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I was in the local Joshin and saw some Joshin brand sable brushes for a pretty good price.

Has anyone used these. How is the quality? Is it comparable to GWs?

I also saw some prime for plastic and metal models - any good?

Basically anything Gundam-oriented, primer or paints, is going to be pretty good quality, at the higher end anyway. I haven't used Joshin's myself - personally I use a brand called Bunseido Woody Fit that they sell at Volk's (along with a full range of Vallejo paints), but really you probably can't go far wrong with sable.