Same for me, I have 5 Da Vinci brushes averaging 5 years of extensive using. Except one which the red paint on the handle blistered here and there - because I left it wet! They're almost in a like-new condition.
The "secret" is:
- Never leave the brushes with emulsion on the bristles, rinse immediately after coating!
- Never leave the brushes wet, dry them (bristles, ferrule and handle) with a paper towel immediately after rinsing!
- Never leave the brushes to dry bristles up, because some water remaining in the bristles may (or "will" if you like) emigrate to the ferrule and maybe to the hande, causing the ferrule retaining rivet to rust - as I experienced with my very first brush. (Can still use it for pt/pd, it's safe as long as you don't wet the rusty rivet...) Leave the brush in horizontal position or hang it from the handle (from the hole in the handle or using a clothes peg) instead, in order to let them dry completely.
Working clean is the key here... (EDIT: I mean you don't really need to coat the ferrule with something - unless you really need the comfort of extra safety. I didn't coat the ferrule of mines with something.)
Hope this helps,
Loris.
P.S. Moisten the brush with (distilled - depending on process) water before coating; you won't have too much emulsion sucked in the bristles that way - we need the emulsion on paper, not in the brush... And, hold the brush perpendicular to the paper while coating, just touch the paper with the tip of the brush; same reason as above...