Cracking Varnish Over Epoxy Sealer?

A shout out to all of the finishing experts, professional and amateur alike. I've been working (for too long now) on completing a sign project done in western cedar and have been having a tough time with the finishing. The planned schedule was relatively simple--an epoxy sealer (Total Boat) followed by several coats of a clear gloss spar varnish (Epifanes) followed by a matte finish spar varnish (also Epifanes)

After sanding down to 220 grit, the bare wood had been prepped with a light wipedown of acetone, to remove any residual surface oils. Then, two separate coats of Total Boat Epoxy Sealer were applied following the directions--the second coat going on just after the first had reached a tacky state.

The epoxy sealer coats were applied in my garage shop, which I heat for short periods with a Fahrenheat 5000, and the interior temp is usually around 55-60 degrees F. But here in the Pacific Northwest the humidity levels can typically be around 50-60% this time of the year. So the second sealing coat took a little over a week to fully cure.

Following a light sanding with 150 grit, I laid down a first coat of Epifanes Clear Gloss Spar Varnish and allowed that to cure for a period of about three days (again, the temp and humidity levels affected the length of the cure time). Then upon application of the second coat (after a light sanding with 320), I ended up with a severe cracking effect. I attributed this to an incomplete cure of the first coat, so I allowed the cracked coat to cure for almost two weeks and the sanded it off using 150 grit. After vacuuming off the surface and then wiping down with a dry cotton rag, I brought the whole project into my house and allowed it to stabilize for a little over 12 hours. Then, I laid on the first new coat of varnish, thinned about 25% with mineral spirits.

The current result was a cracking effect even more pronounced than the first (pictured below). Any ideas what's going on here? I should point out that this did not happen on the back side of the project where I followed the basic schedule described above entirely in my garage shop, and the finish turned out nearly flawless. So why the cracking on the opposite side, could there be some kind of reaction taking place with the epoxy? Or, is it possible that sanding down to remove the initial varnish coats wasn't thorough enough, and I managed to leave enough behind that it reacted (again) with the fresh coat of varnish?

Short of completely sanding down to bear wood I'm out of ideas for how to correct this issue or even why it's happening. Any suggestions would be appreciated.


Attachment 424049

I am not the one to question one's approach to finishing because I am not an "expert", but please enlighten me as to why you are applying spar varnish as a top coat over epoxy. I have always considered both to be top coat finishes. Maybe the epoxy is moving since it is designed for boats and would be used on fiberglass and wood and needs to move or flex. Maybe it's just not compatible with varnish.
I will be closely following this to help me understand if I'm wrong in my thinking.
BTW, please don't interpret this as a snippy type answer. It definitely is not meant to be regarded as such. If you interpret it as such, I apologize in advance.

that Cracking looks like a solvent incompatibility. Perhaps the epifanes is incompatible with your mineral spirits.

Quote:

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Originally Posted by Prashun Patel

that Cracking looks like a solvent incompatibility. Perhaps the epifanes is incompatible with your mineral spirits.

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But it sounds like the first coat (un-thinned) did not crack, while the second coat (also un-thinned) did crack. Re-attempt coat (thinned) after sanding cracked even worse.

Hmmm. Is is possible some moisture/contamination got into the Epifanes in between the first and second coats? Especially since after you sanded way back (which should've taken care of any surface contamination) the cracking still occurred.

The only thing I can think of is to sand back the cracked varnish yet again and try a different varnish. Keep us posted.

Quote:

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Originally Posted by Mike Ontko

...I've been working (for too long now) on completing a sign project done in western cedar and have been having a tough time with the finishing. The planned schedule was relatively simple--an epoxy sealer (Total Boat) followed by several coats of a clear gloss spar varnish (Epifanes) followed by a matte finish spar varnish (also Epifanes)...

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Never used Total Boat epoxy. But WEST epoxy requires many coats followed by a thorough block sanding before the application of varnish. For your epoxy to take so long to cure you must have messed up the resin/catalyst ratio. Humidity was not the problem. I'd strip everything and start over.

