I used to have a nude bodyshop model, but I lost it in a clean-up. It's handy for making skin or tattoos, as they don't get covered up by their undies.
I've been googling and searching sites, but no luck. Does anyone know where this hack can be found, please?

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*edit* FOUND! thank you all!
http://www.pandorasims.net/sim2files.html

You can get it here: http://www.sexysims2.com/showthread.php?t=148652 but not at the moment as "downloads" are off line!

Can't you just make invisible clothes? Like undies or a swimsuit, and colour the alpha file in all black.

Well, when I make a tattooed skin, for example.. it will show up in the custom skin section while I am working on it. The Sim in that section is wearing underwear, so I can't see what's underneath properly.
I've been importing it in occasionally during the process, and testing it on a "nude" clothing. But it was easier when the model Sim was nude to begin with.

There are invisible clothes here:
xo

http://www.hairfish.com/mermaidcove...emaleadult.html

http://www.hairfish.com/mermaidcove..._maleadult.html

you can get to the elders section from this page (the site is closing):

http://www.hairfish.com/mermaidcove/html/2clothing.html

Thanks, but that still won't give me a nude bodyshop model. I'm not looking for invisible clothing. I can make that myself easily enough.
I'll have to wait till sexy sims is back. But I've never visited that site, so I'm not sure where I got my original hack from.
Hang on, let me take a photo, you will see what I mean.. back in a jiff..

This is when I'm in the section for creating genetics, the models have undies on:

The invisible undies are listed in the links that I posted, is that not what you were looking for?
xo

I found some references on Insimenator, and somethign similar to what you ask for seems to be on InsimAdult.

http://www.insimenator.org/index.php/topic,98828.0.html

I found this by Chris Hatch at insimadult:
xo

"You can have true nude in Body Shop. Just create a simple sim (makup etc. won't show when naked) open it with SimPE and change the line in the Age Data where it has activecat= to 0x00000080 (hex 80) when you open this sim in body shop go straight to genetics and you'll see the skin without any clothing mesh altering it. you can test any skin on both genders, all age groups fat/thin morphs".

rhondalatte, thanks for that info. I'll try that later today.
I've been hitting so many dead ends looking for this, sites non-existing or broken. I hope this is what I need, seems to be. I'll be back later.

*edit* oh my gosh! I've been googling the wrong thing. I had been putting "nude bodyshop" into google. I just tried googling "nude bodyshop mannequins" and came up with this site. I'm pretty sure this is what I had before:
http://www.pandorasims.net/sim2files.html

I'll load up Bodyshop now and see if it works, back in a bit!

*edit again*
Yes!! That's the thing I needed. But if it wasn't for you all helping, I wouldn't have thought to put the word "mannequins" into google, as I saw that word on one of the forums from your links.
Cheers, everyone, now I can see what I'm making.