Hi Pedro - Here's my experience using polyester resin ("PR") and an assortment of acids / salts to create patinas:
PR adheres to patinated tubes fairly well, but the bond is weaker than to clean brass. Heavy handed scraping could break the tube free from the blank. Shearing cuts with a skew or gouge have been no problem for me. The patina needs to be sealed before casting or the PR will significantly darken it, losing all the great color you worked hard to achieve.
Having said all that, I no longer patinate the tubes directly. I've been using metal reactive paints (copper, bronze, brass, iron...) to create a textured surface on the tubes before creating the patina. The textured surface looks a lot more interesting to me and the combination of metals gives a much wider variety of colors. Still needs to be sealed before casting. The textured surface should provide a much stronger bond of the PR to the tube.
Ed
