Conversion Varnish Wrinkling Problem

      Wrinkles that appear after applying a second coat of CV could be caused by a variety of mis-steps, some related to the "recoat window." October 1, 2010

Question
I have never really came up with a remedy to the following problem and am wondering if anyone can help. I dye, stain, and put one coat of CV on and then the second coat wrinkles up in places, but when I re-scuff the third coat will lay fine. The thing is, this only happens every so often so I never can figure out what I did wrong this time (if anything) to make it happen. I know some of you have told me about the recoat window.

For several years I have been spraying one day and then letting it dry overnight with the dehumidifier on and then scuff and recoat the next day. This works most of the time but today it didn't. If I spray one coat and don't let it dry overnight and try to scuff and spray the same day then it doesn't scuff very good at all. Itís like the finish just clogs up the pad. We use Highflex 220 grit pads to scuff with, I tried one door recently with a piece of sandpaper and it was much worse. If anyone has any thoughts on this I would sure appreciate you sharing. I am at my wits end on trying to figure it out. Maybe humidity has something to do with it? Maybe the recoat window?

Forum Responses
(Finishing Forum)
From contributor D:
Wrinkles are caused by the second coat grabbing onto the first coat and pulling it away from the wood (adhesion is poor). Apply your two coats to a clean piece of glass using the same method you would use in your normal finishing method. This should tell you if the CV has any issues. If you have a wrinkle here, call the folks who made the CV and get them involved.

If the CV appears ok, move on to making several panels with just your dye and CV, stain and CV, and see if you can isolate the wrinkling problem to the dye or the stain. To me, it sounds like your stain is the problem, areas where the stain is not completely removed can keep the CV from adhering to the wood, and although the first coat may look slick, the second coat will pull the first one away if it can.



From contributor U:
Whose product(s) are you using? Are you scuffing between coats with the 220 plus or is that what you are sanding the substrate with?


From the original questioner:
Contributor B - I think you are right about the second coat pulling the first coat away. Although this mostly occurs on stained surfaces I recently had it to happen for the first time on natural red oak. I use zar stain which is oil based and have for the last several years because it doesn't blotch on maple. I had it to happen on Mohawk stains too though. Contributor U - I am now using Chemcraft cv, I used to use Mohawk, I've had it to happen on both brands. The wood is sanded with 120, scuffing between coats is done with Highflex 220 grit sanding pads. So does anyone think that waiting overnight to scuff and recoat is part of the problem?


From contributor B:
Zar is one of the hardest stains to use in a production setting because of its formulation. Even though the directions may give you a dry time that is meant for the DIY'ers that use polyurethane over it, not professional CV.

For it to happen with Mohawk stains and Chemcraft mean you have an issue in your finishing system. Since you are seeing wrinkling on an unstained surface, you are most likely looking at a reducer issue or something associated with your substrate. Use your CV as you usually spray it on clean, clear glass and see if your wrinkling occurs there. If it does, get another piece of glass and try it again, this time without any reduction. If you don't get a wrinkle then you most likely have a solvent issue.



From contributor W:
I never let it sit all night just because of the wrinkling problem. I'll give it the second coat in about 15 to 20 minutes. Then itís done, and there is no wrinkling or anything else to worry about. I do use vinyl sealer so I'm starting with a good smooth surface. Not much sanding after the first coat. Also I use 320. I think 220 is too coarse for between coats.


From the original questioner:
I always let the stain dry over night. Contributor B Ė I agree that zar is hard to use in a production setting. I started using it because it went on even on maple and because it was available locally. Chemcraft has some stains that I am thinking of switching over to.

Contributor W, I don't use a vinyl sealer, just two coats of the CV. The first coat of CV would still be wet in 20 minutes and wouldn't nearly be ready to sand. My tech guy says that two coats of CV is tougher than a sealer coat and then a coat of the CV so thatís the way I have always done it. The odd thing is, the second coat of CV will dry completely in 30 minutes. Don't know why it takes the first coat so long to dry.



From contributor M:
If you have an adhesion problem it doesn't go away just because you kept it from wrinkling. The finish can flake off in those areas and make a really big problem in the field.


From the original questioner:
I'm still not understanding how you guys can scuff that first coat after 20 minutes, my first coat would still be tacky after just 20 minutes. Perhaps I need to use more catalyst? I am using 12 ounces per gallon but maybe I need to try more catalyst on some scrap pieces and see what that does.


From contributor C:
You must add catalyst according to manufacturers' instructions. If you don't, the properties of the coating can be badly affected.


From contributor V:
Different CVís have different dry rates. Iíve used some that need an hour or more before I could sand them, and some that dry very fast 20-30 minutes. So you might want to try a different manufacturer or product.


From contributor N:
Wrinkling is caused by hitting the recoat window. This is the time after cross-linking has started but before it's completed. Overnight is particularly bad. C-V should be dry enough to sand within an hour - if not something is wrong. Shoot wait and hour, sand, shoot and you'll never have this problem.


From contributor V:
What do you mean by "recoat window"? I spoke to ML Campbell tech line and they were somewhat unclear, they first started talking about the "wet on wet" window which isn't what you mean I'm sure. From what I was told if you can scuff it then re-coat you're ok, doesn't matter if it's one hour, overnight, or three days?


From contributor N:
The term is self explanatory in a way. It's a time window during which wrinkling will occur due to the partial conversion of the coating into a solid mass prior to its complete conversion. It's too far along to be dissolved and not yet far enough along to be completely chemically resistant to the solvents in another coat. All catalyzed coatings have these recoat windows. Itís simply a function of chemistry. Cross linking does not happen instantaneously but over a period of time normally hours. The time between the cross-linking starting and being complete is the recoat window.


From contributor V:
Understood but MLC at least says they do not know when that window is which makes it a crap shoot in a way. I spray their Duravar CV and typically it's about one hour to 'dry to sand' but I'm sure cross linking is very far from complete yet I don't have issues with wrinkling. I also have waited overnight when the piece is too large to spray two coats in a day and allow for sanding with no problems. It would be real nice if there was some rules for application window provided by manufacturers.


From contributor N:
That would be nice but impossible since it all depends on temperature primarily. If you shoot something and then set it into the Phoenix, AZ sun (which I do) the cross-linking is done within an hour. If you shoot something and set it in a damp cold shop in Portland, or the recoat window will be from 12 hours later to a day later. All chemical reactions follow the Arrhenius equation which is logarithmic with temperature.


From the original questioner:
I think you have touched on something that has a lot to do with my original problem here. I have always thought that temperature and humidity has something to do with it. If you are in Arizona then there is very little humidity then I can see being able to scuff and recoat in an hour. But here in Kentucky especially in July and August, it is so hot and humid that a friend of mine that lives in Arizona can't stand to be here because she can't breathe for the humidity. So what I'm saying is that my recoat window may be longer away from the point of spraying due to this humidity.



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