The use of resin probably makes it as hard to sand as fiberglass ...
The more usual constructions using white glue (T2 in our case ) does allow for easy sanding ... and a proper finish ... as seen on many paper mache constructions here...
Thinking that canvas or even fiberglass cloth applied with resin would be as equally hard to sand .... a situation anyone who has done fiberglass work before can confirm ...
Don’t blame the materials , blame the method of application ...