what brand of "outdoor varnish"?

I’d take it back

Also, is it marine spar varnish?

It is Varathane Ultimate Sapr urethane waterbased clear satin

Advice from "the guy in the store" is often worth what you paid for it. Some are knowledgeable, some have no idea outside reading the tint formula for mixing paint. Urethane, in general, is not very UV resistant. But putting "urethane" on the label can indicate just a small percentage of resin in a water-borne finish. These are usually primarily acrylic. But I am not familiar with that brand. I do know that Minwax Helmsman is just a joke for outdoor work. A few years ago, when there was a Wood Magazine Forum, this was common advice posted by a couple of the forum hosts. It used to be posted on the individual's site when Wood closed their forum, but since he's retired, it' gone 404.

I might add I did some work for Steve, the author, when a "painter" messed up a varnish job in a sun room in the most expensive towns in the state. He recommended McCloskey's Heirloom. I went back several years following the initial work when their door failed due to the door maker's defect and the original finish was holding up fine. Unfortunately, that varnish, due to M&A is no longer made. It was a non-poly (alkyd + soya) varnish.

While this was oriented toward "exterior varnish", UV is the enemy of varnish and this would work equally well for a sun-soaked interior window.

Paint On A Clear Exterior Finish
We woodworkers (especially when it comes to finishing) are creatures of habit; we use the finishes we use because they are the finishes that we have always used. But, from time to time it's good to step back and examine old "truths" just to see if they really are true. The use of marine varnish to finish exterior elements in non-marine applications is one of those "truths" that has long needed to be revisited. That is the purpose of this article. Several years ago when I began participating in the WOOD Magazine Finishing & Refinishing Forum we regularly saw questions from homeowners and others asking for advice on the best way to finish exterior doors, especially those exposed to the weather and subject to high UV. The options then were Helmsman Spar Urethane (and its polyurethane look-alike competitors), expensive marine varnish, and exterior paint. Over the years little has changed except that, thankfully, more and more woodworkers today understand just how poorly any finish that contains urethane resin will perform when exposed to UV. Even with only partial exposure to direct sunlight polyurethane will fail quickly, often peeling like a bad sunburn before the end of even a single season. Marine varnish, then as now, was the recommendation of choice offered by a number of contributors. There is no arguing that quality marine varnishes will outperform polyurethane "spar varnishes" if UV resistance is the only objective. But, good marine varnish is very expensive; and, in reality it offers no reprieve from regular, on-going maintenance. You must still tend to the finish every year in full-sun environments; you must inspect, sand damaged areas, and recoat. Further, even those who regularly recommend marine varnish products will tell you that a minimum of 5 or 6 coats is required to obtain the full benefit of these finishes. So, not only are you applying two to three times more varnish; you are applying a product that
costs two to three times more, and your maintenance schedule is unchanged.
Further, it is important to understand that marine varnish is "long-oil" varnish; varnish that is softer and much less resistant to moisture in the form of water-vapor than regular or "short-oil" varnish. Moisture movement into and out of the wood with seasonal changes in relative humidity is every bit of destructive to joinery as
UV is to wood. These quality marine varnishes are excellent finishes in their intended environment. If I owned a wooden boat I would use nothing else. But, we are not talking about maintaining a boat; our objective is to apply a durable finish to a front door and to use a finish that will offer maximum protection along with minimum maintenance. Quality oil-based exterior paint, sans the pigment, is ideally suited to this application. Exterior oil-based paint, after all, is little more than exterior oil-based varnish with a lot of pigment added. Remove the pigment and you have a very durable exterior varnish with additives that benefit the finish on your front door.

