Blob when printing with Prusa Mini+, Prusa PETG, with ironing on top layer only and ultradetail .05.  

Further inormation on BLOB

I tried using .15mm quality and bolb still occured although 2 layers later.  Wierd.  Acts like the nozzle is clogged then spits it out suddenly.  Otherwise the previous layers and following layers print ok.

Just for info: I have mentioned the same issue here.
For me so far, it looks like this problem is impossible (or at least, very hard) to avoid when printing an object that covers large area on bottom. And as you mentioned, it has nothing to do with "correct" first layer, because this continues on layers above 1st layer -and it ends when print continues with details (walls and other vertical protrusions).
Explanation for why that happens is simple (at least from my observations), but that doesn't help much to solve the problem. Actually, the same thing happens (to some extent) with PLA too. With PLA however, blobs don't collect on nozzle, because already printed neighbor lines melt (again) very quick, with filament coming from nozzle, as soon nozzle passes by -not so with PETG. Once PETG cools down, it needs more time to melt again (compared to PLA). And so, when nozzle prints a new line next to old line, and filament can't melt with old line, excess of filament has nowhere to go -it collects on nozzle... and falls down sooner or later.

Right now, I think the only solution is, to reduce amount of extrusion. But I wasn't experimenting with that, so it's only a guess. I'm also about to purchase plated nozzle, which should prevent filament to stick on it -even that doesn't solve the cause of the problem.

Just sharing my thoughts...

BLOB

Thank you for your information.  I have tried a few prints with different settings.  Oddly I printed an item yesterday with a large flat area and there was no problem.  This is just a can cover and it blobs every time in exactly the same place.  I will try it again and have a closer look at the nozzle and report reults back here.

I'm intrigued by @BogdanH's observation. My knee jerk response to the OP's question would have been to suggest a) make sure you have a squeaky clean nozzle and heater block, b) remove any tiny bit of PETG from the nozzle while the printer is waiting to heat the nozzle and bed up to target temperatures, and c) check that the hotend is not leaking. I print some large block-shape objects with large bottom areas all the time, and found blobs to be rare. So I found Bogdan's comment interesting because that may point to other parameters such as temperature, speed, etc. As I mentioned in the link Bogdan was referring, I use nickel-coated copper nozzles but I'm not sure that's the defining factor. And otherwise I stick with the default Generic PETG profile, with some temperature adjustment based on a temp tower run.

BLOB problem solved

Well first of all thanks to the replies to this BLOB blog. 

The suggestions got me looking in the right direction and solved it.  I had done quite a few prints with no problems with PETG until I tried printing the can lid which had a large base.  The print would always deposit a BLOB in the same place whenever I printed it and if I used a different setting, 05mm Ultradetail vs .15mm Quality it always blobbed no matter what but in a different place when using a different setting.  My Z height was set to -800 and I turned it up to -700 and ureka - no more blobs at any setting.  Thanks to all.

@fuchrs
Thank you for commenting -I really appreciate that.
a) my nozzle & heater block is pretty much clean.. there are few small specks of course (is hard to avoid them over the time).
b) I wait patiently with a tweezer while hotend is heating up and remove small ooze just before print starts.
c) 100% no leak on hotend.

...and still, some seems not to have this "problem". But then, every spool of filament can be a little different. Which is why I think, that I should try to play a little with extrusion -so far I was using default settings. Btw. as I'm using Bondtech extruder, I also checked for recommended E-step value (415) and I can say it's accurate enough (my E-step measuring resulted with 411 -so difference is kinda negligible).
I'm aware that plated nozzle can't prevent that issue.. I just hope it would make life a bit easier 😉 

Posted by: @john-13

..My Z height was set to -800 and I turned it up to -700 and ureka - no more blobs at any setting.

Thank you for reporting!
Even my 1st layer is almost perfectly 0.2mm thin, maybe it will help if I make it a tiny bit thicker.

Yes, you can frequently see suggestions that PETG doesn't like to be squished as much as PLA, so easing off on live z makes sense. I must say though that I rarely find it necessary to change my live z when I switch between PLA and PETG. Maybe I'm just a lucky guy 🍀