I've been a pro varnisher for eighteen years. Even among professionals there is no gospel when it comes to brightwork. One will tell you that a 'badger' bristle brush is the only kind you should use, and another, like me, will say that's nonsense and tell you that foam brushes
are just as good.
Geographical location does make a difference in application methods-- most of my varnishing experience has been in Florida
First, I'm no fan of water-based finishes. From what I've seen, they're sensitive to atmospheric humidity, and will 'blush', or cloud if humidity is high. Also, remember that humidity typically is highest just before the dew point is reached when the air is saturated in early morning or evening.
The bubbling in your Bristol finish might have been caused by residual moisture trapped in the wood, released when the sun hit it. That's one advantage of using a heat gun to strip old finishes-- the heat drives the moisture out of the wood.
One common mistake novices make is not thinning whatever varnish they're using. It's almost impossible to get a level mirror-like finish with no brushmarks or lap lines, without thinning.
Often, the instructions on the can will tell you not
to thin, in bold-faced letters, or to thin no more than 5%. Just remember that it's a manufacturer's disclaimer, intended to prevent the user from altering the chemistry of the product, which of course, they cannot be liable for.
But even when varnishing indoors, but especially outdoors in the sun, the varnish will wrinkle, or 'alligator', because the top of the liquid will dry faster than underneath, and shrivel. Thinning prevents this action.
I hope this is helpful, and I invite any interested readers to visit my varnishing blog: Varnish Teak