the brown bag is stronger it will not shrink, it wont let water pass threw it. it is harder then plaster will bond to pretty much anything, but glass and plastic,
the pictures I posted on the crown moulding job I'm doing the crown I made yesterday was made with Durabond 45 and 90. If your having a hard time call USG they should be able to find you a supplier.
here are some pictures of what I do with Durabond in the brown bags, the white bags are just ready mix joint compound just in powder form. with Mica added to make it dry.
These pictures are of how I use Durabond 90, 45 and 20 these jobs all have Durabond as my base coats
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