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I often make use of the blend modes in TVPaint, all across it's a very powerful tool!

Yet, I've found it a shortcoming that the custombrushes have far fewer blend mode options than other tools in the tool panel. Can this be fixed?

I'd mostly like to see the custombrushes get the whole range of blendmodes but it'd be best, I think, if all the tools would get the full range. ( I understand off course that an exception would have to be made for (for example) the eraser )

It all depends on the stamp mode that you are using with the custom brush.
In your screenshot you are using "smear stamp". I guess some combinations just don't make sense, like smear + healing or smear + panto.
I think you just have to change the stamp mode to "alpha stamp". There you have all the options.

Yes, you're right! Thanks for pointing that out, Paul.
My mistake