Can You Dilute Spar Urathane....

[Dennis McDonaugh]

without reducing the UV protection? I replaced the slats on a couple of park benches with new white oak slats. No stain, just planned on three coats of spar urathane. I'd rather wipe it on, but its kinda thick and I'd like to thin it. Is there any detrimental affect on the UV protection? I figure I'll have to put on a couple extra coats to make up for the thin hand wiped coats.

[Jesse Cloud]

I'm not a finishing guru, but my understanding is that the UV protection in varnish comes from the solids, so thinning it will decrease the protection. Having said that, I like to put on a thin first coat, followed by a barely thinned second coat, followed by a coat or two at full strength.

[Steve Schoene]

Two things happen when you thin a finish. Each coat in liquid form goes on thinner, and when the thinner evaporates the remaining solid are fewer. Therefore the thickness of one coat of brushed on "full strength" varnish would require 3 or even 4 coats of wiped on varnish thinned 50:50.

Since the UV protection depends on the thickness of the coating, thinning doesn't call for just a few extra coats but 3 or more times the number of coats. But, if you are willing to do it there isn't any reason not to thin the varnish. (You need to thin it a bit even to brush, and probably should thin the first coat or two to increase penetration.)

There is another problem--"spar urethane" is an oxymoron. If you really need UV protection--like for boat brightwork or outdoor furniture that is in full sun all day, then you want a better varnish. Polyurethane resin is inherently susceptible to UV deterioration and requires lots of UV protective additives just to be OK. Spar urethane, if it has any use, is to provide a flexible finish, not one that is a very good UV protector.

IF the UV part is the important thing, then you want a marine grade traditional spar varnish. Epifanes Clear Gloss is one. Interlux Schooner and Pettitt Captains are others. If it is sold in an ordinary paint store, or a big box store it isn't marine spar varnish, no matter how nautical the name.

[Dennis McDonaugh]

Thanks Steve...I guess. I'll try to find a real marine varnish instead. I don't want to be redoing them for ten years or so.

Dennis

[Michael Dresdner]

The UV protection in exterior varnish comes from two common additives. Various manufacturers add more or less, and the more there is, the more protection you get.

One type of additive called a UV absorber absorbs UV energy, converting it and releasing it as heat energy. The second additive is a HALS, or hindered amine light stabilizer. Unlike the first, it scavenges free radicals formed when photons (light energy) do manage to do their damage. Left unchecked, the free radicals will cause a cascade reaction, degrading the finish much faster. This second compound is used not to protect the substrate from fading, but to prevent the finish itself from degrade. The efficacy of both depends on the amount in the mix, so the more finish you apply, the more UV absorbers and HALS you are also applying. How you apply (many thinned coats vs. one or two thick) is not relevent; only the final mil thickness counts.

Traditionally, the name spar varnish referred to tung oil and phenolic resin varnish. While neither material blocks UV from the substrate, both have natural UV stability. That means they won't prevent fading, but they don't break down easily in sunlight either. The flexibility comes from the oil to resin ratio.

These days, the term 'spar varnish' or 'spar urethane' simply means the coating is formulated flexibly enough to work as an exterior coating.

Urethane resins are not inherrently anything, mostly because there are so many types. (The FSCT Paint Dictionary lists five major categories.) The two we concern ourselves with are the ones commenly used to make interior and exterior urethanes. Exterior urethanes are UV stable because they are aliphatic -- based on a straight carbon chain backbone. Interior urethanes are not UV stable because they are aromatic -- based on a hexagonal ring backbone. It is this critical difference that renders them one or the other. Adding UV absorbers or HALS helps, but you must start with an exterior urethane for it to last.

Incidentally, one of the other five categories of urethane is the two part automotive clear urethanes used over paint and flexible bumpers. I am sure even the most suspicious will accept that this urethane is quite UV stable.

[Charlie Plesums]

Thanks Steve...I guess. I'll try to find a real marine varnish instead. I don't want to be redoing them for ten years or so.

Dennis
If you find any exterior finish that lasts 10 years, let me know. When we owned a boat many years ago, the cheap finish lasted half a season, and the expensive stuff lasted the whole season... and that was in Yankee land where the season was less than half a year. I'm sure finishes have improved, but 10 years sounds like a miracle

[Dennis Peacock]

If there's a resonably priced exterior finish (varnish) that lasts 10 years? I want to know what it is so I can get me some-O-dat stuff. :)

[Steve Clardy]

The UV protection in exterior varnish comes from two common additives. Various manufacturers add more or less, and the more there is, the more protection you get.

