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Youngling
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Discussion Starter · #1 ·
So I’m hitting a wall here and could use some advice.

8 3-Hinge mdf cabinet doors 1.5ft x 3 ft
- Rolled with a solid coat of S-W Shellac primer.
-dried for 4 days
-sanded smooth
-some burn through(less than 20% of surface area)
- everything looked good
-temp 65 degrees 20% humidity in air
-two coats Kem Aqua on backs
-doors warped downwards
-Sprayed one coat on faces
-doors warped back into original shape
-Sprayed second coat on face
-doors warped slightly down again
Why are the doors warping?

sorry for the weird layout of my turn of events. I’m running on 3 hours of sleep with this nightmare of a project.
 

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How are you doing these

Are they laying down flat on benches?

Theyre 3 ft tall I see,are they laying across ladders by any chance with 14 inches or so hanging off on both ends?

Why wait 4 days after priming with shellac?

MDF is heavy,I could see them waring if laying across ladders,I always lay the long ones out lengthwise,takes up more space but theyre fully supported,if they actually warped one way and then another that seems troubling.

I lay my cabinet doors down on top of extension ladders on top of taller sawhorses,I always lay the taller doors out lengthwise,I know MDF isnt lumber that can be wet or "green" so it cant be moisture 20 percent humidity is normal,right?

Id make sure they were fully supported layed down,paint em and get them hung back up stat and hope for the best,or ask a good carpenter
 

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The face with the higher moisture content will swell causing it to bow in the opposing direction. I mentioned in an earlier thread that I’ll sometimes wet the back sides with a damp towel when finishing horizontally, thus minimizing the chances for developing a permanent bow. It’s also important to have the workpieces supported along their entire length and avoid uneven airflow when drying. Also try to avoid applying all top coats to one side which will minimize bowing…it’s best to apply 1st top coat to both sides before moving to the 2nd top coat to minimize risk.
 

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Youngling
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149 Posts
Discussion Starter · #8 ·
How are you doing these

Are they laying down flat on benches?

Theyre 3 ft tall I see,are they laying across ladders by any chance with 14 inches or so hanging off on both ends?

Why wait 4 days after priming with shellac?

MDF is heavy,I could see them waring if laying across ladders,I always lay the long ones out lengthwise,takes up more space but theyre fully supported,if they actually warped one way and then another that seems troubling.

I lay my cabinet doors down on top of extension ladders on top of taller sawhorses,I always lay the taller doors out lengthwise,I know MDF isnt lumber that can be wet or "green" so it cant be moisture 20 percent humidity is normal,right?

Id make sure they were fully supported layed down,paint em and get them hung back up stat and hope for the best,or ask a good carpenter
Two benches with 1inch x 3inch x 8ft long boards spanning across them.

about 10 inches of overhang each side.

Was sick so had to delay the finish coat for 4 days.

I’ve sprayed doors like this before and I usually use buckets or an a-frame storage rack for drying but it wasn’t needed on this job.

This is my first time doing bare solid mdf cabinet doors and the risk seems way higher to me then new poplar/mdf doors.
 

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Youngling
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Discussion Starter · #9 ·
The face with the higher moisture content will swell causing it to bow in the opposing direction. I mentioned in an earlier thread that I’ll sometimes wet the back sides with a damp towel when finishing horizontally, thus minimizing the chances for developing a permanent bow. It’s also important to have the workpieces supported along their entire length and avoid uneven airflow when drying. Also try to avoid applying all top coats to one side which will minimize bowing…it’s best to apply 1st top coat to both sides before moving to the 2nd top coat to minimize risk.
Good to know. We did 1st coat backside and didn’t notice anything crazy, then we did second coat backside and saw the excessive warping.

Doing the front side with a coat helped to “unbow“ itself.
Good tip on the wet rag moisture on the opposing side.
 

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So I’m hitting a wall here and could use some advice.

8 3-Hinge mdf cabinet doors 1.5ft x 3 ft
- Rolled with a solid coat of S-W Shellac primer.
-dried for 4 days
-sanded smooth
-some burn through(less than 20% of surface area)
- everything looked good
-temp 65 degrees 20% humidity in air
-two coats Kem Aqua on backs
-doors warped downwards
-Sprayed one coat on faces
-doors warped back into original shape
-Sprayed second coat on face
-doors warped slightly down again
Why are the doors warping?

sorry for the weird layout of my turn of events. I’m running on 3 hours of sleep with this nightmare of a project.
What do you mean by this: "-some burn through(less than 20% of surface area)?"

futtyos
 

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I burned through the primer exposing some of the mdf but no more than 20% of the surface area of the door had burn through.
I am somewhat confused here. You say that these cabinet doors are raw, unsealed MDF. You then say that you sealed them with SW shellac primer, then sanded them down to the point that 20% of the MDF was showing. This tells me that wherever you "burned through" the shellac, the MDF was now "un-sealed" and ready to suck up water, in your case from the waterborne Kem Aqua (aqua being another name for water). This being the case, why did you not seal the "burned through" areas of the very water absorbable areas of the MDF? What made you think that these "burned through" areas were not going to readily absorb water and warp the doors? How did you sand such that you describe areas as being "burned through?" Sandpaper, sponge sander, random orbital sander?

