Problems with Lacquered Hardwood Plaques Sticking Together

Finishers discuss why a finished pieces would stick together even after months of drying. June 17, 2009

I have been manufacturing wooden award plaques (oak, ash, etc.) for several years for a local school district, usually applying a couple of coats of nitrocellulose lacquer (Kwall high build gloss lacquer) thinned with 30% lacquer thinner. The finish is fast and comes out excellent, but if I put the plaques in a box for shipping or just lean them against each other, they stick, causing the lacquer to come off some of the plaques. This happens if I put them together the day after spraying or weeks later. Recently one of my customers complained that they have found some stuck together that I had delivered six months earlier. Does anyone know of a way to make this lacquer dry completely so this will not occur?

Forum Responses
(Finishing Forum)
From contributor S:
I'm not familiar with the brand, but check with the manufacturer to see if it can be force air dried.

From contributor T:
Normally you should let dry for 24 hours, but that applies to pre-cat. It probably takes longer to cure. Try SW pre-cat high build - it flows well and is not sticky after 24 hours.

From contributor R:
You're adding too much thinner. Check manufacturer's thinning limits. If your spray gun can't spray the finish, the tip is the wrong size.

From contributor I:
Try putting wax paper between them or switching to conversion varnish.

From contributor J:
Just use a pre-cat such as Magnamax. You can pack it up the next day no problem at all. It will spray easy for you and it looks as good as regular nitro.

From contributor G:
After scouring the Kwal website, I couldn't find any reference to N/C lacquer at all. They have acrylics, epoxies and alkyd/urethane blends. If what you are using is indeed N/C, then it is an evaporative finish and will dry but not cure. I suggest switching products. A post cat lacquer or varnish will cure and eliminate your problem.

From the original questioner:
Boy, you guys are quick to respond and on point. You are correct about Kwall not making nitro lacquer. Here is the info off the can of lacquer: Excellence Gemini, Kad A Lac high build gloss lacquer; manufactured by Gemini Coatings Inc., in Oklahoma. I once wrote them an email, but got no response. I actually found a 1-800 number on the can and I'll be giving them a call. I'll also try to get informed and try using pre-cat lacquer. By the way, the plaques are not sticky to the touch, even after a half hour of drying - they only stick when they touching each other. Thanks.

From contributor T:
The finish on your plaques needs time to cure, and this could take up to a day or two before you can stack them or lean them up against each other. The advantages of a catalyzed coating, be it post or pre, is that the catalyst scoots up the cure time. Either change your choice of coatings or let them sit idle for a longer period of time before you attempt to stack or pack. The addition of forced air via a strong fan will certainly assist in flashing off the solvents.

From contributor B:
Maybe the lacquer and thinner are not 100% compatible; maybe the first coat isn't drying enough before the second. Maybe the lacquer (whatever it is) is too old. I've done what you're doing lots of times and it was usually okay, but we had to watch out for what kind of plastic we wrapped pieces in. Paper wrap or separators worked okay. The other suggestions you got seem very good. In the old days, we used to cut NC lacquer almost 50%, but I guess VOC rules have changed that.

From contributor L:
What he's using is indeed nitro lacquer. It's a brand name for that particular company or area. He needs to let the plaques cure more before stacking or change to a catalyzed product.

From contributor C:
Nitrocellulose lacquer is a solvent release type coating, which means the solvent re-wets the film. This is great for adhesion but the downside is that lacquer finishes off-gas for a long period of time (up to a month). This off-gassing of solvent will re-wet the lacquer film, causing your plaques to stick together. (Putting something between the pieces, not foam, helps if you don't leave them packed for a long period of time.)

Pre-cat lacquers can be stacked much sooner. Chemcraft's Opticlear could be stacked after just four hours. The downside to these is most of them are CV hybrids and they off-gas formaldehyde.

From Paul Snyder, forum technical advisor:
If the plaques are sticking together after more than a few days, something is wrong. It may have more than just nitrocellulose (NC) lacquer in it or is formulated with a plasticizer that is subject to migration over extended periods of time. I'd switch brands.

