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How to correct board cupped by shellac

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Forum topic by mds4752 posted 12-13-2013 04:37 AM 1514 views 0 times favorited 16 replies Add to Favorites Watch
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mds4752

48 posts in 1170 days


12-13-2013 04:37 AM

Topic tags/keywords: shellac milk paint

Help me understand where I went wrong tonight. I built a 6 board chest and finished the outside with 2 coats of commercial milk paint. Chest sat idle & completed for about 2 weeks now. Tonight I applied a 1 pound cut single coat of shellac (made 2 nights ago w/dewaxed tiger flakes) to interior for protection and the lid cupped so fast I could hear the battens straining the cut/clinched nails holding them on. The lid is 3/4″ pine and was 2 boards I edge glued together.

Why is this happening & where did I go wrong? And can it be fixed?

-- "Live each day as if it were your last; one day you're sure to be right." -- Lt Harry "Breaker" Morant


16 replies so far

View Loren's profile

Loren

8295 posts in 3109 days


#1 posted 12-13-2013 04:40 AM

Leave it alone and observe it. You can strip off the
shellac in a couple of weeks if you don’t see an
improvement.

Did you account for wood movement when fastening
the battens down? If not it could be the moisture
in the alcohol is indicating a problem with the construction
method.

View Earlextech's profile

Earlextech

1159 posts in 2151 days


#2 posted 12-13-2013 01:43 PM

You don’t need “protection” on the inside. The milk paint was allowing the wood to breath and move. The shellac sealed it and most likely pointed out a flawed assembly.
Pictures would help us understand. It can be fixed.

-- Sam Hamory - The project is never finished until its "Finished"!

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bbc557ci

589 posts in 1535 days


#3 posted 12-13-2013 02:28 PM

About 20 years ago I built a Cherry table with leafs/extentions and a smaller table made of Ash. I used poly on both. I applied 3-4 coats of poly to both sides of the tops figuring expansion/contraction would be equal, and reduce cupping. That worked well.

-- Bill, central NY...no where near the "big apple"

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a1Jim

115202 posts in 3038 days


#4 posted 12-13-2013 02:42 PM

Typically you want to have finish on both top and bottom to equalize moisture infiltration ,in this case you have what I’m guessing is water base on one side and alcohol base on the other creating more of a moisture barrier on one side than the other. It’s also possible you used wood that was not completely dry in the first place.

-- http://artisticwoodstudio.com Custom furniture

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mds4752

48 posts in 1170 days


#5 posted 12-13-2013 03:56 PM

I’ll add some pics this evening or weekend. Thanks for the theories folks; I appreciate any and all input or suggestions.

Wood used was pine from lumber yard—was S4S or S3S. Standard stuff, so I think it was as dry as a routine situation. Milled it to thickness with my planer. Jointed the edges of two 12” boards and glued it up using cauls and clamps. After glue cured, I cut it to final dimensions and planed it smooth and flat with hand planes. I then assembled the chest (uses snipe hinges for affixing the lid, which is 18” x 40”) and gave it 2 coats of commercial milk paint. Once assembled, the lid was very flat in all directions and remained that way for the 2 weeks after assembly and before putting on the shellac on the inside. It looked great.

Read a few articles and considering the chest will be used to store kids’ outdoor toys, I thought a thin coat of shellac on the inside would help protect the wood from water, moisture, mud, dirt, grass, etc. So, I bought some dewaxed tiger flakes and mixed up a 1# cut and applied it to all surfaces of the the inside of the chest.

I obviously failed to take into account the impact it would have on the lid I guess in combination with the commerical milk paint. In any event, the lid cupped and bowed immediately after applying the shellac.

-- "Live each day as if it were your last; one day you're sure to be right." -- Lt Harry "Breaker" Morant

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mds4752

48 posts in 1170 days


#6 posted 12-15-2013 01:12 AM

As promised, here is a before pic (shows lid nice & flat) and an after pic (shows cupping after milk coated exterior and shellacked interior).

I guess it’s a lesson learned that this is a poor combo choice I made.

