Can you thin this resin down with acetone? I was hoping it was similar to hxtal in this regard.

XTR has a faster cure time than HXTAL, so it's a little more dangerous to thin it with acetone.

You can thin it down. Still, you'll need to ensure a good air source to keep air moving across the surface to help evaporate the acetone out of the epoxy before the chemical reaction starts. If there is still acetone in the mixture, it can interrupt the chemical reaction so the epoxy won't set.

Having a fan or other air movement on the surface will help.