These additives, intended to discourage insects such as wasps and wood boring bees, and prevent the growth of mold and mildew, would be useless in a marine environment; but, we aren't talking about a marine environment. We are talking about your front door. With this as a background permit me to introduce you to my friend Jim Kull. Jim was the owner of a successful refinishing shop in Southern California prior to his retirement and move to Texas. His retirement gave him a bit more time to experiment so he conducted and posted the results of the following test on the WOOD Magazine Finishing & Refinishing Forum where he served as the host. When Jim decided to step down from his host duties he was instrumental in my becoming host of that forum.Here then is Jim Kull's original post edited slightly for clarity:
"In a recent post my friend, Steve (Mickley), made reference to my tests of
doggie sprinkling on exterior finishes. I figure after almost a year of testing it is
time to post some interesting discoveries. As a preface, allow me to set the
stage. Almost daily there is a posting about clear, exterior finishes for doors,
chairs, signs and such. Responses run the gamut from diehard marine finishes
to apply a coat of primer and then paint. Each of these has a bit of a
problem. Marine finishes are not always the easiest to find, and it grieves me to
think of a lovely oak, teak, mahogany, fir, redwood or similar nice wood door
painted in mauve goop.
Bob (from Florida) inspired me with his continuing and accurate statements
about the failings of a clear coat and the advantages of a good quality exterior
paint. I decided after lots of reflection that he really was right but there was
always the picture of mauve in my mind. So, how could one take advantage of
his advice and yet capitalize on the beauty of a nice wood? I began to reflect on
the characteristics of paint. Now comes the boredom...
There were several things I knew about paint:
Exterior paints contain a mildewcide and a fungicide that a (marine)
varnish does not.
The best quality paints will contain a UV (inhibitor) and trans-oxide
pigments in very high percentages.
Almost all paint is custom mixed by the store. The retailer maintains a large
supply of base products that are used to achieve the desired color.
There are generally four base products and the specific one for your paint is
determined by your color choice. These base products are either named or
numbered. They are named pastel, deep, tint and neutral. If numbered it is
cleverly 1, 2, 3 and 4 with the exception of Olympic who numbers 1, 2, 3 and
5. Olympic is unaware that "4" comes before "5". Pastel and/or 1 is virtually a
pure white and used for the lightest of colors. The others are slightly color
altered from white and more translucent than pastel. These are used for
succeeding deeper colors. All of this comes to neutral, 4 and/or 5. [ed. note: some vendors call this deep base and extra deep base, e.g., Sherwin-Williams] These are
clear and used for (mixing) the darkest colors. In the can they are somewhat
opaque but dry more or less clear.
Now comes the testing. I bought 4 oak exterior doors. Each door was given one
coat of the same MinWax Stain. On 3 of the doors, I applied 2 coats of "base"
to the 6 sides of each door (3 coats on the top and bottom edges). Each of
these three doors had a different type of exterior neutral, 4 or 5 base. The
fourth door was finished with a consumer "spar" varnish from my local friendly
paint/hardware store. The bases for the 3 painted doors were an exterior
semi-gloss acrylic, an exterior semi-gloss oil-based polyurethane floor paint,
and a semi-gloss oil-based trim and siding paint.
The doors were set up, slightly inclined, in mostly direct sunlight under a pecan
tree in the backyard. (My wife just loved that one.) Daily, the sprinklers
managed to hit the doors. The birds in the pecan tree used the doors for target
practice. And, yes, the dogs did anoint the doors on a regular basis. My blonde
Cocker, Zazu, was particularly enamored with the doors. Over the course of
the test the doors experienced lots of Texas sunlight, rain and snow. The
temperature went from below freezing to over 100. The advantage to the
inclined position of the doors was the snow, ice, water from the sprinklers and
the rain tended to collect in the raised panel areas. I feel these doors were
subjected to far more severe environmental conditions than would be expected
from normal use.
The results were interesting. The "spar" varnish (initially) looked fabulous; but,
after about 2 weeks it began to develop small cracks. In rapid order the door
began to turn black, started to mold and the smell was enough to knock a
buzzard off of a manure wagon. The water-based acrylic is milky in the can
like a water-based poly. It dried to a more or less water clear surface but was
a bit cloudy. It tended to wash out the stain a bit. Over time it became
cloudier and ultimately become almost white. But, it remained solid and
protected the wood. The oil-based bases are also a bit opaque in the can but
dried to a clear finish that is almost identical to a spar varnish - they added an
amber tone to the doors. Both the oil-based poly floor paint and the oil-based
trim and siding paint remained "clear" over the entire test period.
The testing came to an end with a bit of encouragement. My wife said
something clever like,"Get those damned doors out of the backyard!" She does
not understand science. The floor poly had some minor checking and a thinned
coat of the same base over the surface made that disappear. The door with the
oil-based trim and siding paint was perfect, other than it had lost a bit of the
gloss.
So, I am with Bob - paint the door. My preference is the oil-based products. If
you are predisposed to a water-based use an acrylic rather than latex.
One thing you will find when you go out shopping for your product is a lack of
knowledge on the part of the salesperson. Not many of these folk are aware
that their neutral or 4 base will dry clear. If you want to have some fun,
spring it on them. They will suggest you are full of Donkey Dust. Ask them to
shake a can and put some on a stir stick. Dry it and voila, it is clear."
Jim Kull
One final admonition; if you decide to try the paint solution you must understand that you are applying it like varnish, not like paint. Use a good natural fiber brush, keep your coats thin, (emphasis added; keep the coats thin! We recommend thinning with paint thinner to improve flow-out and leveling.) and brush the paint-base out into a thin, uniform film. If you apply the paint-base too heavily you will get a cloudy finish.
Addendum to Earlier Article
Several important things have changed since this article was originally written. Perhaps the most important development has been the advent of low VOC products. Many of the oil-based exterior paints still on the market have been reformulated to meet the more stringent VOC requirements. While these products will still work it is important to understand that thinning these products introduces new requirements. The low VOC paint bases can not be thinned with mineral spirits/paint thinner. You must thin with naphtha. If you do not; if you attempt to thin with traditional mineral spirits/paint thinner the finish will remain tacky literally for days and will never cure properly.
The second and somewhat more frustration development is that oil-based products are becoming more and more difficult to find. They are available, primarily in paint stores that cater to the trade; but, they will require more searching, possible even beyond city, county or even state borders. The oil-based paint bases remain superior. Even though some water-borne finishes will work they simply will not last as long.
Copyright 2003-2010 Steve Mickley, Copyright 2007-2010 Hardwood Lumber & More…Ltd. All rights reserved.

One final admonition; if you decide to try the paint solution you must understand that you are applying it like varnish, not like paint. Use a good natural fiber brush, keep your coats thin, (emphasis added; keep the coats thin!
We recommend thinning with paint thinner to improve flow-out and
leveling.) and brush the paint-base out into a thin, uniform film. If
you apply the paint-base too heavily you will get a cloudy finish.