Problem with topcoat adhesion -- over stain

      Wood finishers help troubleshoot a comrade's problem with topcoat adhesion on stained work. April 22, 2000

Q.
Does anyone out there use ML Campbell's Windsong wiping stain, and would you care to share some information with me?

We are using this product on Northern soft maple and ash, spraying it on and wiping it off within five minutes. We use Campbell's vinyl sealer, and Duravar topcoat. We are experiencing poor adhesion of the sealer and top coat. We can scrape off the coating easily with a fingernail. We have let the stain dry overnight and still have the same problem.

The moisture content of the wood is normal, and the wet coats of material are normal, also. The distributor is baffled.

Any suggestions or similar experiences?



Are you letting the finish cure at least 24 to 48 hours before you scrape it with your fingernail? Any finish can be gotten into right off the line, without a chance to cure.

Also, your distributor is not much help if he cannot solve this problem. Ask him the time required for a total cure of the finish. If he doesn't give you a specific time, look for another source! Most finish systems take at least one to two weeks for the finish to completely cure. It is after this time that you would want to try and scratch the finish off, do cold check tests, and cross-hatch adhesion tests.

Some of the problem could also be in your vinyl sealer. Some vinyls are a modified vinyl containing some nitrocellulose resin, to keep them from cobwebbing. They may have sent you something like that and you're not getting good adhesion. You also could have gotten a bad batch of material and your distributor is playing dumb. There are many reasons for this type of problem, but first check the finish after complete cure.



I think the problem lies within the vinyl sealer.

You may be putting too much on or some problem with the sealer itself may be causing the poor adhesion. Why not change to Campbell's water white vinyl sealer? Also how much are you reducing the sealer you're using now?



How fine do you do your white-wood sanding? Is it 150 grit or 180 grit? You should not go finer than 180 grit.

Duravar reaches 85 percent of its cure cycle after about 48 hours. But are you maintaining the temperature at 65 degrees F (min.) for that first 48 hours? The remaining 15 percent cure takes place over the course of the next three weeks, but that first 48 hours is critical when it comes to a minimum temperature tolerance.

Are you adding only 10 percetn catalyst BEFORE reduction of the Duravar?

If you are using the ML Campbell 25 percent vinyl sealer, there is no nitrocellulose in it. But that sealer coat should not reach more than .5 dry mil.

If you think that you are sanding too fine, mist on an application of "grain popper." Mix a 50/50 alcohol-to-water solution. This will open up the wood pores if you think that your sanding is burnishing them closed. Is your sanding done with dull paper? This can also burnish the wood.

Are you spraying 4 to 5 wet mil of vinyl sealer and Duravar?



Another issue to bring up is the dry time between sealer and top coat. Make sure the sealer is flashed off enough, it should only powder up when sanded, and there should be minimal gumming or loading of the papers. If you notice the paper gumming you need to wait longer for it to dry. This dry time is extended considerably depending on the humidity and the amount of reducers added to the sealer.

Another problem with the maple and ash is that their surfaces are extremely hard. Avoid, if you can, going past 120 grit with final sanding. If the 120 grit affects the color too much, you can always lighten it. Also, scuff the surface of the sealer with 320 RIGHT BEFORE applying the Duravar. Make sure the surface is clean. Spray 5 mil, three times. Get a mil gauge if you don't have one, and use it until you get the feel for how HEAVY 5 mil is.

Remember, it is a mechanical bond you are dealing with. Reduce your first coat of Duravar about 10 percent after the addition of catalyst. The second coat about the same.



From the original questioner:
Thank you for the quick responses. I would like to explain a little more.

We white-wood sand the maple to 180 grit and the ash to 120 grit. We sand by hand and also with the Dyna orbital sander by Dynabrade. We try and keep the finishing process as simple as possible, meaning a stain application, sealer application, and topcoat application.

We had been using Sherwin Williams dye stains, which worked well but we were too dependent upon the sprayer to put them on correctly; we wanted something less operator-dependent.

When this problem surfaced, we did a comparison where we sprayed a dye stain and a wiping stain next to one another and sent them through the stages together. We experienced good adhesion with the dye stain, but not with the wiping.

