Does GOLDEN make a hardener to create a very hard and sandable acrylic surface?
You should try the GOLDEN GAC 200 blended into your glazes. This is a specialty polymer medium formulated with a much harder resin than the softer and more flexible ones used for most of our other products. The maximum addition of GAC 200 in a mixture is 75% of the total blend. Higher levels pose the risk of films cracking during the curing. Also, it's important when sanding acrylics to note:

• Acrylics are thermoplastic in nature: sanding increases the overall film temperature which makes the acrylic softer. Avoid sanding in one area too much.
• Even though they touch-dry quickly, acrylic films take much longer to fully cure. Until the layer cures, sanding can be much harder. Therefore, allow as much time as you can (within reason, we understand wanting to not have to wait too long between layers) before attempting to sand.