Expert: Greg Scholl - 3/15/2009

Question
QUESTION: Hi,
I am building flat panel, mission style, kitchen cabinet
doors using cherry and plan to finish with 3-4 coats of
polyurethane.  

I've done this once before and had problems with the panels getting adhered to the frames by the poly. The smallest
amount of shrink/swelling on the frame causes unsightly
seams where they meet the panels.  To avoid this problem I
am contemplating the following:

applying the poly to the panels BEFORE assembly;
then assemble, glue up, and mask off the panels;
then poly the frames.

Do you recommend another method of avoiding the problem?

Thanks very much.


ANSWER: Hi David, Two things come to mind, First is to eliminate as much possibilty of shrinkage by selecting and using properly kiln dried/aged lumber. Use veneered panels for the doors, (plywood), and remembember to store that lumber and the finished constructions in a climate monitored situation,too. Lumber can take on, (or give off), a lot of moisture pretty quickly in some storage situations... The second is your method of finish application, (and the finish itself). One reason why spraying excels at this type of finishing, is that it has the ability to be applied very evenly and lightly in a controlled manner, 90 degrees to the surface to keep from pooling and coating too heavily. You didn't mention your application method, but if you're brushing, there's no way to keep the finish from getting into the joinery through capillary action, and then hardening there to cause the cracking and the other conditions you describe. Even with your 'masking the panel' method, you'll risk this problem. You'll also be adding a lot of time to the project. The application method is key, to being able to lay down a controlled, thin, even, film of finish, with no pooling and other problems associated with other application methods,it makes a tremendous amount of difference. If you aren't accomplishing that with your spraying technique, then you're applying it too heavily, or it isn't thinned properly, (or both).... Post back if need be, but that should help with the issue a bit....regards- Greg

---------- FOLLOW-UP ----------

QUESTION: I appreciate very much the info.  

Spraying application really isn't an option for me right now.  So given I will be brushing do you have any recommendations on method? i.e. should I use a thinned coat
on first, second or all coats?

I plan to use Minwax Clear Satin poly.

Thanks much.
David

Answer
You're going to run into these problems if you brush, and thinning really won't have any effect, and it can make matters worse by penetrating even deeper into the joinery where it will cause those issues you've outlined. If you are planning to use Minwax, (just an ok choice for kitchen cabinets), you could buy it in spray cans, and their spray cans have a good nozzle allowing one to get really good results. The problem is that it's fairly expensive to buy a lot of cans, and to get good results you have to have good technique....applying a wet enough coat to properly flow out, but not too wet to pool and cause the issues you're trying to avoid...it's a FINE line...but a better choice than brushing. Also with Cherry, you shouldn't need that many coats...the thicker you apply the final film, the more chance of cracking at the joints, etc. The bottom line is that if you're building Cherry cabinets, an expensive proposition, this isn't a professional quality finish for this type of work, and you would be much better off using a finish that will withstand the kitchen environment, and something that's KCMA rated...like a precat lacquer or a conversion varnish, that will give the durability and maintenance free finish for 20+ years that a cabinet should have. Most of these also have UV inhibitors (sun screen) to keep reactive woods like Cherry from drastically changing over time. These finishes also make a chemical bond between layers, ending up with a film layer that is essentially one cohesive layer, where the Minwax Poly, as other conventional poly's and varnishes, only have a mechanical bond between coats....prone to premature aging and failure. Your other option is to have them sprayed by a shop that does these finishes. One other choice might be a wipe on finish like WaterLox, but it depends on the look you're after...hope that helps- Greg

http://www.waterlox.com/site/431/default.aspx