Product data sheets for finishes often include the dry times for the product. You'll find dry to touch, dry to sand, dry to recoat, and dry to stack times. The dry to stack time tells you the minimum amount of time to wait before it's safe to allow two pieces to come into prolonged contact with each other without sticking together (blocking). The times listed are usually for ideal temperature and humidity conditions and will take longer if the temperature is lower or the humidity higher. In a production environment, stack time is an important factor to maximize output.

The link below is to the data sheet for Sherwin Williams high-build lacquer. I could not find one for Kad-A-Lac lacquer, so I'm using this one as an example. They do not provide a dry to stack time for room temperature, but lacquer is usually safe to stack after overnight drying. Using a slow evaporating solvent (lacquer retarder), cool temperatures, and high humidity can stretch the time out by a day or two. If you apply too many coats, too fast, it can take even longer. The data sheet does say you can place the sprayed items in a drying oven warmed to 140F and reduce the dry to pack time to 60 minutes.

You say the plaques stick together after weeks and months of drying. So it's not a problem of dry time; it's a problem with the lacquer itself. The lacquer may include ingredients that cause the sticking. For example, latex paints made with vinyl acrylic/polyvinyl acetate (PVA) resins are prone to blocking indefinitely. Lacquers that include these types of resins can exhibit the same problem.

Or it could be the plasticizer that's used to give the lacquer some flexibility. Lacquer dries very hard (brittle). That's why it can be polished to a high gloss. Without adding a plasticizer, it would crack as it dried and hardened. Plasticizers migrate/evaporate out of coatings over time and the coating eventually ends up crazing and cracking. The plasticizer that's used in some rubber and plastic is a solvent for NC lacquer and when the rubber/plastic stays in contact with lacquer for a while, the two items will melt together as the plasticizer migrates from one to the other. It may be the case that the lacquer you're using includes a plasticizer that fuses two pieces together when they stay in contact with each other. That could make it very hard to open cabinet doors if you used that lacquer.

But both of these possible sources of the problem are just guesses. The bottom line is the brand you're using sticks together after weeks and months. If it's not past its shelf life and you're applying it correctly, then the best option in my mind is to change brands. Chemcraft, ML Campbell, Sherwin Williams, and Becker Acroma are widely available and all have pre-catalyzed lacquer that has a dry to stack time in the 4-6 hour range. One of these would be a good alternative. More durable catalyzed finishes, like conversion varnish and polyurethane, can have dry to stack times in the 8-24 hour time range (and remember that's under ideal conditions).

From the original questioner:

Thank you. I have taken all of your advice to heart and hopefully I'll come to a conclusion soon. A few notes: the nitro lacquer I'm using is newly purchased; I usually let the plaques dry at least two days; and I usually coat them with two coats of lacquer with no sealer. I'm beginning to think it might be the product I'm using especially after the last response. I noticed what might be an answer from a previous forum thread...

"I switched to pre-cat Gemini about 5 kitchens ago. No comparison with the old NC lac. This stuff is pro grade, compared to the old school lac. The only trouble I have is convincing folks that I didn't buy them at a store and actually made and shot them myself. No blushing trouble, no trouble stacking parts after one hour because they don't stick together anymore. Cost about 50 bucks a kitchen more and worth every penny."

Sounds like he had a similar problem and switched to pre-cat. Problem is that I just bought 5 gallons of this nitro lacquer and the price was out of this world. So I'll be contacting Gemini tomorrow to see if they can help me out.

From contributor K:
Gemini makes a pre-cat lacquer. I spray my doors with it and you can stack parts in as little as one hour without problems.

From contributor E:
High build should never, ever be used for what you are trying to finish. I highly recommend you move up to the Gemini 700 series of high solids lacquers. Your issues will go away.

Gemini's High Build has plenty of plasticizer and maleic resin, which makes it great for new-home construction in areas where durability and longevity are not required. Which Kwal store is selling this to you? I am the sales rep for Missouri and will be glad to help you address your issues on a one-on-one basis.