Definitely looks like water exposure to me. Did you have a lot of humidity and dew where the boat was kept? What kind of cover did you use? Was it watertight or breathable?

Laszlo

how frustrating.

i agree with laszlo, looks like moisture....and maybe an intersection with freezing and Pre-kote.

the first thing i would want to know prior to suggesting a fix is, what is in the 'bubbles'?  you describe them as hard....but if you cut into them....does water come out?  is it an ice bubble?, is it white chalky substance (e.g., pre-kote)?  after cutting a bubble off and you sand the area ...do you get to clean, pristine glass/epoxy or is it deeper than that?  we are focusing on determining if it is it just the paint layer or is there any involvement of the underlying glass or wood..... and excising a couple bubbles and looking at whats in them or below them will help determine the best corrective strategy.

if you could do that and let us know....we are here to help.

   It was stored outside in the PNW, so yes, a fair amount of dew, moisture from the ground splashing up etc.  It was under a plastic tarp, not air-tight strapped, just draped over the hull on the trailer. 

When I scrape the bubbles off, it just looks like a little pit with some more paint down at the bottom, not all the way down to the epoxy coat.  It is not chalky or icy or wet inside there as far as I can tell. I haven't yet sanded them down yet so we'll see what I get down to when I do sand the hull in the garage.  

I'm not sure the best way to prevent it in the future if it's just to atmospheric moisture besides keeping it housed in a building or garage, but I don't have enough room for that, alas. 

I guess I hoped marine polyurethane paint wouldn't be so fussy? 

Hi JFtee, 

fwiw....marine poluyethane is not particularly fussy.....but it is top-side paint so the only issue you should have with this kind of paint is really about sustained water contact.

i have poly painted boats that i have kept outside in my unheated garage for years and also get a lot of paddling time....and have never had a similar paint problem.  that said, they are not typically exposed to dew on a recurring basis.

but based on what you have shared, i would not give up on poly just yet.  based on what you described, there may have been an issue in the initial paint application ….especially the observation that the blister may be between the layers of pant applied.

intelux has a checklist related to blistering that I would read through:  https://www.interlux.com/en/ca/support/boat-paint-problems/blistering-bubbling

h