The Smooth-On product is good. Being an epoxy, it allows you time to shape before it sets. You can smooth it with drops of water or alcohol. I even used Windex and my finger so it wouldn't break up while smoothing. It has a fair amount of micro-balloons but you could always add more. I found a black dye that works with polyester and epoxy so it can be colored. I'm sure there are others. You don't need to add much. Drops will work. Here's the link on that stuff.
https://www.fibreglast.com/product/B...nt_43/pigments. I like their products too.
I used it for the canopy build to keep it light weight yet strong. It can be drilled and tapped like wood. Without the dye, it has a light gray primer color. I go back and forth between epoxy and polyester fairing compounds. Can't make up my mind. Always looking for stuff that mimics balsa for sanding.
Sometimes in modeling, you find stuff like this and it works well. I have no financial interest in these companies, but I was intrigued with Smooth-On's "FX" type products used in the movie industry. i.e. stuff for making molds, masks etc. I bought the small containers parts A and B and it lasts a long time. Being white and grey, it's easy to see when it is mixed. Here's the link.
https://shop.smooth-on.com/free-form-air.
Let me know how you like it.
PS I just noticed Smooth-On had a sample pack of small jars for mixing colors. I just ordered it to give it a try