Topics

Drywall compound for cowl mold?


Steve Stearns
 

Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 


Ryszard Zadow
 

The key to using drywall mud is to make sure your foam plug is as close to the shape you want before applying it. (which is the case no matter what material you use) 

This is my preferred method and it works well. Once the drywall mud is on you can sand it very smooth and then I paint it with rattle can enamel. Great way to pull a mold. 

Here's a pic of the cowling I built for my Acroduster. 2 ply 3K carbon using soda straw stiffeners. Top cowl covers an IO-540 and weighed in at 3.8 lbs. Bottom was 4.2lbs. Hand layup nothing was bagged. 


On Monday, January 11, 2021, 08:58:03 AM CST, Steve Stearns <steve@...> wrote:


Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 


Jim Evans
 

Rich:  Good use for the plastic straws although you may still be chastised for not using paper. :)

Jim

On Mon, Jan 11, 2021 at 10:28 AM Ryszard Zadow <ryszardzadow@...> wrote:
The key to using drywall mud is to make sure your foam plug is as close to the shape you want before applying it. (which is the case no matter what material you use) 

This is my preferred method and it works well. Once the drywall mud is on you can sand it very smooth and then I paint it with rattle can enamel. Great way to pull a mold. 

Here's a pic of the cowling I built for my Acroduster. 2 ply 3K carbon using soda straw stiffeners. Top cowl covers an IO-540 and weighed in at 3.8 lbs. Bottom was 4.2lbs. Hand layup nothing was bagged. 


On Monday, January 11, 2021, 08:58:03 AM CST, Steve Stearns <steve@...> wrote:


Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 



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Airport: 804-443-5885
Fax:     804-443-6891
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Greg Norman
 

Steve,
I've done three cowls with drywall mud over foam strips. You can build up that stuff up pretty high. A couple days for stuff over a half inch in moderate weather. Yes it will crack a bit but who cares. Spray the surface with cheap laquer paint (dries fast) and use packing tape. Not foil tape. No need for mold release goop. 

Greg Norman 
N540GN

On Mon, Jan 11, 2021, 9:58 AM Steve Stearns <steve@...> wrote:
Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 


Bill Allen
 

Good work Ryz!

I’d head that using spiral wrap instead of straws made it easier to lay them down over curved surfaces, and that they could be pulled out afterwards. 
Looking for an excuse to try it ..... or have someone else try it :^)

Bill

On Mon, 11 Jan 2021 at 16:28, Ryszard Zadow <ryszardzadow@...> wrote:
The key to using drywall mud is to make sure your foam plug is as close to the shape you want before applying it. (which is the case no matter what material you use) 

This is my preferred method and it works well. Once the drywall mud is on you can sand it very smooth and then I paint it with rattle can enamel. Great way to pull a mold. 

Here's a pic of the cowling I built for my Acroduster. 2 ply 3K carbon using soda straw stiffeners. Top cowl covers an IO-540 and weighed in at 3.8 lbs. Bottom was 4.2lbs. Hand layup nothing was bagged. 


On Monday, January 11, 2021, 08:58:03 AM CST, Steve Stearns <steve@...> wrote:


Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 

--


Tim Andres
 

Also know your different drywall compounds. The quickset stuff (30-60-90 min) doesn't shrink as it dries, but also doesn't sand very well. Tape mud has more glue in it and also doesn't sand well. All purpose, lightweight sands nice, but shrinks as it dries.
Tim A   

On Jan 11, 2021, at 6:58 AM, Steve Stearns <steve@...> wrote:


Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 


Ryszard Zadow
 

I use the cheapest stuff I can find.. and haven't had any trouble with it cracking or shrinking. The key, as mentioned was get your foam shape as accurate as possible. Ideally all you want to do is use the drywall mud to fill the foam pores, not use it to make shapes. 

RZ

On Monday, January 11, 2021, 10:59:16 AM CST, Tim Andres <tim2542@...> wrote:


Also know your different drywall compounds. The quickset stuff (30-60-90 min) doesn't shrink as it dries, but also doesn't sand very well. Tape mud has more glue in it and also doesn't sand well. All purpose, lightweight sands nice, but shrinks as it dries.
Tim A   

On Jan 11, 2021, at 6:58 AM, Steve Stearns <steve@...> wrote:


Greetings all,

I'm doing another iteration on my cowl modifications and last time I glassed over aluminum tape over carved pour foam (with pluses and minuses).  Afterward, I removed all the foam and tape.

This time I'd like to put an additive layer on top of the roughed-out foam shape with a finer pore structure than the foam and, preferably, something that I can wash out with water (or, at least, something easy enough to remove once the glass has cured.  And then wax and polyvinyl alcohol on top of this for the surface I glass.

