Finishing for hot dry and wet applications
Hello all, my first post here and I will start by apologizing if I am posting in the wrong area.
I recently made a wooden handle for an espresso Portafilter out of a nice piece of well dried oak. I finished the wood with BLO and then a good layer of paste wax. After two days of use on the espresso machine, the wood had dramatically shrunk and it now feels like it has never seen a piece of sandpaper AND the wood looks and feels incredibly dry in spaces. As the shrinkage is so bad, I will have to make a new one. So, does anyone have some advice on how I should treat the wood for this particular use? Key things in my mind: the wood gets quite warm over an extended period of time due to the coffee machine (there is a metal rod running through the length of the handle that heats it up), but it also gets splashed with a lot of water when I clean it.
How long did you let the BLO dry after application?
This might be a good spot to leave the handle unfinished.
How warm is “quite warm?” I was thinking of more of a film finish that would withstand both the heat and the moisture. It looks like most epoxies that are readily available and could be used for both bonding the handle and coating the outside have a max service temperature of about 160F. Polyurethanes look to withstand closer to 200F from some quick research.
You could glue the handle on with Gorilla Glue and finish the outside with poly.
Additional thought : if you ever want to get the thing off, you can undo epoxy with a heat gun. With Gorilla Glue I think you’d need to break away the wood and grind away the glue residue. Might be preferable to buy a new portafilter in that case.
You don't think that poly would crack with the temperature changes? the rod that holds the handle on runs the length of the handle and is threaded, so I just drilled a hole to the right size and screwed the handle on - works like a charm.
The BLO didn't dry for long - just a light coat and dried for a few hours, then I hit it with the paste wax. leaving it un-finished is not in the cards as coffee would horribly stain the handle....
The main issue is the shrinkage of the wood - you can see and feel the wood fibers all around the handle. It looks worse than unfinished/unsanded wood.
In terms of temperatures: the brewing head gets up to 200 degrees (93c) The rod that runs through the length of the handle which is roughly 5 -6 inches (130-150mm). I would need to dig out my old thermodymanics texts to tell you what temperatures it is encountering on the inside. In any case, when the machine is on, the handle is dry and is heated from within. I typically have the machine on for 1-2 hours at a time. Thus, I suspect that the wood is drying out more than normal.
The porta filter will get quite hot. If you are set on a wood handle Cocobolo or Gaboon Ebony might work well. They are both dense and oily with very fine grain. Neither would need any finish at all as they will polish well. A little pure tung oil (thinned) might work on the ebony, I don't think the Cocobolo will absorb anything. I would not use wax as it offers no protection against heat or water.
A few hours isn't remotely long enough for BLO to cure. A few days isn't enough.
A bunch of things might have happened. I'm guessing the heat might have caused the uncured oil to migrate outward, and combined with the wax, caused your issues.
If you want to stick with oak, and really want a finish, try oil based poly or varnish. Skip the wax.
You have 2 things going on here. The wood is shrinking because "well-dried" to normal room temperature humidity levels is not nearly as dry as heated wood gets. So the handle still loses a lot of moisture, and thus shrinks. Neither a light coat of BLO nor paste wax significantly inhibits the transfer of water vapor in or out of wood. And if your oak happens to be red oak rather than white, moisture will go out of the open pores of the wood in the end grain. To make a handle that doesn't shrink, you will need to dry the wood further, probably in an oven at a moderate temperature (200º?) to get it to the dryness it will be at in use. Then make the handle from it and keep using it so it doesn't swell up.
The surface issues have the same root cause, which is the lack of water resistance of your BLO and paste wax. So water is soaking into the surface, heating up and swelling the fibers, causing them to separate and roughen.
To be honest, I can't say for sure what finish would be best. However, I would try multiple (5-7) coats of a durable wiping varnish that is designed to penetrate the wood first, then build up a waterproof thin film. My go-to finish for this is Waterlox Original, which I've used a lot. It stays soft enough to move with changes in the wood, and won't crack or craze.
I don't think oak, especially red, is a particularly good choice for this handle, as it is not very stable, and in particular shrinks a lot. Walnut would be a better domestic choice. But even with it, you will need to get it much dryer than for normal furniture use to keep it from shrinking when heated by the rod.
Your best solution is to abandon the use of wood and re-make the handle out of some synthetic material such as acrylic or some of the other man-made meterials used for counter tops. Also, check out some of the synthetic materials used by pen makers.
I was leaning in the same direction. This is not wood's best environment.