The sealer used is what the distributor recommended with the dye stain. He did not change that when we started using the wiping stain. We do not reduce the sealer because we spray with an air-assisted airless from a 55-gallon supply.

We sand sealer with a sponge pad (fine grit) but we always did.

The Duravar is catalyzed before reduction, at 10 percent. We topcoat spray with an Accuspray gun and pressure pot. None of this has changed since we were using the dye stain. The only thing we did change, at the recommendation of the distributor, is the use of a reducer with the Duravar. We are now using a butyl acetate reducer. The theory is that it reduces faster without breaking the solids down, thus getting more solids on the surface. However, we still got better adhesion with the dye stain than the wiping, so I don't think the reducer is the problem.

One point the distributor is trying to tie to this is that we are using a stearated sanding paper. But a lot of wood people use this and no problems, and it is the same paper we use with the dye stain, so I don't see a connection. I think he is reaching to drag other manufacturers into this.

We have let the topcoat dry for 24 hours before testing, but it has been two weeks since we first discovered this problem and the same item still has adhesion problems. Not to beat a dead horse, but again, the dye stain item we sprayed alongside the wiping stain item does not have the problem, but the wiping stain item does.

I'm baffled and upset. I really don't want to change materials, ML Campbell is a good product.

I've been spraying for years and have not had this kind of problem with other wiping stains, but we are production-oriented and cannot ponder on this problem for very long.



Since you added a reducer -- not recommended by the M.L. Campbell tech sheets -- to the Duravar, I do not see them standing behind their product, or even offering much help. The distributor is a different story, but the product manufacturer is pretty doctrinare about their tech sheets.

The ML Campbell lacquer thinner is formulated to work with their products, yet be on the weak side (unlike a "hotter" thinner). If a hotter lacquer thinner were introduced, then you should have to consider what the recoat window would be when you apply the Duravar.

But you did similar performance tests using two different finish schedules, with the only variable being dye stain vs. the MLC Woodsong pigmented stain. And in doing these, the schedule with the wiping stain is failing and the dye stain is holding up. The Woodsong wiping stains are reduced in a xylene vehicle and they have an alkyd binder, so there is nothing inherent in the stain itself to cause the problem, unless somehow you have a strange batch. Have you tried a Woodsong stain from a different lot?

I have a few other questions and no idea of their relevance: Is the vinyl sealer being stirred on occasion? Are you using ML Campbell's water-white vinyl sealer (24 percent solids)? Is that vinyl sealer no more than 0.5 dry mil thick? Are you putting the hot solvents into the vinyl sealer as well?

I would opt out of the hot solvent to achieve spraying viscosity. Instead I would warm the finish materials by ten degrees or more from whatever they are. I am not suggesting hot spray lacquers. But the warmer the material, the thinner it is with no loss in solids, and no ire from the chemists at ML Campbell.

Are the wiping stains wiped clean of their excess or is there a layer of color left on the woods as if you were using a glaze?

In a previous post, I mentioned that the temperature of the sprayed items must be at least 65 degrees F for at least the first 48 hours after application of the topcoats.

Last, the shelf life of the catalyst is about one year. You may have a batch of catalyst that has expired. Try a fresh batch.



Your added info leads me to believe that your problem is in your wiping stain -- not so much the stain itself, but other factors.

With a wipe stain being pigmented and containing more solids, your color is on top of your substrate, where with a dye stain, with virtually no solids, the penetration is better. Also, a wipe stain needs a lot more time to dry than an NGR does. So these are two good possible causes for your problem.

Possible solutions would be: (1) your sealer may be too thick or not sprayed wet enough, and is not getting a good bond to the wood through the wipe stain; and (2) the wipe stain is not dry enough, preventing your sealer from bonding to the wood.

If you are finishing on a moving finishing line your dry time is a lot longer on your wipe stain. I really need to now how long you are letting that wipe stain dry and by what means, i.e., air dry or oven dry.

You are correct with your assumption that your ditributor is trying to drag other suppliers into this problem. Stearated paper has no effect whatsoever on what you are doing. He is trying to find an answer but is wanting to misdirect you on something else to take some of the heat off himself. I see no problem with your sanding schedule just maybe your process or material. This should be an easy problem to take care of. Well, it is for me -- I've been down this road too many times!



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