I don't know, if this has been brought up, before, here on the boards but it's a "new experience", for ME, nonetheless.............
I recently had BOTH my arms forced behind my back, by that tough guy we all know as "TyroneFly" and while dislocating my shoulders, he made me buy one of those fantastic "Dyna-King all aluminum head cement reservoirs".
Well, I got it and have to say..........."once my arms, are back in shape after being so abused by T-Fly, I'm going to love using this thing for head cement. No bottle to knock over, ( a hobby and favorite past time of the family cat, when he comes to see what I'm doing at my tying table), no lids left off of cement bottles, etc.
Once properly filled to the correct level, a perfect sized drop of needed cement each and every time. A GREAT device! Well worth the sheckles it cost.

However, as the title of this thread, suggests......... I did have a rather unpleasant surprise, when I first filled the new holder with a new product I ran across we're all pretty familiar with.......... "Sally Hanson's Hard As Nails".
EXCEPT, I found a bottle labeled, "ADVANCED" Hard As Nails", nail polish finisher". I thought, since it's as clear as the regular, but had added to it, "Nylon and Retinol", it might be a little longer lasting and tougher, than the regular, which I use on all my salt water flies, to seal the heads and/or, coat over the eyes.

Well after testing it, IT IS a lot tougher, than the regular Hard As Nails.
Great!
But, it also..............sitting in my D-King cement reservoir just over night, it DISSOLVED, not only the bodkin-sealing o-ring, but also turned the o-ring on the lid threads to a "gooey, black, goo"!?!
I don't know, what ELSE is in this "ADVANCED", Sally Hansen's, but it's pretty powerful, whatever it is!?!
Oddly, after a week, of being dried on my flies, there's no ill effect to them at all!?! Just the 0-rings.
Lastly, for "KUDOS" to Dyna-King............. I emailed them and explained everything and asked the price on three sets of new o-rings, for my reservoir. "Karen", emailed me back and simply told me, "I've put 3, new, sets of lid and bodkin o-rings in the mail to you, today, Paul...no charge!"
Can't beat customer service like that!