Friday, November 23, 2007

Sap Issues

Pine has a lot of good qualities for chairmaking, it grows into large trees quickly, carves beautifully and has beautiful grain. However, when I was learning to paint chairs, it presented a great difficulty. The abundance of sticky sap that resides in pine does not like to be painted, and often the paint will chip off from the sap filled pores during the burnishing process, revealing little white patches in my black seats! This is problem really comes to light because I prefer to burnish hard to get a high sheen. For those content with a matte finish, this may be a non issue. Here is a photo of a nasty pitch pocket that I came across while making the prototype from my recent "Frankenchair". I chose this piece of pine knowing that it would be going into an experimental chair.

alt=""id="BLOGGER_PHOTO_ID_5136035643841279458" />

The problem comes from milk paints quick drying. This creates a dramatic surface tension that can form a weak bond to dirty or sappy wood. The photo above shows the sap pocket after heating with a heat gun to liquefy the surface sap. Next I rinse the seat with naptha twice (wear gloves!) The photo below shows the seat after rinsing.

Finally I stain the seat with a water based walnut stain, which raises the grain. As you can see, the stain covered the sappy area easily and gives me confidence that the paint will hold as well. I do use extra-bond in the first coat on the seat, sort of a belt and suspenders approach. Subsequent coats are plain milk paint. Following this process has yielded me trouble free results for years, regardless of the sap content of the seat.


Anonymous said...

Hey Pete, while your on the subject of seat wood. I came across a very large butternut that a guy is sawing into seat wood for me. If I air dry these boards how long typically til I can make chairs from this? I have a moisture meter so is there a target percentage I should look for? Thanks, Greg P.

Peter Galbert said...

I generally put green butternut in my open air storage building (an old school bus) for about a year or over the span of a summer. Then I bring it into my basement to finish seasoning. There is no hard and fast rules, just don't dry it too fast or it will check. I don't often use a moisture meter but when I have, I've used wood that reads up to 11 percent. Once again, it is more important that the wood is acclimated to your enviroment than any magic number. Seat blanks tend to warp when carved because of the wood being removed from only one side, excessive warping can indicate that the wood is too wet. My advice is to saw it, coat the end grain and forget let it sit until you can't stand to wait any longer, or until it stops losing weight, whichever comes first. It is also a good idea to keep the wood in the shop for a month or so before working it to help ensure that it is not going to harbor any big suprises. Good luck.