I have used West Systems Epoxy for many years. i am using the 5:1 mix with 206 hardener. However the last two batches cured rubbery and soft. Very difficult to sand. What did i do wrong? is the epoxy getting too old to use?
I suggest you switch over to Polyester "laminating" resin.
You can buy the stuff by the Qt., at most Auto Body supply stores, you can buy hardener separate, AND use simple Acetone to thin it down.
You can add micro-baloons, from a mix that can be used as glue, to a dry mix, that can be used as a filler.
I'm going to catch hell for this suggestion, so I might as well add this, I've been using Polyester resin, probably longer than most have been in this hobby.
Now, go ahead and lie about how long you've been in the hobby!
Issues, I've never had an issue with Polyester resin, and as I've said, I use it for everything, including the making of this cowling for this Gee Bee Z. You can also see it used as a filler on this same model. This pattern ship was glassed with Polyester resin well over 15 years ago, and still looks great today!
Polyester resin, Polyester resin, Polyester resin, has a nice ring to it also.