This tip is especially important for co-polyester users, but it's good to understand regardless of your preferred filament type. Because I wasn't used to the BigBox infrared leveling scheme I've gotten some rotten prints because the first layer wasn't sticking well enough. This has been particularly true with colorFabb HT. Hopefully my analysis of where I went wrong will help someone. Currently I do everything with the various Eastman Chemical Amphora co-polyester variants: Taulman nVent, colorFabb nGen, colorFab XT, and for the last week, colrFabb HT. (I believe E3D Edge is also one of these.) What all of these have in common is a low viscosity, low surface tension tendency to ooze and string. This is usually manageable by tuning your retraction settings and it has no effect on the first layer except for some mess due to strings. Or so I assumed! The problem is that, after a print when the hot end has cooled, the ooze almost always will have left a tiny bump of hardened filament just below the nozzle tip. This is no problem for the next print, but what if you've changed something and need to rerun mesh bed leveling? Now we are in a world of hurt. Typically, leveling needs to be good to +/- 0.1 mm. I can pretty much guarantee that little hardened droplet of filament on the hot-end tip is thicker than that. This will significantly throw off the feel of your thickness gauge when doing MBL. Long story short, when doing mesh bed leveling, make sure there is no filament residue below the brass nozzle tip. The easy way for me is warm up the hot-end, and then set it to cool-down. I use a metal scraper to remove the ooze until it cools sufficiently that no more ooze appears. I hope this helps someone else.