Question
A week ago I sprayed doors and drawer fronts with white precat lacquer and they remained slightly tacky to the touch hours after they were sprayed. Even after three days I could not stack them because they would weld together. It was 98 degrees and 98% humidity when I sprayed. Could this be the cause or is it bad paint?
Forum Responses
(Finishing Forum)
From contributor A:
Did you add retarder? High humidity is a major issue. Luckily I live in Phoenix and don't have to deal with it. When I lived in Pittsburgh spraying was difficult.
Here's the solution:
After the items have flashed off, set them in an environment where a fan is running, i.e. blowing air over the flashed surfaces. Note that there should be no turbulence at all when a surface is flashing. It is OK to have air movement only after the finish has flashed and knitted itself together into a film (what originally was a liquid when it was applied). This running of the air will help but it does not always work. The theory is that the moving air will help to carry away the solvents as they continue to gas off out of the formed film of finish. If you can, in the same room, raise the temperature (turn on the heat). In a closed setting (a fixed space), a room with the higher temperature has less relative humidity than the same room with the original temperature. The room seems drier because it is. Condensation forms as temperatures lower. When it is cold enough, then you reach the dew point. A hot room can hold more moisture in the air than a colder room can.
You will have two competing conditions:
1. You are exhausting the solvent-filled air out of the room.
Balance the two and your coatings will cure and not print. Or, wait it out. Additionally, you might want to explore the possibility of using a lacquer thinner rated as quick. These lacquer thinners are usually used in cold environments because at some point cold ambient conditions also inhibit a coating's ability to knit together properly and form a dry solid film of finish.
Note that waterborne coatings do not actually knit together, they coalesce. That's just a little informational aside brought to you by Bob Flexner who has recently revised, updated and re-released his finishing book "Understanding Wood Finishing".
I have been adding acetone to my lacquer thinner, with varying degrees of success. The downside to speeding up the flash-off is that you can get a reduction in the amount and quality of the flow-out of the finish as it begins the process of knitting itself together into that film of finish. The problem with adding just acetone is that it is a solvent which evaporates from the finish quickly. I am still left with tail solvents which are latent in the finish and which are the cause of the printing problems. It is these latent tail solvents which are gassing off after the finish has flashed. The finish is flashed, more or less and it is still going through its shrink-back process as the tail solvents work themselves out of the coatings. This is the gassing off. This is where the fan and its movement of air across the flashed surfaces becomes most useful, carrying these solvent gasses away making room for new gasses to surface and exit the film. Fast drying lacquer thinners are blends of solvents and this is what we finishers need, good and balanced blends because we do best at mixing, not playing formulators and guessing.
After adequate flashing, the dissolved resin (in solvent which is leaving the picture) begins to polymerize, or knit together in long, stable chains of molecule, forming the film. This is the second stage. Now imagine a scenario where during the middle of the flash stage the film starts to form. What does this mean? Once the solvent has flashed, the film starts to form. If you picture the coating as 10 feet thick, imagine that the first foot flashes and a film starts to form. This film inhibits the 9 feet of solvent underneath it from evaporating. At this point a couple of things can happen:
1. The evaporating solvent trapped below the film builds enough pressure to tear through the film which leaves a bubble frozen in the coating. This is called solvent pop.
2. The solvent continues to evaporate but at a much slower rate. This can lead to tacky precat.
As Contributor B pointed out you want exactly the right amount of air movement (typically minimal) during the flash period to let all the solvent emerge from the bottom up. If the coating skins over you will have a longer dry time. If you add retarder you will slow the evaporation rate and hold the coating open or wet longer. Retarder can be used during high humidity so that excess water vapor (humidity) in the coating can make its way out. If the film forms on top too quickly, water can become trapped and leave a grey cast in the finish - called blush.
My guess is that you sprayed too heavy a coat so that even in optimal drying and curing conditions you didn't let the bottom solvent out. If it’s a precat you can try spraying everything with straight retarder. The logic here is that the slower solvent will dissolve the film that has formed and allow the trapped solvents to come out. I have done this successfully to remedy blush with precats. When you start spraying postcats, there are other challenges to face.
If your finish is not drying, there's something wrong beyond the heat and humidity. It could be a bad batch. Get your supplier to replace the lacquer and see if it doesn't work better. If it doesn't, I'd switch brands.
Comment from contributor A:
