Minwax Spar Urethane question

[Greg Parrish]

I've been reading and reading and not finding my exact problem so I figured I would see if anyone had thoughts. I built an outdoor potting bench using cypress. After it was finished and sanded, I applied minwax spar urethane as a coating. All was going well applying thin coats with light sanding between coats using 220 grit paper. I have the sides to the point that they were finished with 3 coats but the top needed more coats to completely cover it.

Long story short, I sanded and applied a thin coat on Saturday morning and allowed it to dry over night. On Sunday morning, and I should have known better, I applied another coat while it was overcast and without thought to the weather. Well starting Sunday night tropical storm Collin dumped a ton of rain on us over the last few days and even though that coat seemed dry by then I'm having some white haze appear in a few spots. What's weirder is that as the sun comes up during the day and the bench top warms and the dew dries off, the spots go away. But at night they come back.

What do I do? Live with the spots? Assume the cypress got so drenched on the uncoated wood (bottom of the boards) that it is still wet and this will stop once it dries out? Sand it all the way back down to the raw wood and start over? Sand just that last layer down? Wait till it dries for a few more days before deciding to see if it goes away? Not sure where to start but don't want to make a bigger mess.

Thanks for any insight.



Here you can see the spots and haze yesterday morning while it was covered with rain water




And here is a shot yesterday afternoon once the sun was up and warm and most of the spots and haze appear to be gone

[Prashun Patel]

White spots can sometimes happen when moisture gets trapped in a finish. If the film is sufficiently thin and not cured, the moisture can sometimes escape over time.

I would let it dry thoroughly, lightly sand, and continue.

The pattern of spots around your nails makes me think the finish has not skinned over there, and moisture is wicking in through the end grain of those penetrations. Unless this can be sealed, it may continue to be a problem.

FWIW, on my outdoor furniture, I no longer try to build films unless I'm prepared to bring them inside in extreme weather. Know that extreme includes both sunny days and rain.

[John TenEyck]

I think the water soaked into the wood, especially the unfinished bottoms, and now it's coming back out. When the sun hits it it dries up the top surface enough for the haze to disappear, but then more water comes out after the sun goes down.

Now the really bad news. I think you are pretty much cooked no matter what you do. The only way to prevent failure of an exterior film finish is to coat every surface that will be exposed to water. With your piece you would need to varnish top, bottom, and ends of those top boards to have any chance of the finish surviving. And you would need to apply another coat or two over the top to cover where the nails penetrate. Even then, it would need yearly maintenance.

If you aren't up for all that, you might want to reconsider what finish, if any, you put on it. An oil finish would be a lot easier and require less maintenance. It won't preserve the wood as well as a film finish, but it will look better over the long haul with a lot less work. Or no finish at all for the ultimate in low maintenance.

John

[Greg Parrish]

Thanks guys. I was afraid of that. Since it is glued and nailed with stainless staples it will be nearly impossible to disassemble the cypress to coat the lower sides without damage. I might be able to coat everything except the spots that contact other wood but I'm not sure that is practical. The odd thing is that it wasn't doing this before the very last coat, so I was assuming it was something to do with that last coat.

I guess at this point I'll just let it sit for a while to see what happens. It won't get hit with rain like it did this past few days very often. We had 5+ inches in 1.5 day span due to the tropical storm. As long as the problem goes away when the wood dries out I guess I can just live with it. It will have to be a live and learn project that we use as is and redo later if needed.

The initial reason for the coating was not just for looks but so that my wife could do plant potting and garden work on it while keeping an easy to wipe down surface. The opening has a sink that drops in and is supplied through a garden hose.

[Prashun Patel]

Greg-
Don't despair. The good news is that you can let mother nature remove the finish for you. Minwax spar urethane is not very durable at all and I bet within a season you will get significant peeling and fading if you leave it outside. Every so often, you can give it a gentle sanding. With time, it will all be removed.

Cypress is a fine outdoor wood even untreated. You just have to be willing to enjoy the silver that all these woods turn.

[Greg Parrish]

Greg-
Don't despair. The good news is that you can let mother nature remove the finish for you. Minwax spar urethane is not very durable at all and I bet within a season you will get significant peeling and fading if you leave it outside. Every so often, you can give it a gentle sanding. With time, it will all be removed.

Cypress is a fine outdoor wood even untreated. You just have to be willing to enjoy the silver that all these woods turn.

Thanks Prashun.

I just went ahead and dropped the sink in and will just use it as is. It looks pretty nice to me regardless of the small hazy spots so I'm not too upset.