My inclination is to put varnish where it goes and paint where it goes, and not mix the two. Otherwise you've got a can of worms with the compatibility between the varnish and paint. 

Are there known problems with the Interlux products, assuming the varnished surfaces is clean and lightly sanded?

Sounds like a waste of time and varnish to me, covering varnish with paint.  I did paint on the hull of my 17LT then varnished the deck and it came out great.  Here is what I suggest:

1.  Mask off the areas to be varnished.  Make sure you use good fine line tape on the edges.

2.  Prime and paint following the directions on this site. Be careful when sanding near the masking tape so that you do not harm it.

3.  When all the painting is done, remove the masking tape and remask for varnish.  Take your time and be very precise with the fine line tape so the you leave just the smallest amount of paint showing at the seam.

4.  Varnish per the directions.  Again, be careful sanding near the masking tape.

5.  When done varnishing, remove the masking tape and look at the seem.  If you were careful when masking, it will look good with only a very small overlap.  On my boat, the overlap is about 1/16th and barely noticable.  I had originally planned to cover the seam with automotive pin striping but decided it looked fine without.  

my experience is there is no problem painting over varnish if it is properly sanded/prepped.

that being said, i would not do it over broad areas - why waste the varnish.  but i would recommend the approach when applied to the deck of a kayak (or a shell) and the hull is painted but you want to bring the paint up to to wrap onto the deck for an inch or so....as shown in many of the clc pictures. 

in the case of the deck example referred to above, you would only apply the varnish to the deck with no masking of the hull becuase the deck/hull sheer line allows you to easily avoid getting the varnish on the hull.   you would then mask off the deck with fine line tape where you want to apply the paint....and then lightly sand  the varnish and sheer line (use a scotch pad, not sandpaper to avoid harming the fine line tape) and then paint the hull and hull/deck overlap as part of the same paint application process.  

the main advantage of this is only one taping is required vs the other approach which would require taping twice.   inevitably, unless you are a total pro, you are going to have overlap, and this approach only creates one line where as the double taping will create a bump between the two taped lines. 

this approach is faster, uses less fine line tape and has less chance for mucking it up by adding additional steps.   the fineline tape over a varnished surface also tends to create a very crisp line as the varnished surface is nice and slick and creates little opportunity for the paint to get underneath it.  

the final advantage, in my experience, is its very easy to wipe off any paint that drips on the varnish with a quick swipe of a paper towel with mineral spirits.   spilling paint on a 'prepped for paint/varnish' surface is harder to clean up. 

anyway...that's my 2 cents.

Howard

Thanks for all the great advice!

I should have mentioned that I only want to paint a narrow stripe on edge where hull and deck meet. There it seemed easier to apply  the paint last after the varnishing was all done. I fully agree that it would be a waste of varnish to do this for larger areas.

In that case, I think you should let the varnish cure for about a week, and that's just a guess. Varnish continues to cure for a year, give or take a month or two. But in the first few days, it's going through a lot of physical and chemical changes, wihat with solvents evaporating and oils crosslinking and all kinds of things I don't understand but have read about. If you rush it, you may find your paint doing strange things, like wrinkling or pealing off in a sheet.

Artists have been varnishing over paint for several hundred years...that's what's on a lot of those paintings in all those museums...but in many cases, they're using the linseed oil and alcohol and pretty basic, known stuff.