Hello,
when do you put the airlock on? In the first stage when the beer is in the plastic bucket? Or in the 2nd stage when its transferred to the carboy? Or both? Thanks,
I do it at both stages. Some people say that during primary fermentation so much positive pressure is created by the carbon dioxide production that you don’t need to worry about outside yeasts floating in on the air. However, at the very least, you’ll be keeping out larger outside contaminants like dust, insects, and so forth. During primary you can opt for something less likely to clog like a hose draped into a small container next to the carboy/bucket, with water in the bottom to act as a physical filter.
I’ve done that during the primary fermentation for highly aggressively fermenting beer/beer likely to produce a lot of clogging material, like wheat beers. Every time I start out with an airlock on those, I come back later to find the airlock clogged up with gunk and foaming aggressively. Starting with a hose is so much cleaner.
I do my fermenting in a carboy, and as such I don’t use the airlock until the initial orgy is over, otherwise the airlock just gets all gunked up. Instead of the airlock, I use a PVC or vinyl hose directed right into the bar sink.
Note: you’ve got to check it once in a while. Last year it got clogged and as a result the stopper blew and I lost several cups of beer to the ceiling tiles and carpet :(.
If you have sufficient head space (space above the wort, below the airlock/bung stopper) you can, and should use the airlock through all stages of fermentation. If you feel you may have excessive foaming (krausen), I recommend that you use a blowoff tube.
A blowoff tube is nothing more than a different airlock. I use a piece of vynil tubing that fits one end into my carboy’s neck (I use glass for the entire process, primary & secondary fermentors), then the other end is placed in a mason jar full of water. Some plastic primarys are 7+gallons and provide plenty of space for a standard 5G batch.
Though you’d likely be able to get away with no airlock on your primary plastic fermentor, IMO it’s not worth the chance of infecting a batch of beer. If you are late in placing the airlock on the cover following the vigorous fermentation period, you certainly risk this. You’ve spent $30 or so on the batch, and a couple of hours of your time cleaning/sanitizing & boiling, why risk it?
FWIW, the secondary fermentor isn’t really doing additional fermentation. It’s really a clearing tank, which also provides some bulk aging time. The purpose of the secondary is to give you a clearer beer. If you don’t care about clarity (I don’t really care myself) then you can bottle directly from your primary (transfer off of the trub to the bottling bucket). If you are going to dry-hop your brew, you’d be better off with a secondary aging/clearing.
Most folks won’t have a blowoff using standard pitching rates, and 5gal batches (in appropriate fermentors), but you can run into trouble if you’re making high gravity beers, using a lot of steeped grains, or using a starter. I’ve only had one blowoff, and that was on my most recent California Common, a 1G starter, and slightly higher than recommended temps for initial fermentation.
If you do get your airlock clogged by blowoff, simply remove, clean, and replace… hopefully before the pressure has built up to a point where the airlock leaves the lid. 
I have multiple airlocks so that I never leave my fermentors un-airlocked in the event that one gets clogged, and I need to clean it.
RDWHAHB. 
Relax, have a homebrew.
Cleaner is always best, any chance of wild/diferent yeast strains contaminating a batch is problematic with any fast fermenting product like beers/ciders.
If you are seriously considering pursuing this google charlie papazian.
He wrote a book called The Complete Joy of Homebrewing, pretty much the bible of homebrewers worldwide.
YMMV I’m a mead/melomel guy, who has a batch working on 2 years in the carboy. Starting to sound like time to bottle.