protek9543 wrote:
I plan to do the wood in my 15' a little different. First I'm going to stain the wood, then I'm going to epoxy it with West System and then I plan to use a polyurethane clear coat.

HullBlister wrote:


I'm not sure I can agree with the use of poly in a marine environment. The concern here is the cycling of the wood. When wood gets wet it swells, and when it dries, it shrinks. Despite your best efforts, it is VERY difficult to completely encapsulate the wood. Even then different temps can also cause similar cycling.

To the rescue comes spar varnish, and similar 2 part finishes. In the case of the spar varnish, it never completely cures with the net result being that it is fairly flexible. This allows the finish to move with the wood and help prevent cracks and chips from happening (which would allow water intrusion).

Many poly's on the other hand are relatively inflexible and somewhat brittle. In a household environment, this is not such a big deal, but the exposure of a marine location can amplify the movement of the wood. Moving wood + inflexible and brittle finish = bad things.

Additionally, many poly's have little or no UV protection. Marine spar varnish does. UV is responsible for the majority of finish breakdown.

HTH,
Blister

Binkie wrote:
I agree with hullblister. Why anyone would use anything but spar varnish is a mystery to me. If the exotic finish fails after a couple of years it is impossible to remove.

rich

protek9543 wrote:


To each his own. I am not pushing my process on anyone. I have a busy life and 4 kids so anyone who wants to keep stripping and re-applying varnish every year is a mystery to me.

HullBlister wrote:


A couple of points...

1) If you are stripping and reapplying varnish every year you are doing something very wrong. At most all you need to do once you have a good base is to scuff sand and apply 2 coats every year.

2) Automotive urethanes are in fact used in the boating industry by some pretty high profile names like Pershing, Aprea Mare, and Riva. However, their use is limited to interior applications and only on veneered surfaces that have a plywood or starboard-like substrate. Considering the mass production nature of these manufacturers you can bet if they could get the same results with a faster application product like an automotive urethane, they would use it everywhere they could.

3) The ability of automotive urethanes to flax with body panels is completely different than the flex associated with the hydrostatic pressure cycling of wood in a marine environment. Since you claim familiarity with the auto products you mention, you will understand this analogy. if you dent a car, the paint and clear coat generally will not crack, chip or peel without some other force being applied to it. This is because the coatings are supported by the steel (or other) substrate. However, if the dent were to come from the inside out, the coatings have a much greater chance of flaking or cracking. In this case they are not supported, and in fact are being stretched and pulled. In a very similar way, the hydrostatic pressure cycling of wood has an outward expansion that presses against the varnish. Because it doesn't completely harden or cure it has torsional and expansion give that also allow it the follow the wood back down as the moisture content reduces. Automotive urethanes do not have this ability to "shrink" and therein lies one of the major problems with them in the marine environment.

4) An automotive urethane - like varnish - will not last forever. At some point you will need to remove it and start over. This is especially true if there is a breach in the coatings. With varnish, a little stripper and some elbow grease will get the stuff off. With an auto product (and like a 2 part product like Interlux Interthane) it must be removed mechanically. Personally I think that mechanically removal is the best option for varnish, but I am in the minority of that opinion; mostly because of the time reasons you cite in your other post.

5) If I was looking at a boat that had an auto finish on the wood, I would discount the price I would be willing to pay by the cost of removal of that finish.

Blister