I am not even sure why the poison topic is being debated. Paladins do not accept or condone the use of poison, it's an evil act - period.
PHB is a core book and such books take precedence over other books unless explicitly contradicted. As such, since it’s indicated in the PHB, using poison would go against a paladin’s code and would cause a fall. There’s no subjectivity involved. A paladin is held to a higher morale/standard than the culture they come from.
Edit: Since it’s part of the paladin’s code, a paladin using poison would technically warrant a powers check, same as if they were lying or cheating. Not because it’s necessarily evil but because they are breaking their code.
BOOM! PWNED, OWNED, thanks very much, and goodnight everyone. Question answered without the need for all this wishy washy perspective nonsense, can we all go back to smiting evil dragons, and not using poisons with our paladins and have some fun.
Now moving away from
poisons, the question in the OP was about
varnishes. This warrants a discussion in perspective and religion, and I think varies from Paladin to Paladin.
While I am sure if I had a paladin of Tyr who would embrace any advantage in warfare to serve the 'greater good', I have a paladin of the Creator who does not believe in the greater good, and whose faith and personal opinion finds the use of brains, hearts, guts, flesh, bones, and other
body parts once belonging to a living creature abhorrent and evil. He has, since coming to ravenloft, made logical concessions to use varnishes made from non organ reagents - such as dust, ash, residue, and stone parts - But this is a tough angle to take, because you have to suspend FR paladins and the concept of the greater good, and realize that a man who believes power and purity is derived of all of creation is going to find alchemy evil or at the very least, iffy.
The greater good is a concept that suggests that it's OK to do something dark or evil if it serves the forces of good (like using body parts in a magical ointment on your weapon, or killing all the Jedi children in order to join the dark side and protect your wife and babies or whatever Vader's contrived reason was). however, if you believe there is
never a free pass to do something dark or evil for the sake of good, and that there is a wrong path and a right path and you have to make hard choices (this is why you're a paladin and not a Divine Champion), then you're probably not going to use varnishes either if the very practice of using brains and guts blasphemies creation.
This is an example of how a culture or a religion can cause someone to view Alchemy like they view poisons. Like I started out, a paladin of Tyr would not agree at all. Being a paladin is not easy, and you have to make hard choices even if they are against your advantage in order to stay true to your paladin's unshakable faith.