I am fermenting a oatmeal stout and it has been three weeks. Thinking I should have done a secondary after doing some reading. Is it to late to do so?
You don’t need to do a secondary. Check your specific gravity now and again in 2or 3 days and if at final gravity, call it done.
Your stout is most likely done fermenting after three weeks in the primary. In this three weeks the beer most likely has cleared of excess suspended yeast and particulates. Just the same as it would in a secondary vessel. When you rack just hold the tip of the siphon just above the yeast/trub layer. I find it helpful to slightly tilt the fermentor the day before racking. I use a piece of 2 × 4 to do this. I think it helps getting more beer out with less risk of disturbing the yeast bed.
There is a LOT of debate over the use of a secondary. Really, it kind of comes down to personal preference unless you’re doing something like bulk aging or the like. Part of the reason for the debate is the risk of contaminating your beer or oxidizing it. I’ve tried it both ways, and my personal preference is to risk it and transfer to a secondary. I believe I get a cleaner, clearer beer that way and it works for me. I usually leave beer in the primary for 2-3 weeks before transferring to a secondary, and it usually stays in the secondary for 2-4 weeks. Realistically, that’s longer than most beers need before bottling, but the only way to tell for sure if a beer is done is to take gravity readings, which are important whether you secondary or not. Take a reading, wait a couple days, then take another. If it’s the same, you’re most likely done and can bottle it.