The downside of drywall compound, from what I understand, is that thicknesses have to be built up in layers to avoid cracking and there is enough drying time to slow things down.

I'm particularly interested in tips for using the drywall compound for the purpose I've mentioned and/or suggestions for a different material that's easier to work with for the same purpose.

And I'm also open to other ideas/suggestions.

Steve Stearns
O235 Longeze
Boulder / Longmont CO
 


Steve Stearns
 

Ryszard, you said, "The key to using drywall mud is to make sure your foam plug is as close to the shape you want before applying it." and I think this hits the key to the problem that I'm trying to solve.  Maybe you have some more guidance for me.  My key issue is that I suck at getting the foam plug "close to the shape".  I invariably, when cutting, sanding, shaping the pour-foam (X-30) end up over-shooting or gouging etc. and need to add "just a bit"  (possibly over a wide area) and, with pour-form adding "just a bit" ends up with a big blob and I'm back where I started.  I was hoping having an additive material that both sands better and is easier to control would help.

Sounds like I'll still need a combo of the two approaches (and that I just need to keep upping my skills but I'm impatient...)

Steve Stearns
O235 Longeze
Boulder / Longmont CO


Ryszard Zadow
 

I use pour foam without reservation, but not quite like your attempting too. It's great to fill areas and build up a big spot, but little or thin stuff isnt really a good application for it. If I get messed up and find I oversanded or didn't' contour correctly I just cut out the section and plug urethane foam back in. 

Another good technique is use templates and grid lines as much as possible. You have to have a way to keep things symmetrical. 

The "rough out" is the most important part. Then it's details. The horizontal groove is routed out and is the overlap of the two cowlings. Laid up the nottom, then covered the plies in the groove with duct tape, then laid up the top cowl. The part line was the groove and made a perfect overlap. 

Used the same technique on the IO-240 for the Varieze, but I pulled a mold off it and vac bagged the cowling halves. Lots of work for a one of a kind.. 

On Monday, January 11, 2021, 12:01:59 PM CST, Steve Stearns <steve@...> wrote:


Ryszard, you said, "The key to using drywall mud is to make sure your foam plug is as close to the shape you want before applying it." and I think this hits the key to the problem that I'm trying to solve.  Maybe you have some more guidance for me.  My key issue is that I suck at getting the foam plug "close to the shape".  I invariably, when cutting, sanding, shaping the pour-foam (X-30) end up over-shooting or gouging etc. and need to add "just a bit"  (possibly over a wide area) and, with pour-form adding "just a bit" ends up with a big blob and I'm back where I started.  I was hoping having an additive material that both sands better and is easier to control would help.

Sounds like I'll still need a combo of the two approaches (and that I just need to keep upping my skills but I'm impatient...)

Steve Stearns
O235 Longeze
Boulder / Longmont CO


James Russell
 

Hi all:

Another way to get a part without building a tool is to lay-up a "flimsy" - a 2 ply layup over your prepped buck.

Obviously, the lay-up won't maintain it's shape if you pull it, so you have to build an elaborate support structure, Bondo it on,
pull the flimsy, prep sand the inside, and finish your lay-ups.

The downsides are:

1) You're 2 plys outside the OML to start with, then you lay-up plys going inside the OML - so mating flanges are an issue, as well as
tight tolerances, since the inside lay-ups aren't against the tool face.

2) Both sides of the part will need to be finished, since no plys were against the tool face.

I was lucky to work at Bell when the old hands were still there, and they taught me how to make plastic-faced plaster masters. It's super fast and relatively cheap,
though I haven't priced the epoxy surface coat that cures in the presence of water.

1) Put surface coat on prepped buck.

2) Let cure a little until tacky, crumble hemp strands onto surface coat - to promote adhesion to plaster.

3) Apply plaster w/ hemp reinforcements until quite thick - 2-3 inches minimum.

4) Add steel rod reinforcements and a base stand of wood w/ plaster soaked hemp.

5) Let cure 4 hours, pull the tool, release, and lay-up the part.

I got to watch them sweep plaster over templates to generate the buck as well - they were good and fast!

These methods are covered in the USN training booklets from WWII:

"Templets & Layout Mock-Up" - Parts 1 & 2

Fascinating reading if you're into sheet metal airplanes and how to build them...

Once BAAM gets cheap enough, 3-D printing will obsolete conventional tool making. I've sanded prints to 220 grit smoothness to lay-up on
successfully.

My $0.01

Regards,
James