" Poly " is varnish - the one part polyurethane varnishes are made with alkd resin with a little urethane resin added and are no more water resistant than the varnishes without the urethane resin. Waterlox Original or Behlen's Bar Top are made with phenolic resin and either linseed oil or tung oil and are both harder than polyurethane varnish. I have a dining roon table that has a shellac finish that has been there over thirty years. While it doesn't get the wear of a kitchen table with a little care shellac is a great finish.
True, Poly is a varnish. Suffice to say that old school varnish uses natural resins while poly uses synthetic.
I use shellac quite a bit, usually as a washcoat but have, on occasion, used it as a top coat on things that don't see a ton of use. i.e. a book case, fire place mantel. Easy to work with and I like the results, but never thought it hard enough for more extreme use.
I have to disagree on the hardness and water resistance of poly, respectfully, of course.
Here is what wiki has to say on the subject:
Polyurethane materials are commonly formulated as paints
coats to protect or seal wood. This use results in a hard, abrasion-resistant, and durable coating that is popular for hardwood
floors, but considered by some to be difficult or unsuitable for finishing furniture or other detailed pieces. Relative to oil or shellac varnishes, polyurethane varnish forms a harder film which tends to de-laminate if subjected to heat or shock, fracturing the film and leaving white patches. This tendency increases when it is applied over softer woods like pine
. This is also in part due to polyurethane's lesser penetration into the wood. Various priming techniques are employed to overcome this problem, including the use of certain oil varnishes, specified "dewaxed" shellac
, clear penetrating epoxy
, or "oil-modified" polyurethane designed for the purpose. Polyurethane varnish may also lack the "hand-rubbed" lustre of drying oils
such as linseed
or tung oil
; in contrast, however, it is capable of a much faster and higher "build" of film, accomplishing in two coats what may require multiple applications of oil. Polyurethane may also be applied over a straight oil finish, but because of the relatively slow curing time of oils, the presence of volatile byproducts of curing, and the need for extended exposure of the oil to oxygen, care must be taken that the oils are sufficiently cured to accept the polyurethane.
Unlike drying oils
, after evaporation of the solvent, upon reaction with oxygen
from the air, polyurethane coatings cure after evaporation
of the solvent
by a variety of reactions
within the original mix, or by reaction with moisture
from the air. Certain products are "hybrids" and combine different aspects of their parent components. "Oil-modified" polyurethanes, whether water-borne or solvent-borne, are currently the most widely used wood floor finishes.
Exterior use of polyurethane varnish may be problematic due to its susceptibility to deterioration through ultra-violet
light exposure. It must be noted, however, that all clear or transluscent varnishes, and indeed all film
, synthetic plastic
, etc.) are susceptible to this damage in varying degrees. Pigments
protect against UV damage, while UV
-absorbers are added to polyurethane and other varnishes (in particular "spar
) to work against UV
damage. Polyurethanes are typically the most resistant to water exposure, high humidity, temperature extremes, and fungus or mildew, which also adversely affect varnish and paint performance.