I've had the opposite experience.
First tried bristol out about two years ago. The exterior version. Did a bunch of work - other boats included - and the finish is still as deep and rich as it was the day I put it on. One of the benefits to the Bristol Finish polyurethane finish is its quick cure time. Traditional varnishes usually have a minimum cure time of 12 hrs, with sanding before laying another coat on. Sure, you can sometimes get it faster than that with japan drier or other accellerators, but you still have to wait till the next day to get another coat on. Hopefully the weather is still agreeable. With the Bristol, you can usually start on the second coat but the time you've finished the first coat. First coat I usually thin 50/50 with MEK, applying a very thin coat. Then comes a full-strength second coat as soon as the finish is such that when touched, you see a fingerprint, but nothing comes off on your finger. Rinse, repeat. At this rate, on a good day you can easily get 4 coats on in a single day, eight in a weekend. So long as you don't go more than 48 hrs between coats, no sanding is necessary. However, for the final coat I give the whole thing a good once over with some 240 grit, then do the final coat.
As for the bubbles, tipping with MEK before they get too cured will alleviate any sanding issues.
And a last note: direct sunlight, as with any varnish project is not recommended. Rig up some kind of tarp if you can to keep the direct sun off, and that will also reduce bubbling.
Every six months or so, I sand a layer or two off and put on 3-4 more coats.
It's worked great for me.
I don't have the patience any more for traditional spar varnishes.
cheers,
Jay
the photo is about coat number four on the original round of coats. It still looks as good.