I've never had such good first layers. Replaced my warped ass Ender3S1 bed with a mirror.
I'm actually not afraid of printing things larger than a few square inches on the bed.... Waited way too long for this. Tightening up the eccentric screws on the bed carriage so it doesn't wobble also helps.
@bdonvr you've probably heard this before...... But magnetic PEI build plate is the best adhesion I've had.
Used glass on my CR10S but it's night and day vs PEI on the ender 3 S1 plus.
Keeping it clean is key but copious quantity of IPA and a very light sand has worked so far.
That's what I was using before, but my problem is my bed is fairly significantly warped... Inflexible glass fixes this entirely and I've had absolutely no issues with adhesion. At least not with PLA or TPU.
Although thinking about it I could clip the PEI to the glass giving it a flat bed...
I've been using a 1/4" thick sheet of carbon fiber for years now. Once warmed up to temp prints stick hard on it. As soon as it's cool a breeze can push a print of the bed. Plus it won't shatter like glass does eventually, boy was that a surprise. You still have to wipe clean with some rubbing alcohol before printing.
Although thinking about it I could clip the PEI to the glass giving it a flat bed…
Having had a similar issue, actual bed more warped than a TV preacher, and a dead, impossible to replace leveling sensor. I moved to a glass bed. But, now that you mention it, this seems like a great way to get then PEI adhesion and have the bed actually level. Just ordered some larger clips and I'm gonna try this out.