Probably won't help with your varnish worries, but my advice is to skip it (assuming you have a few coats already) and get that boat on the water.  I never even varnished mine and it was quite attractive and held up for 5 years stored inside and outside (uncovered) before I brought it in to retrofit a new deck on it.

Have you thinned your varnish at all? 

FrankP

Are you using the better quality foam brushes?  Have you tried a touch of 333 in the varnish? Maybe a slightly heavier coat that has time to flow out. How about using a good bristle brush for the last coat.  I think varnish can be vindictive.

I have found that any varnish problem I've had has been from the epoxy (mostly curing times). Your varnish is getting contaminated somehow. Either the thinner you're using doesn't mix right (I've actually have better luck with regular mineral spirits then 333 with Schooner), something your using to clean the wood in the prep or the actual surface, whether it's epoxy or oily/waxy wood. I'd do test patches and try different combinations of thinner, foam brush, roller, varnish brush, etc.

I've tried four different varnishes with combinations from full strength to thinned with recommended thinners (and non-recommended). And the only thing that changed the outcome was stripping the varnish off and scrubbing off the epoxy blush (the silent killer). The only time I had problems with non-epoxied wood was when I coated it with boiled linseed oil first, then tried to varnish over top that. 4 years later and I still have problems with that varnish adhering.

Thanks.  Actually Interlux was really helpful.  See thier response below. Frank, you're prba bly right about just getting it in the water, but I worked too hard to run my hand over the deck and feel a pitted surface.

(From Interlux) Hi David,

We are sorry to hear about the bubbling issues which you are experiencing. Based off all of the information which you have provided there are 3 potential culprits which would be creating these bubbles. The culprits would be:

1.     Excess moisture/residual moisture on the surface as the varnish is being applied. If the varnish is applied overtop of moisture, you would likely experience bubbles as the moisture begins to escape but essentially ends up getting trapped within the paint film, you could also experience extended drying times/possible lack of gloss. (Allow the surface to dry for 1-2 hours or more prior to wiping down with the 333)

2.     Excess 333/residual 333 on the surface as the varnish is being applied. If/when applying a varnish onto a surface which still has remnants of the 333 due to the slower evaporating properties of the 333 you would essentially trap the 333 onto the surface and as it attempts to naturally evaporate it gets trapped within the paint film and creates these bubbles. I would recommend that you allow atleast 1 hour to pass on by from your wiping down with the 333 prior to beginning the application of the varnish.

3.     Foam brushes are sometimes a bit too stiff and could very well agitate the coating a bit more then ideally desired. When over-agitating a coating during the application, you end up introducing bubbles into the coating which will likely remain in the surface. Therefore if using foam brushes, I would recommend that you keep your brush passes to a minimum. Or you can switch on over to a china bristle/badger hair brush which will help to lay the varnish down a bit more.

Also, not sure if you are thinning the varnish at all, but if not I would add 5-10% which will help to assist with the application and flowing of the varnish as well, 333 would be plenty suitable for this.

Hope this helps!

All the best,

Jay Smida

Interlux Yacht Finishes

After sanding I've taken to wiping the boat down with 333 on a rag, then using varnish thinned with 5% mineral spirits and a splash of penetrol (a flowing agent) applied with a foam brush. This on the advice of Joey @CLC and it works great!

Dan

I second what danthaler said above on thinning with mineral spirits and penetrol. It worked very well for me and took the worry out of a great looking varnish finish.

I also needed to thin my finishes in order to get them to flatten out. Never having done this before I wasn't sure how to get a certain % of thinning liquid. I've been just dumping a little in. 

How do people measure the amount of thinning solution used?

While others may use a graduated cylinder stolen from chemistry class, I use dixie cups.  I use the cups to scoop from the varnish can, pour into a paper filter that sits on top of a cheap clear plastic cocktail cup. One dixie cup for varnish and one for thinner.  I'll usually use 2-3 scoops of varnish and a quarter to half scoop of thinner or whatever ratio I want to use for that period.  I try to get the wide plastic 6 oz cups so my 3 in foam brush easily fits in it. Don't use those same cups for epoxy, they'll melt in place. For that I use chinese take-out soup containers.