Ed,
Sorry for the delayed response, I was a bit busy...yesterday was my wife's Birthday.
I second Darwin's suggestion, it is always best to test something before spraying up your templates, and it also gives you an idea of how much paint is discharged using the spray can. It's all too easy to over do it, just light coats is good, and, yes, it's best (at least for me) to do it before assembly. I haven't found any real problem with PVA bonding on the coated cardstock using the acrylic matte spray; can't say whether a gloss or other product would have the same results, though.
As to the barrels, JSC seems to rely upon the use of outside materials for some parts, like wire for some of the masting and flag staffs and wood for barrels, etc. If you want, you could try rolling them using regular weight paper around a thin wire or pin. I have a pin mounted in a 1/4" diameter dowel that I use for lots of things, and rolling small tubes is one of them. I used this to make the carronades on the Constitution. You can also use brass rods for the same purpose, and they come in different sizes. It might take a bit of practice to impart the slight increase in diameter to the tube that your barrels might require, but it becomes easier as you do them; I found that out after making about 20 barrels for the carronades. :lol:
Also, you can add a shorter strip at the breech end where the barrel is stepped; the JSC illustrations should give you the right proportions to work with. Of course, you can either use paper you've pre-colored using your printer or paint it after the barrel is done. One benefit is you should end up with a hollow end, although perhaps not exactly to scale, but I personally like to see barrels with an open end...unless, of course, you are intending to add a tampion to the end as some ships use (that's the round cover that goes over the end of the barrel when the gun is not being used, for those who were guessing what the heck I was talking about

).
I also like to coat the whole thing with a bit of white glue to firm it up a bit, but I do that while it's still on the rod I rolled it on to hopefully keep the barrel straight...you should probably paint the body of the barrel while still on the rod too, touching up the ends when that paint has dried. I'm sure there are other ways of doing this, but this is the way I have been doing it lately. I think you get a smoother surface on the barrels when doing it this way than a wooden dowel/toothpick might give you, but I would suggest you try making the barrels a few ways and see which one you like the best.
Well, hope this is not too late to be of any use to you, Ed.
Cheers!
Jim