Pretty sure Epiphanes requires a special thinner, not mineral spirits.

Not taken as snippy at all, and thanks for the follow up. I was following a similar project, "Durable Outdoor Finish" posted on YouTube by Mark Spagnuolo, aka, The Wood Whisperer. In that video, he follows a similar schedule for sealing and then topcoating. The epoxy product used is Total Boat Epoxy Sealer, a penetrating product intended for sealing, reinforcing, and preserving wood and not for use a casting or topcoat resin. The intention is to first seal the wood from future moisture intrusion and then to further protect it and give it a matte finish by applying multiple layers of spar varnish. Epifanes has a long standing reputation and is one of the few that appears to come in a matte finish (which by the way is just as "dead flat" as Mr. W. Whisperer states in his vid.).

I thinned the first coat on the reverse side with about 50% mineral spirits. The first coat on the second side was actually full strength--I felt confident enough in the epoxy sealer that I could go with a first coat at full strength (Epifanes recommends thinning the first coat when applying to bare wood).

So, I'm thinking there must be some kind of issue with the epoxy cure on the second side.

Would applying a coat of unwaxed shellac as a seal coat before applying the oil based spar varnish be a viable approach?

Andy, you're correct that Epifanes recommends their own special thinner. But members on SMC back in 2011 identified that product simply as a proprietary blend of mineral spirits, so I went with what I had. Regardless, it was the second recoating that was thinned and not the first. The first time around it was full strength without thinning, so I let it cure and sanded back down to the wood (sealed with the epoxy) and then went with a 25% thinned varnish, and got an even more pronounced crackle.

I suspect what you've mentioned--that the epoxy didn't do it's thing, either because the ratio was off (though I'm pretty sure it wasn't), the mixing wasn't thorough enough, or the temperature and humidity levels hand an effect on the cure.

Epoxy cures because it is catalyzed--there's no way to prevent it from curing once that has taken place. Cold temperatures will slow curing somewhat, but certainly not days. And humidity won't stop it either. There is a probability bordering on certainty that the epoxy was not properly catalyzed. Also 1 coat is virtually useless; it requires multiple coats to create a vapor barrier.

Similar problem with the varnish--it requires its own special thinner, regardless of what you've heard. And never have I heard of thinning Epifanes 50% after first sealing with epoxy. Straight out of the can is how the pros do it, except under special circumstances, and then only up to ~10%.

First post. I’ve really enjoyed and appreciated all the information available here and hope I can give a little bit back.
I’m not familiar with the brand of epoxy sealer but I do have some experience with epoxy and varnish as a boat owner. It sounds like you may have an issue with amine blush on your epoxy. It can be more pronounced when using epoxy in cool humid conditions. It will mess up any topcoats if not removed. I tend to scrub it off with warm water before topcoating. Sanding it before removing the blush usually makes the problem worse. Also, some epoxies can take a week to fully cure.
I don’t undercoat brightwork with epoxy as I find it just adds to the work when the inevitable happens and I need to refinish. I aim to have 10 or more coats of varnish on a piece for protection. I try to give a light sand and add a new coat or two at least once a year. I am able to go ten years or more without stripping and starting from scratch by maintaining the integrity of the varnish this way. Any chips or worn spots will lead to finish failure in my experience.
I find that multiple coats of low gloss varnish can give a cloudy or muddy appearance so I build up my film thickness with gloss varnish and use the low gloss for the last coat or two to get the look I want.
I have had Epifanes develop a wrinkled finish when I’ve put it on too thick or top coated too soon. When you work outside you sometimes have to push the limits!
Good luck with it!

Mike,
I wonder if there is an 800 number on the can? If so, maybe you could call Epiphanes and see what they think happened. Might be worrh a try.

Good luck.
Fred

Pretty cool info. Thanks. I never knew about amine blush.

I too question whether the epoxy was properly catalyzed, in which case you will have to remove it and start over.

Try a test with the Epifanes on bare wood or over shellac, just a small area. If it cures ok, the problem is in the epoxy sealer coat.