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