Around a year ago Stephan from CNC Kitchen on YT did a bunch of scientific tests to show how to best bond layers and attempt to print without inclusions. Maybe check that out. It requires higher temps and super dialed in squish. Printing perfect PETG requires double the amount of tuning and effort as PLA. It also sucks up moisture to the extreme. Like you can print fine with kinda wet PETG, it will make lots of zits, stringing, and terrible seams. If you want it perfect, the stuff has to be dried to the extreme and printed from a drybox. I hate PETG. I'd rather print any other filament if I can. If I really need to use it, I dry it, but I also manually place every single seam on the inside of my part and in the middle of any back walls. Almost all issues with PETG will occur due to or around seams. If you can hide all of these, the outside of the part can look nice. Like I'll go into my design and add a tiny chamfered groove in the outer surface if I have no choice and must have an external seam. Just forcing the extrusion anchor inside the part by a few millimeters can help mitigate the issues. All the problems are due to moisture expanding and causing ooze combined with PETG being extremely sticky when molten.

Are you printing your own fountain pen grip sections? That's wild.

I haven't found a reason to get into 3D printing... but I do love fountain pens.

This probably isn't what you want to hear, but if I need something watertight, I use a material that can be smoothed during post-processing using a vapour bath (I prefer PVB, but ABS or HIPS should also work). Smoothing the print effectively causes it to self-coat. PETG may simply not be the best choice for your application.

It's hard enough to successfully print something that holds water properly. If it's at all possible, the best way to handle it is coating the print with something to seal it afterwards. If it's not, making something airtight would require having quite a few walls and especially surface layers to make sure all small gaps eventually get blocked off.

If i want quick and easy watertight i'll coat it in a paint sealer (or similiar) from the craft store. I have yet to manage printing a watertight print right on the bed.

More perimeters and smaller layer height?

100% agree with adding a coating agent after printing, but if you’re looking to minimize small holes and fissures, consistent line width is super important. High speeds can make thinner lines than when it slows down at starts/stops and corners.

Printing PETG slower than PLA is already a common recommendation, but unless your printer supports input shaping and linear (or pressure) advance, I’d go as slow as you can bear. As a perk, if 245 is the optimal temp for your usual speed, going slower will make 240 work better.