Atheling I just use simple polyfilla powder mixed with water to the required consistency and then spread it onto the base. I let it dry overnight and then sprinkle a sand and grit mix on top over a slightly watered down PVA.
Ah, I don't use the PVA watered down as I have had more warping issues if I do add water
I’m not sure why your bases warp as I’ve never had that problem myself.
I'm beginning to think that it just be the filler/spackle. It must shrink. I definitely use a very high quality PVA.
Hopefully if you try the Warbases bases it might help. They are laser cut and quite strong. Worth using the craft knife to cut some criss cross grooves on larger bases as it helps stop warping and the filler sticks to the smooth surface better.
Which is what I also do! This is getting very odd- I must be going wrong somewhere but I cannot for the life of me figure out what it might be!!??
A mate did suggest using something called Modge Podge glue that is water soluble so I might try that without any filler/spackling at all.