Question
I am starting to see more and more cases where maple and birch veneer faces show blotchy spots after stain is applied. Before the stain was applied the face appeared clean and ready to finish.
I have two questions: What can be done to correct the problem if it is not detected until after the stain is applied, but was applied with no sealer? And what testing can be done to tell if the blotchy spots will appear before I put a stain on the face?
By the way, the panels in question have already been installed as wall panels and are being finished on-site.
Forum Responses
Blotchiness on birch or maple veneer is most likely due to poor sanding. Also maple and birch are notorious for having a blotchy look after staining.
Your veneer should be sanded to 180 prior to applying the stain. With the limited amount of information provided I am assuming that you are using only a wiping stain to achieve your color - if not, please post exactly what you are doing in your staining process.
A washcoat will help eliminate this problem. Mix one part sanding sealer to one part lacquer thinner and spray a wet coat, let dry, then sand with a 220-grit stearated paper or 3M fine-grit sanding sponge prior to staining. You will be amazed at how even the color looks after a washcoat and proper whitewood sanding.
I also recommend using an N.G.R. dye stain and/or toner prior to the washcoat to add depth to the finish and even more color uniformity.
Also, when you say you sanded with 280 does that mean that you sanded in progression or did you just sand with 280 only before finish? Generally speaking you start with a 120 then go to 150, then 180 then stop right there because that is about as fine as you need to go on your white wood sanding. If you did sand with 280 only prior to finish then you just wasted time and sandpaper.
As far as a simple solution or fix to this problem there really isn't one. If I am wrong here I would like to be corrected if anyone else has any advice on this.
There are still some unanswered questions as to exactly how you stained the panels.
Obviously, if there is an existing topcoat, it will need to be prepped appropriately before you should proceed.
Are you working with conventional finishes or catalyzed finishes?
The last poster is right about sanding. There is no need with any lacquer or conversion varnish to sand above 180 grit when you are white wood sanding. If you are sanding a washcoat, then 280 grit is O.K. (more abrasive than that and you are apt to sand through the washcoat).
Comment from contributor A:
When painting trim in home interiors, we use Liquid Sander/Deglosser. It promotes uniformity in the liquid acceptance of the area to be stained, and removes grease. I've found mineral spirits or lacquer thinner just spreads the grease and oil around, while this liquid neutralizes it or dissolves it when wiped down completely.
