Urethane vs Varnish

[masonc]

My paint supplier recommended two part polyurethane, whats the opinion on that? I bought some but never got around to using it.

[redsky49]

Urethane or varnish? Maybe you mean traditional oil-based (tung or linseed oil) varnish or polyurethane?

There are not great differences, but certainly some notable minor differences.

In my opinion, there are certain clarity advantages to traditional varnish. These are subtle, but noticeable by comparison.

If varnishing a piece with carved portions for example, I have better success applying the film thick enough to protect, while not filling in the lower areas, with traditional varnish.

There is a definite color tint with traditional varnish that ranges from light amber to almost a rust red. These begin to make a difference in multiple coats. Polyurethane is typically clear, with any tint coming from either the flattening agents (if used) or the UV shielding additives. Some polyurethanes are also water based. I am not aware of any trad. varnishes that are water borne.

It is easier (faster) to recoat a traditional varnish (there are exceptions). Polyurethane varnishes demand a scuffed surface as the prior coat does not dry the same as trad. varnish. Trad varnishes dry in a typically linear fashion. Poly's seem to sort of "kick off" at a certain point. It can be more difficult to maintain a "wet edge" with polys.

Trad. varnish dries due to the evaporation of the solvents. Poly dries due to a chemical reaction once exposed to air and moisture.

Speaking with factory reps, the consensus seems to be that the better polyurethane blends seem to be 15-20% tougher film surface hardness than the better resin varnishes. This does not include products such as gym floor finishes which are not suitable for marine use.

Spar varnishes are formulated to account for the expansion and contraction of the underlying wood. They (spar varnishes) are typically used on the exterior. Note that the same movement of the wood, occurring on the outside of the boat, will also occur inside of the boat. While varnished exterior teak or mahogany is normally monolithic - toe rails, eyebrow trim, etc, - interior woods are frequently used in components constructed from multiple pieces, with multiple joints. These joints will also open and close based on temperature and humidity, potentially affecting your finish in a negative way.

Poly varnishes are more brittle than trad. varnishes and more subject to this separation at joints. Still, they may be a better choice for table tops and counters than trad. varnish.

I have used turp for thinning trad varnish, but generally use mineral spirits and/or brushing thinner with polyurethane. 240 to 320 grit sand paper between coats.

While I have had favorite badger hair brushes that I have loved, used and preserved for years, when in a hurry I am quite satisfied using foam brushes. This is only true though with the USA made Jen brushes. The Chinese brushes are worthless.

For critical or final finish coats I also usually decant and strain the varnish. Give serious thought whether you really want to return the unused varnish to the original can. And use a tack cloth! Twice!

I spent several years doing custom furniture finishing, working my way through college and after, including some museum restoration pieces, and have continued working with varnishes, as well as paints, for going on 45 years. This is based on my experiences. Offered as my opinion.