the resin when replacing a used sanding disc it will once again stick to the wheel for you.
Its a way of getting more mileage out of each disc.