One type of additive called a UV absorber absorbs UV energy, converting it and releasing it as heat energy. The second additive is a HALS, or hindered amine light stabilizer. Unlike the first, it scavenges free radicals formed when photons (light energy) do manage to do their damage. Left unchecked, the free radicals will cause a cascade reaction, degrading the finish much faster. This second compound is used not to protect the substrate from fading, but to prevent the finish itself from degrade. The efficacy of both depends on the amount in the mix, so the more finish you apply, the more UV absorbers and HALS you are also applying. How you apply (many thinned coats vs. one or two thick) is not relevent; only the final mil thickness counts.

Traditionally, the name spar varnish referred to tung oil and phenolic resin varnish. While neither material blocks UV from the substrate, both have natural UV stability. That means they won't prevent fading, but they don't break down easily in sunlight either. The flexibility comes from the oil to resin ratio.

These days, the term 'spar varnish' or 'spar urethane' simply means the coating is formulated flexibly enough to work as an exterior coating.

Urethane resins are not inherrently anything, mostly because there are so many types. (The FSCT Paint Dictionary lists five major categories.) The two we concern ourselves with are the ones commenly used to make interior and exterior urethanes. Exterior urethanes are UV stable because they are aliphatic -- based on a straight carbon chain backbone. Interior urethanes are not UV stable because they are aromatic -- based on a hexagonal ring backbone. It is this critical difference that renders them one or the other. Adding UV absorbers or HALS helps, but you must start with an exterior urethane for it to last.

Incidentally, one of the other five categories of urethane is the two part automotive clear urethanes used over paint and flexible bumpers. I am sure even the most suspicious will accept that this urethane is quite UV stable.



Welcome to the forum Michael!!!!!!

[Jim Becker]

I don't want to be redoing them for ten years or so.

This one jumped out at me, too...even the best film finishes invariably fail and need restoration far sooner than ten years. Two-three max. The best finish for this kind of thing is no finish, or a penetrating oil product like Sikkens or Pentofin that is easily and quickly renewable. Regardless, your finish will not stop the natural silvering of the wood.

[Cliff Rohrabacher]

If there's a resonably priced exterior finish (varnish) that lasts 10 years? I want to know what it is so I can get me some-O-dat stuff. :)

"[R]easonably priced" is a rather subjective concept.
I submit that reasonableness is obtained when one factors quality of a finish over time, durability, application issues, and the financial cost together.

It's entirely reasonable to pay a lot of money for a lot of product quality.

The things that might keep me from paying the high premium for a high-end 2-part urethane, an epoxy, or a precatalized urethane are diffuculty of application coupled with questionable need for finish that will stand up well to full out door all season exposure. The latter can have a complex decision tree.

[tod evans]

i`d like to welcome michael dresdner also. for those who don`t know, michael if kind of a finishing guru who regularly contributes to various woodworking publications.. tod

[Jim Becker]

The "longest lasting" "clear" exterior finish that I'm aware of is accomplished by using exterior oil-based deep tint base paint...it dries clear, although not necessarily "super shiny" like varnishes or epoxy.
------

And I'm adding my "welcome" to Michael Dresdner, too! I hope you'll check in once in awhile in the Finishing Forum.

[Steve Schoene]

Urethane resins are not inherrently anything, mostly because there are so many types. (The FSCT Paint Dictionary lists five major categories.) The two we concern ourselves with are the ones commenly used to make interior and exterior urethanes. Exterior urethanes are UV stable because they are aliphatic -- based on a straight carbon chain backbone. Interior urethanes are not UV stable because they are aromatic -- based on a hexagonal ring backbone.


Michael,
Can you identify any of the aliphatic urethane varnishes that are available to the DIY market as one-part products. I thought almost all the single-part DIY products were combinations of aklyd and polyurethane resins. Those were the subset that I was thinking of, at any rate.

I know is Interlux Perfection a two-part aliphatic polyurethane product for marine uses, both clear and pigmented. West Marine sells it for $60 a quart. The thinner is almost $20 a quart.