Soloing is dead boring, though. I get a kick out of saving a teammate's hide, not dooming them. I love seeing a guy who has been split up from the team, standing between us is a bunch of clots, crawlers, gorefasts and a bloat. I nail the crawlers close to them through the pack and then charge through with the shotgun, largely disregarding my own safety to get through the pack so I can link up with them, give them a heal, then turn 180 and clear a path back to the group.
Anyway, welding. When planning to hold a zone, you want to give yourself room to move and direct everything towards 2-3 definite entry points. Any less and it will be very hard to keep them back. Any more and the team will have to split up too much to hold every front. To that end it's often effective to weld doors further out from the area you're holding.
Example: Holding Biotics spawn zone + right. Entry points are left from spawn, left from main corridor, end of the main corridor, trader hallway (both end and above). What you want to do is prevent/minimise specs coming from the left side. Leaving the right open is recommended since welding is useless if the glass gets broken, and all specs there will be coming from trader hallway anyway, so you can form a looped run for kiting between main and right so long as you can keep it clear. This way the majority of specs are coming from the short stairs and the trader hallway, and depending on your position you can have 2 teams with room to retreat when needed. If the door left of spawn gets busted your best option is to charge through there and move through the rest of the map, assuming there isn't too much left in the wave when that happens.
But how to secure the left side is important. You have 3 doors for 2 entry ways to the same corridor left from the spawn. Some people holding the area will just weld the door left from spawn, letting the other door funnel more specs into the main corridor. This crowds the main corridorand difficult to hold and killing your open loop. So often the left door in the main corridor will also be welded.
But if you enter the room and weld the door to the left hallway instead, you're blocking the spec's entrance to that room and keeping more distance between you and the welded door, leaving them more likely to path further down the hallway. You can weld the door on your side to buy time should the door on their side get knocked down, or you can leave it open since ammo pickups occur there. I find the latter works better; whenever you're checking for ammo, check on the door. If it's getting battered down, consider welding the door in the main corridor at that point.
There aren't an awful lot of double door situations like this, but they serve more purposes than just increasing the time it takes for specs to reach you. The reason doors welded randomly don't get knocked down during the wave is that specs will take a different path rather than knock down a door if it isn't going to send them halfway across the map to do so.
