Quick green bowl question

[Scott Barton]

I am experimenting with "finishing" the bowl in it's green stage so that it can warp all willy nilly.

Trying to understand the actual "finishing" part, I was thinking I would sand to final grit and apply some penetrating oil and then let it sit and do its thing. Then, after it dries for some bit, come back and buff it.

So, I tried that. Now, I am seeing some cracking and checking. Now, if that is the story the wood is going to tell, I don't care and I love it. But it is normal to expect that?

there is so much info out there on turning green wood, but not so much about finishing it.

 

[hockenbery]

Scott

I let my green bowls dry without finish or sanding. This takes 3-5 days for those 1/4 or less wall thickness and a couple of weeks for those with a 1/2 wall thickness.
Then I sand them mostly with a 3" Velcro disks and an angle drill. Then any finish works . A favorite finish is 3-6 coats of waterlox letting it dry over night between coats. I wipe off the excess oil after each application.
I sand the raw wood to 320, the first coat of finish with 400, 0000 scotchbrite between other coats.


To avoid cracking I follow these guidelines
1. use wood that isn't already cracked. If I rescue a blank from the fire woodpile I expect it to have cracks in it.
Wood I have just cut I keep from drying either by sealing the end grain or putting it in a plastic bag.
I turn it before it begins to crack. A few day, weeks, or months Depending

2. turn a form with flowing curves, avoid sharp angles.
A bowl that has flat bottom meeting vertical side walls at 90 degrees just wants to crack
A hemisphere bowl can just move and twist and almost never cracks if rule 3 is followed.

3. turn an even wall thickness all around the rim can be a little ticker than the side wall.

4. Keep the bowl wet while it is on the lathe either by turning quickly or misting.

Have fun,
Al

 

[Scott Barton]

Ok, good advice. You mention you put it in a plastic bag. Does that allow mold? Are paper bags acceptable?

 

[Thomas Stegall]

In the case your describing its simply a case of checking as it dries unevenly. That fact that it has finish on it, is not relavant to the drying process. Your putting finish on it was unfortunately, premature. You want to make sure that you have a piece completely dry before begining the finishing process.

Recently Lowe's home improvement store started carrying a very affordable wood moisture meter for about $10 instead of the typical price of one for $80-200. I have one and it appears to work reasonably well. And yes wrapping in a paper bag (grocery bag) is an good way of slowing the drying process to reduce the chance of checking. Depending on the wood type you might want to seal the endgrain AND use the paperbag. Open grain woods like Oak can dry faster than other species. In Al's response mentioning the plastic bag, I think he may have meant wrapping the endgrain (when in log form) not the whole piece, and yes wrapping the whole piece will often mold within days depending on the species and moisture content.

 

[Scott Barton]

Your putting finish on it was unfortunately, premature. You want to make sure that you have a piece completely dry before begining the finishing process.

No worries. So I messed one up. Chuck it up to lesson learned. I don't think I would have understood the consequences as vividly as I do now.

I still had a blast turning it!

 

[hockenbery]

Ok, good advice. You mention you put it in a plastic bag. Does that allow mold? Are paper bags acceptable?

Scott,
I wasn't especially clear

1. The plastic bag is for the unturned blanks.
This is for wood you plan to turn in Days

Blanks left in plastic for a month in the summer may be rotten.

The main thing is to keep the blanks from drying before you turn them
Drying is usually accompanied b cracking large thick blanks can't bend as they dry so the pull apart ap leaving cracks.

2. Finish Turned bowls and hollow forms can be put it a box or paper bag for the first few days.
Big hollow forms I want to get some insurance on I put in a cardboard box: flaps closed first day, partly open the second day, fully open the third day, and out of the box on the 4th day. The box makes a humidity chamber that promotes even drying.


Al

 

[MichaelMouse]

Since nobody else did, I guess I'll ask where you had the piece after dipping. That's the key. You want to keep the relative humidity high for a while - 65% or better perhaps - so that the outside won't start to check and possibly pull apart to a crack. Too high for too long is mildew time.

No unperforated plastic for the rough more than overnight. I wouldn't use wet shavings in a cardboard box or paper bag, either. Dry is good. That way they start equalizing with the piece and buffer the letdown. Change when damp. When the wood's lost dimension appropriate to the trip from saturation to equilibrium, weigh. Week and weigh. Same weight, it's as good as it gets for where it is. Moisture meters are for planks stored somewhere else by someone else, not for stuff in your environs where you have a hygrometer.

If you're going to turn once and let go, turn under a half inch thick body and turn a piece with good slope or curve to the sides so it won't split near the bottom. Don't worry about that "equal thickness" myth. It's how wide, not how thick that ruins things down there. you can leave some wood for a low CG with no worries.

FWIW, the wet warp stuff doesn't really ever look smooth after it's dry, so I sand dry and then finish for a reasonable surface. If it's "period" you're after, don't sand at all.