-- "Live each day as if it were your last; one day you're sure to be right." -- Lt Harry "Breaker" Morant

View woodenwarrior's profile

woodenwarrior

203 posts in 1655 days


#7 posted 12-15-2013 01:44 AM

Are you sure it was the shellac that did the deed and not your milk paint? I was looking at your unfinished lid and both edge glued pieces are flat sawn. Once you applied that water based milk paint to the lid it would seem ( to these eyes) that the lid took hold of the water in the paint and went bananas on you. Shellac being alcohol based should not do that. If it were me, I would build a new lid with either rift sawn or quarter sawn pine. It may take some looking in the bin at the hardware store (I assume this is where you bought it) but it will be worth the effort.

-- Do or do not...there is no try - Master Yoda

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a1Jim

115202 posts in 3038 days


#8 posted 12-15-2013 03:41 AM

From what I can see of your photo you have trim glued to each end of the top ,if that’s the case you are not allowing for wood movement and that’s what’s causing the bowing unless they are taken off the next thing you will have to contend with is a cracked top

-- http://artisticwoodstudio.com Custom furniture

View Loren's profile

Loren

8295 posts in 3109 days


#9 posted 12-15-2013 03:55 AM

The moisture in the shellac probably cause expansion of the shellacked
face. It may dry out and go flatter.

View tefinn's profile

tefinn

1222 posts in 1898 days


#10 posted 12-15-2013 04:39 AM

mds4752 – I totaly agree with Earlextech. What happened here is the pine boards you used have a higher moisture content than you might think. The milk paint allowed the wood to breath. When you sealed the inside, only the outside of the box was still allowed to breath. As it lost moisture the outer side of the boards shrunk, causing the cupping. With higher humidity it may flatten out. Your best bet would be to sand off the shellac and see if it straightens out with time. If it does leave the inside unsealed or seal the entire box, inside and out.

-- Tom Finnigan - Measures? We don't need no stinking measures! - Hmm, maybe thats why my project pieces don't fit.

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mds4752

48 posts in 1170 days


#11 posted 12-15-2013 05:06 AM

The trim on the ends of the lid are battens attached with clinched cut nails, so I believe that will allow some movement of the battens and lid.

I think I’ll sand the shellac off the inside of the lid and see if it flattens out. I haven’t noticed the sides or bottom cupping but they’re pinned in pretty good with rabbets and nails and glue.

I guess I totally underestimated how the boards would react. The shellac was only a 1# cut too.

My other option would be to put shellac on the outside of the entire chest, but I think that would change the appearance of the milk paint.

-- "Live each day as if it were your last; one day you're sure to be right." -- Lt Harry "Breaker" Morant

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a1Jim

115202 posts in 3038 days


#12 posted 12-15-2013 05:27 AM

Nails usually allow for some flex but clinched nails and on top of that they are cut nails take away alot of that flex if not all of it.If you feel the battens are not the problem then try dampening the top of the top with water you can ether mist it on with a sprayer or do a light wipe down with a damp sponge and see if that flatens it out then let it dry and coat the top with the same cut of shellac to the top.

-- http://artisticwoodstudio.com Custom furniture

View Aj2's profile

Aj2

687 posts in 1259 days


#13 posted 12-15-2013 06:59 AM

I’m with woodenwarrior on this one use quarter or rift sawn for a lid that size.You would be lucky if it didn’t cup with flat sawn pine.Esp with two different finishes.

View TCCcabinetmaker's profile

TCCcabinetmaker

930 posts in 1816 days


#14 posted 12-15-2013 08:04 AM

Well, if you have any sun lay it the lid out in it, crown side up, this should help pull some of the moisture out.

I noticed something in your design though…. You need to totally seal this chest, the grain of the boards on the sides of your chest move in a different direction from those on the front. This is a problem with wood movement with wood left to breath, as the direction of expansion on the sides can actually split the boards on the front and back. My cedar chest that I built for myself is where I learned this painful lesson, as the back totally split as the sides expanded. Now, the chest is not a pile of splinters and is still quite functional, but you may have issues with it.

-- The mark of a good carpenter is not how few mistakes he makes, but rather how well he fixes them.

View Tennessee's profile

Tennessee

2410 posts in 1975 days


#15 posted 12-15-2013 02:17 PM

+1 for woodwarrior, and also the flatsawn board will warp.
After refinishing for 12 years back in the 80’s, it only took us about 2 attempts to learn to stop using milk paint due to this exact problem. Since we were refinishing items where we had no control over the wood initially used in the original build, when you ruin one or two with milk paint you learn to stop using milk paint. Too much moisture.

-- Paul, Tennessee, http://www.tsunamiguitars.com

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