Originally Posted by eastoak
diacetyl is very detectable so if you don't have it you don't need to do anything to remove it. just pointing this out for someone who is new to lagers that may read this and think it's something they have to do.
Actually, that's the thing. In low amounts, most people miss diacetyl. It's a "slick" mouthfeel or an oiliness on the teeth or tongue, but without flavor. If you actually taste diacetyl, that means it's loaded.
And it gets worse with time, not better. So a slight oiliness on the tongue after primary may be a huge butter bomb after lagering, when it's too late to fix it.
And some people are genetically unable to taste diacetyl even in large amounts. So tasting for diacetyl isn't really effective, unless you are an expert at tasting it in very small amounts.
Doing a diacetyl rest as a matter of course, whether strictly necessary or not, won't harm the beer a bit and may help. Even for ales, it's recommended.