Conversion Varnish Delamination

      Too fine a sandpaper may be the reason conversion varnish is losing inter-coat adhesion. November 25, 2008

Question
Has anyone had a problem with Sherwin Williams conversion varnish delaminating? I have had a problem with the finish on a couple of pieces of furniture that I built and am concerned with other pieces now. A client called me about a coffee table that a grandchild scratched the top of. I noticed that once scratched, the finish peels off. I have since noticed it on a piece I built for my wife.

I sand in between coats with 320 grit paper. Maybe that is too fine. I have tried either spraying a coat on and sanding within 30 minutes to an hour (when dry of course) and recoating, or spraying a coat on at the end of the day and sanding the next day and recoating. I do not thin the varnish, and catalyze at 3%. Is there an adhesion promoter or other product that I can add?

Forum Responses
(Finishing Forum)
From contributor J:
320 may be too fine for CV. Most manufacturers recommend 220. The problem is that not all sandpaper is created equal as far as grit goes. One brand's 320 is closer to another brand's 240 or another's 400. Sanding technique can also play a part. Most people try to use the same piece way too long. Sandpaper loses its grit fairly quickly and using a worn out piece of 320 is like using 400 or worse.

Doing all your coats in one day whenever possible will also help inter-coat adhesion tremendously. If this is not possible, then at least scuff sand immediately before recoating. Do not scuff one day, then recoat the next.



From contributor R:
From SW website for clear Kemvar:
Sand with 220 grit paper and remove all sanding dust.

For pigmented conversion varnish: no recommendation except this one.
For full sharp gloss appearance, sand intermediate coats with very fine (400-600) grit paper to prevent telegraphing of sand marks.

I always use 220.



From the original questioner:
Thanks for the info. I do tend to overuse the sandpaper which goes back to my lacquer days due to burn through of the corners. I will try using 220 sandpaper today and see how it works out. I did notice after further review of the top of the table that it delaminated between coats. I think this will solve the problem.


From contributor M:
I think we need to know your entire finish schedule in order to help. What grit do you sand the wood? What kind of stain are you using and how are you applying it? Are you using a glaze? Toner or shading? These things could all have an effect on adhesion. Also it would help if you could get a magnifying glass and look closely to see where the finish is delaminating. (Is the finish coming off at the stain layer or some other layer?)


From contributor L:
I use SW products, just because it's near, not because it's best. I had that happen to me with CV as well as a cab. The CV problem was just how you described it, and it was because I shot it on early and the temperature was well below 70. Out of the same batch I shot that day the only pieces that did that were the ones I did that early morning. With all that said; was the temp above 70 when you put it on? I also found that with a few of their products and some of their CVs, it's best to use it as a self-seal system.


From the original questioner:
Yesterday I sanded the top of the table down to bare wood and refinished it. I found that the finish delaminated very easy in some areas and not at all in others. The delam pattern looks like a wiping motion, which could be areas sanded with worn out sandpaper or loaded paper. Also I do not wipe the surface with any chemicals, just a clean dry rag. I do notice that the sandpaper loads up with finish dust as I sand on all of my pieces during the finishing process. The paper loaded up yesterday sanding the top which was sprayed a year ago.

I do not stain the wood, I only use natural woods such as koa and mahogany. The wood is sanded to 220 before finish. I then blow off the wood with air through a filter to eliminate water spray from the compressor, then wipe with a rag. I then spray a coat on, let it dry, and sand with 320 grit paper (which I am switching to 220). Then apply a topcoat.

I usually finish the whole piece in the same day. Here in San Diego it is usually warm and dry so there is not often a problem with cold or moisture, but if it is cold or raining I wait for a warmer day of at least 70.

Should I consider using a vinyl sealer first, then CV on top? The sandpaper I use is Norton's equivalent to 3M's tri-m-ite paper (which is kind of a light gray color). Do you have any recommendations on other paper?



From contributor R:
I guess we go back to... where was the delamination? If between coats, then I suspect too fine a sandpaper. If it was between the wood and finish, then I still think too fine a sandpaper. 220 is way too smooth for before a finish. Leave some tooth! On coarse woods I stop at 120. On maple and such I sometimes go to 150 but I would worry about 220.


From contributor J:
I would have to agree with contributor R - 220 is too smooth to sand the bare wood. There are some exceptions to this rule, but in general 220 is where you stop sanding wood and start polishing it. The rag wiping may also be suspect as a source of contamination and is probably unnecessary at best. A good blast of clean compressed air should be all that is needed.

As for sandpaper recommendations, if you are already using Norton, ask your supplier for some A412 or what used to be called "Champagne." It's expensive but it is hands down the best scuff paper I've ever used.



From the original questioner:
I forgot to mention that the delamination is in between coats. I do believe it was a combination of too fine a grit (320) and worn out sandpaper. The finish is adhering well to the wood. As far as I know the finish should soak into the wood to adhere well. I am going to check into contributor J's recommendation on sandpaper. Next time I spray I will spray a test board to test adhesion and different grit papers. Thank you to all that have responded and helped me solve this problem.


From contributor G:
I use 320 all day long with no problems. Maybe you're not getting a good scratch since your sandpaper is getting loaded up.


From the original questioner:
I have used 320 for all scratch coats on all of my finishing work since starting finishing over 20 years ago. However I used lacquer most of that time. I did spray automotive finish for guitars for several years and we also used 320 before the final coats. When I spray the final coat of conversion varnish (after first coat is sanded) I add a third coat on table tops only within 5 to 10 minutes after the finish has been sprayed to give the CV time to flash. I do not sand in between this coat as the finish should be somewhat soft, which is something I did not mention before. That is the way I have always sprayed the final coat. Is that the way you do it, or do you sand in between each coat? I have also switched to Klingspore sandpaper, but do you have any recommendations on the type of paper (besides 3M) that would be best?


From contributor J:
In all honesty 320 grit should be fine if you are spraying, scuffing and re-spraying the same day. It's when you let a catalyzed seal coat cure too long that 220 becomes necessary for a mechanical bond. This is one of the reasons that I use pre-cats on 80% of what I do, along with the fact that they make glazing simpler and easier. In my opinion, conversion varnish is overkill in some cases. Not everything requires a finish that is as resistant as CV.


From contributor G:
There's no need to let the finish flash for that long. A minute or two will let it set enough not to run off edges. Just make sure both coats together are around 4 mills wet. I don't agree that 320 is too fine when the specs say 400 - 600 is needed on glosses. I've used 500 Abrilon myself many times. I've found that 220 will telegraph scratches. Just make sure you get an even scratch when you sand and things will be all right. Look into other causes for your problem - 320 ain't it.

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