Specifically for bases but any terrain where I wanna do some minor sculpting and filling.

I'm looking to embed washers (for magnetized trees) onto some plastic card, and keep things fairly low profile.

Looking for something that is gonna be easy to sculpt and contour, but will dry light weightish and somewhat flexy. Any pointers?

I'd say either greenstuff or milliput, or a combination of both, depending on your needs, greenstuff stays somewhat elastic once cured, there's a bit of a learning curve to get the best out of it though, milliput can be sanded or drilled after it's cured and is cheaper so is suited for bulky stuff or bases where detail is less important, you can always mix them to get the best of both.

not sure why you would need it to be flexible though if it's just to embed some washers

Well the plasti-card base has some flex, so a thin layer of something stiff will crack and flake off.

Never tried greenstuff, this may be my chance...

8 hours ago, Mr Tough Guy said:

I'd say either greenstuff or milliput, or a combination of both, depending on your needs, greenstuff stays somewhat elastic once cured, there's a bit of a learning curve to get the best out of it though, milliput can be sanded or drilled after it's cured and is cheaper so is suited for bulky stuff or bases where detail is less important, you can always mix them to get the best of both.

not sure why you would need it to be flexible though if it's just to embed some washers

I did my first sculpting recently, using Das air drying clay. Super easy to use, but it does dry rigid and hard, in couple days.

Spackle / wood filler is also an option. Check out The Terrain Tutor on YouTube. He can tell you lifetimes about what you need to know.