Spraying a piece with nitrocellulous pre cat lacquer, have about 50/50 results over the past 2 gallons. problem is that i have to wait for a "nice" day because i have to spray outside. thought today would be fine here in the northwest, hasn't rained for days, temp got to 45-50 degrees. and damn got a blush finish! i'm ready to try my hand at another type of finish, I've heard of conversion varnishes...what are they? sprayable? durable? would i get in the same trouble with say an Oxford waterborn lacquer? I'm always dealing with temperature and humidity problems where i live and would like suggestions. thanks
Not to sound glib, but nice days are a real luxury when finishing. A perfect day would be 65 degrees, 30% humidity, no breeze (at all), no bright sun, and having all of those conditions stable for the entire day.
Where I live it is high humidity almost all the time (80+ percent is not at all uncommon), the temps easily move 25 degrees or more in a day, and if we aren't having thunderstorms it is a drought, with harsh sun and 100 degree days. Welcome to South Texas.
Blush is moisture, period. It can be moisture on the surface of the material you are spraying, it can be moisture in your air lines (you didn't say what system you are using to apply) that will give you blush at the start of the spray, your moisture trap may be full, or a few other things.
I hope you are keeping a log of your efforts so you will know what works and what doesn't. You should note all the variables mentioned above as well as the temp, the amount of solvent applied, and the temp fluctuations during the cure time.
For what ever reason it may be, the material "blushes" because moisture is trapped underneath the lacquer as it dries. The dried top coat doesn't allow the lacquer to properly outgas the solvents, including any moisture that will be a part of the application process.
So taking for granted that you have already made sure your gun is clean, your solvent is clean (and isn't really cheap crap), your moisture traps are clean, etc., etc., take a look at one of the most common variables that cause blush.
Incorrect solvent mix, coupled with incorrect application. In your case, it may be something as simple as the surface of your project being much cooler than your applied material
Depending on the thickness you spray, the bottom of the film will dry at a different rate as the cold surface will slow down the curing process, causing it to be behind the curing process as compared to the top of the film which will freely outgas its solvent to start to cure right away. That was hard for me to learn, and until I did, I didn't believe it.
It is almost impossible to diagnose long distance, but these tips
project, but not the whole thing. (This tip was worth the price of admission when I got it!)
I respray the areas with a mix of 20% lacquer material and the rest lacquer thinner at about 6 mil thick. Sounds like suicide... but it may be the ticket for you. It works best if sprayed when the lacquer is at "thumbprint" dry. You would be surprised how well that works. It softens and redissolves that top layer and allows the moisture to escape. This is the most valuable tip you can get for a lacquer finish when you are dealing with stained or dyed woods that cannot be sanded.
The other thing I do after I sand off the blush and quit swearing is to start from scratch. Since lacquer resolvates, it is hard to mess up if you have a grasp of how it works.
I sprayed out big mahogany doors at a restaurant, outside in the elements when it was in the upper 90s, and the relative humidity was at our normal 80+ percent. I started with my lacquer thinned with low VOC (L2 - not hot) at 50%. It was like spraying water. I put it on heavy, and it dried nice. I got it up to about 75% or so lacquer to 25% thinner before I started to see the finish acting up. I cut it back to a 70/30 mix and it was fine.
But until I got my mix right, it took me several hours of work just to spray one side of one door. When I got blush, I sprayed with super thinned material and it fixed it. I wanted to move inside or take the doors to the shop, but the door couldn't leave the premises as is the case so many times. I was trapped, and the elements were killing me. I wound up spraying out on the large veranda of the club in the open air during off hours and it was just painful I was there so long.
But for the rest of the project, I had my notes with temps and batching mixes with the time of day and it was a breeze.
Too long to explain here, and too easy to Google. There are precats, post cats, CABs, conversion lacquers, conversion varnishes, etc. It is too easy to confuse the names and their applications.
Try starting here, and explore the site. The goal of good finishing is reached by a lot of different methods, and you won't find a more knowledgeable library than this:
This from the guy that finally got me to shoot poly, told me how to "hang" a second coat successfully (I thought it was folklore) and you are telling someone to stay away from a finish?
Too damn funny.
But on the other hand, it could be good advice....
I just figure that is someone else is doing something day in and day out (like finishing with a certain product) I ought to be able to at least get in the game.
I've found "dew point spread" to be more reliable than looking at temperature and humidity.
A 70F day, with a dew point of 52F is an 18 degree spread. A 70F day with a 62F dew point is an 8 degree spread. The dew point is commonly available on most weather web pages. I live near two small airports, so I dial up the automated weather systems, which are updated once a minute. Weather Underground (wunderground.com) will grab local airports and rapid fire update it, based on the user's zip code.
Many NC lacquer vendors recommend 20 degrees spread, I've gotten away with 15 several times. Most product information sheets, available at the finish mfg's web site, will contain such information, much more than is usually printed on the can.
On colder days with the proper spread, fast thinners will work fine in NC lacquer, with no blushing. People sometimes get caught on cool days by what they think is low humidity, but in reality, the dew point is too high.
I don't spray water base under 72F without warming the finish in a water bath. I'll use the water bath down to ~ 65F. Below 65F, I don't spray WB at all. 1000 watt worklights will also slightly warm the work surface, helping WB flow-out. WB does not seem to be very humidity sensitive, but the folks who live in the humid places like Houston, or the very dry deserts see extremes that I don't see.
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