 

[KellyDunn]

Scott, Turn the outside. If its sopping wet put some air on it for a bit so you can sand it. Then put some finish on the outside. I use a wipe on poly. Be liberal. Let it sit a few then wipe dry with a paper towel. thats only so you dont have too much of a mess to buff off later. If it actually soaked up the finish put some more on. Take down your inside and sand that. The outside finish keeps the moisture from evaporating quickly. Finish your bottom then put finish on the whole piece. Same thing. put on wipe dry. Let it sit and do its thing. Depending on the wood it may still open up and and move like old leather. Or it may do very little. Or something in between. I give David Ellsworth credit for teaching me the put on finish on the outside before doing the inside. It was for hollow vessels but works on bowls just as well. I have a few examples of green turned burl on my website. Not sure the photos will really give you an idea of all the movement. Robusta burl and blue gum burl.
www.kellydunnwoodturner.com
After its done all the moving its going to do I may put more finish on it. Most all my work gets buffed and waxed.

 

[Scott Barton]

Michael--

I have my lathe in the basement. I took the bowl upstairs. and set it on the counter. I didn't know about putting it in a bag until the second day. I put it in there and haven't looked at it since then.

Kelly-

If you put the poly on the exterior, what do you if you find that the exterior shape needs to be adjusted slightly as you hollow?

 

[MichaelMouse]

I have my lathe in the basement. I took the bowl upstairs. and set it on the counter. I didn't know about putting it in a bag until the second day. I put it in there and haven't looked at it since then.

Presume you're heating the upstairs to human comfort? If you had a hygrometer, you'd probably show something close to mine's reading - 45%. That's an 8% moisture content, and a whopping difference for a piece which is, at best ~30% moisture content. Thicker you left it, bigger the difference between interior and surface. The outside of the bowl is under stress as the wood contracts, and it only contracts across the grain, not along. Thus a dry spot on the end grain can make a check which is opened by mechanical stress, ruining your efforts.

You need to keep the loss rate from the surface in proportion to the interior, so you keep the whole at an elevated RH through close containment with wax or a bag, or box. Or put it out in the unheated garage where the RH will be a bit higher and the molecular activity a bit lower because of temperature. It'll come down slower out there, and you can feel and see the loss.

Here's what happens when you leave a fresh rough overnight on the table under a heating duct. http://i35.photobucket.com/albums/d160/GoodOnesGone/f2267132.jpg Almost no distortion. All the tension has been relieved by the crack.

If you let things happen slowly, the wood will distort, but not crack. http://i35.photobucket.com/albums/d160/GoodOnesGone/Edge-Droop.jpg
Love the garage as a seasoning spot during heating season. Summer they can stay on the basement floor in a corner for a week or two.

http://i35.photobucket.com/albums/d160/GoodOnesGone/Bowling.jpg

The stack of oak underneath is now 1 2/3 captains' beds. Working one now, which is why my hands are full of splinters.

 

[Scott Barton]

Michael--

Ok, your last post puts it all together. I understand where we are going now. I feel like I have a much better understanding of what is actually happening. Along with everyone's posts, I think I have a decent process laid out now.

Predictably, the first bowl I turn cracked. While I don't mind it on this first one, I think I'm more informed so avoid this on my future work.

Thanks!

 

[robo hippy]

I turn green to final thickness, let them dry and warp, then sand and finish. It is easier to sand out when dry, though there are a number who promote putting oil finish on them and wet sanding. I may have to give that a try some time. Taking the bowl from the basement to the house could be a big change in temperature and humidity. I would keep it in the basement for several days before moving it up to the house. I do make sure to round over the bowl rims as the sharp/square edge is more prone to cracking. I don't don't use the shavings method though it does work. I have had several cases where damp shavings colored the bowl, and not in a good way. Summer in your area (I grew up in St. Joe) will not require bagging due to the high humidity, though air conditioning can make the inside of the house a lot drier. For me, down on the floor to start and then up on a wire shelf. I also wrap the rim in plastic stretch film. A bit of compression on the rim helps prevent cracking. Do make sure to turn out ALL cracks.

robo hippy

 

[KellyDunn]

Scott, I would say dont put finish on the piece till your happy in the 1st place. The finish will help keep the piece from going oval as you do the inside. And help prevent cracking. But depending on the wood anything can happen. This method has worked for me for about 26 years now. Nothing is foolproof in our medium though.

 

[hockenbery]

Scott
I has been my experience that experienced turners have very few pierces crack less than 5%
Beginning turners depending on how fast they learn tend to have a much greater percentage of cracking.


Poor quality wood
Poor form
Uneven wall thickness
And drying out on the lathe

When you have been at it a while
You'll choose good wood, make flowing curves, turn even walls and work quickly.

Hooking up with a local club or taking a course in bowl turning can speed your development.