I can't remember what the item is...

[Grendelkhan11]

I seem to recall there being a 3.5 item that was like a flask of paint or a liquid that you could coat blades with to deal fire damage (but would also hurt the weapon some) for a while but I don't remember what its called or where it was from? If anyone remembers, please tell me. I would also appreciate if you found any other items that can be applied to melee weapons.

[Fizban]

Since you mentioned a flask, I expect you're looking for Bladefire from Arms and Equipment Guide. Its pretty terrible though, at only 1 point of damage per hit for a few rounds after a full round action to apply it.

There's also Capsule Retainers from Complete Adventurer, which attach to the weapon for quicker use and last for only one round, but deal enough damage to notice and don't waste your time.
Trollbane from Dungeonscape to bypass regeneration.
Silversheen (wondrous item) from DMG, bypass DR/silver.
Bladeshimmer (wondrous item) from Complete Scoundrel, make item invisible (wears off after one attack).
Ectoplasm (sticky) with Ectoplasmic Stabilizer from Ghostwalk (the effects of Ectoplasm are listed under the feat)
And maybe Blister Oil from Races of Stone, which is clearly meant as a trap but doesn't actually say you can't put it on a weapon.