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In article ,
says...
I used to prefer(ed) to lay down the first few (4-5) coats with tung
oil based varnish for 'penetration' , let fully cure; then a few coats
of urethane based varnish for UV protection, let cure a month or two
then apply a 2-part clear (Interthane, etc) on top of that for
durability ..... and 3-4 years later remove it all when the first coat
lost 'adhesion'.

Just an observation.

There are two based types of Varnish. Water or oil based. The difference
is that water based is thinned with water and the oil based with mineral
spirits, paint thinner. Even if not stated you can tell the difference
by looking at what is called for in the clean up instructions on the
can.

All oil based varnishes contain tung oil or some other type of curing
oil. These days it can be a modified soy oil of some sort. The resins in
a varnish, without the curing oil, would form an extremely brittle and
easily damged surface. The oil modifies and softens the final results
making it less brittle.

Penetration into the wood is dependant on the viscosity of the varnish
not the type of resin used. I agree completely on sealing the wood with
a coat or two of varnish thinned fifty percent. However, once that has
been done and the coats cure, there is no further "penetration by a
finish no matter how much it is thinned. The wood cells are already
sealed.

Urethane and polyurethane are simply the types of resins used in the
varnish deliniating them from non urethane varnishes Non urethane
varnish can use natural resins, rosin, amber, or man made resins that
form a weaker chemical link then the urethane resins.

Spar or marine varnish, in addition to UV inhibitors not found in other
varnishes, is what is termed long oil varnishes. They contain a higher
ratio of curing oil to resins so they are more flexible and better able
to accomodate, without cracking, the higher level of movement that wood
experiences in an outdoor enviroment.

The use of a non spar/marine varnish under coats of spar varnish can
actually defeat the purpose of using a long oil varnish by failing and
cracking long before the a spar varnish would have if it was used
without the undercoat of the short oil varnish.

NOTE; I am just giving some observations of spar and non spar varnish. I
am unfamiliar with the acrylic finish you refer too and am not making
any observations in regards to an acrylic finish Vs the spar/marine
varnish one finds off the shelf.

Take care

--
Mike G.
Heirloom Woods

www.heirloom-woods.net