Greetings fellow Space Engineers!

This guide is about creating an airtight docking portal between two large grid ships, or a large grid ship and a station.

Q: Well that's nice and all W0lf3y, but, why do I need an airtight dock?
A: Because ice is a valuable resource! /sarcasm

Merge blocks will be necessary. You simply cannot create an airtight seal between two different grids without them.

Q: Won't that make them one grid then?
A: Yes. That's the goal.

Q:Well, won't my ship become a station then?
A: It may. And depending on map settings and what your docking with. Once you you turn the merge blocks off, your ship will either automatically become a ship again or you may need to open the info panel and trigger the option to "Convert to Ship"

There are several blocks you can use to achieve a seal. The most important aspect of them though is that they lack mounting points on the appropriate edge(s).
You can use:These all lack mount points on the oposing edge.

The goal here is to have ONLY the merge blocks holding the two grids together. If your ships / stations won't separate after turning the merge block(s) off, then either you messed up, or you need to use the "Convert to Ship" option.
Here's a few images of sample constructions.

and the examples of what it will connect to:
Airtight Hangar Door's:
45* Slopes:2x1 Slope Tips:

These are not the only way to create a sealed dock, they're just examples I cooked up within a few minutes for writing this guide.

I'd like to thank you for taking the time to read this guide.
I hope you find it usefull in your future creations.