Just curious,

futtyos
 

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Most cabinet shops will spray 2-3 coats of primer on bare wood, because yes you almost sand back to bare wood after 1st coat. In the future I personally would recommend atleast 2 coats primer prior to topcoating. Hope you got it worked out.
 

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Even if the shellac wasn’t sanded through, the DPM solvent in Kem Aqua combined with KA being alkaline has a tendency to soften and/or melt shellac, allowing moisture into the substrate, being that shellac is soluble in alkaline water and slightly soluble in DPM solvent.

It was particularly noticeable when having used a lot of DPM containing waterborne clears on wood sealed with dewaxed shellac. When applying top coats, the sealed wood beneath always soaked up moisture darkening the wood which would later lighten to its original state as it dried, which wasn’t so with the commercial catalyzed sealer I often used. It also resulted in significant grain raise which was another indicator of the shellac not holding out moisture under alkaline/DPM containing top coats.

I had to be particularly careful of warping due to finishing a lot of floor-to-ceiling grain-matched veneered cabinetry…if one panel, drawer front or door were to warp, the entire grain-matched set would need replacing, which only happened once whereby one door warped as part of 5 piece grain-matched set, all 5 components needing to be changed out due to one minor warped door edge which exceeded the tolerances written into the cabinetmaker’s warranty.
 

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Youngling
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149 Posts
Discussion Starter · #15 ·
I am somewhat confused here. You say that these cabinet doors are raw, unsealed MDF. You then say that you sealed them with SW shellac primer, then sanded them down to the point that 20% of the MDF was showing. This tells me that wherever you "burned through" the shellac, the MDF was now "un-sealed" and ready to suck up water, in your case from the waterborne Kem Aqua (aqua being another name for water). This being the case, why did you not seal the "burned through" areas of the very water absorbable areas of the MDF? What made you think that these "burned through" areas were not going to readily absorb water and warp the doors? How did you sand such that you describe areas as being "burned through?" Sandpaper, sponge sander, random orbital sander?

Just curious,

futtyos
Futtyos,

As always you have a critically thinking mind when it comes to problems. I appreciate that about you.

I researched mdf and online it says mdf doesn’t swell with water based primers(No I don’t trust everything I read online). I am new to painting raw cabinets but have had success with Stix primer on one kitchen that had raw mdf /poplar cabinets with no swelling to be seen.

sanding sponge, 3m stickit sander, and a random orbit sander(briefly).
The reason I simply painted once I burned through…I was out of time and at my wits end.
 

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Youngling
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149 Posts
Discussion Starter · #16 ·
Most cabinet shops will spray 2-3 coats of primer on bare wood, because yes you almost sand back to bare wood after 1st coat. In the future I personally would recommend atleast 2 coats primer prior to topcoating. Hope you got it worked out.
Thank you Kevin. I ran out of time on this job. In the future I’ll plan to do that.
 

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Youngling
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149 Posts
Discussion Starter · #17 ·
Even if the shellac wasn’t sanded through, the DPM solvent in Kem Aqua combined with KA being alkaline has a tendency to soften and/or melt shellac, allowing moisture into the substrate, being that shellac is soluble in alkaline water and slightly soluble in DPM solvent.

It was particularly noticeable when having used a lot of DPM containing waterborne clears on wood sealed with dewaxed shellac. When applying top coats, the sealed wood beneath always soaked up moisture darkening the wood which would later lighten to its original state as it dried, which wasn’t so with the commercial catalyzed sealer I often used. It also resulted in significant grain raise which was another indicator of the shellac not holding out moisture under alkaline/DPM containing top coats.

I had to be particularly careful of warping due to finishing a lot of floor-to-ceiling grain-matched veneered cabinetry…if one panel, drawer front or door were to warp, the entire grain-matched set would need replacing, which only happened once whereby one door warped as part of 5 piece grain-matched set, all 5 components needing to be changed out due to one minor warped door edge which exceeded the tolerances written into the cabinetmaker’s warranty.
This is what I was looking for.

Even my high end, carpenter/cabinet making, brother-in-law said he had no idea what I did wrong.

Thank you all for your help.
 

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My only other thought on this: Room temperature humidity have a big affect on woods in general. Most contractors will have their hardwoods delivered to jobsites long before install so that it can "climatize" . Keeping that area heated while doing so is important.
 
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