I have been lurking on here for a long while and have been thoroughly impressed with the wealth of information that exist here. This truly is an amazing group of people. So I want to say thanks beforehand to all the people on here willing to share their expertise.

I am planning to make my own vb clothing out of a light shell and then coat the shell with something to prevent it from breathing. I was wondering if anyone has done this before, and what their experiences were. I was considering McNett Tent Sure tent floor sealer, but I wasn't sure how long it lasts especially when washing it. Should I use silicone or urethane?

Again, thanks in advance.

That would almost surely be more expensive than using silnylon. It might be heavier too.

By shell I mean a cheap windbreaker with a hood which I stripped the lining out of (basically a ripstop nylon shell). This is extremely cheap, around $3 at a thrift store etc., and easily modified to fit correctly. Silnylon would probably be lighter, but I have very little experience with MYOG clothing, so it is a lot easier starting with something that is already made.

Andrew Skurka tried it on a GoLite Storm on his most recent Alaska trip. I don't think he had much success.

From his website gear list:
pre trip comment - GoLite Storm (treated with McNett Thundershield) Avoids failure of insulation due to perspiration

post trip comment - Not sure Thundershield completely effective

I think the issue would be that it is a temporary addon treatment that would start breaking down the first time you wear it and would have to reapply frequently.

If you are handy with a sewing machine or want to become handy with one I would recommend Stephenson's Warmlite VBL fabric called "fuzzy stuff" very odd company but the quality is there. I bought their VBL shirt, pants, and socks this fall, they are getting used this weekend for the first time. If I were to do it again I would buy the fabric and sew my own clothing, their socks I think are going to prove to be top notch though. They are worth the purchase.

Here is my threat about the Stephenson's gear with an initial impression.

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=55433&skip_to_post=476535#476535

I've actually had a good deal of success using Anti Gravity Gears UL rain jacket as a VB jacket.

It doesn't have the venting options to be used while active but for sleeping or sitting around camp it's great! I contacted AGG about a custom made UL rain jacket with pit and forearm vents but haven't heard back from them.

http://www.antigravitygear.com/antigravitygear-ultralight-rain-jacket.html

Yeah, I didn't think the thundershield stuff would work. It is in an aerosol can so I don't know how you could get good coverage.
I did notice that Skurka used McNett Tent Sure which is used for tent floor sealant. Skurka's gearlist says "VBL pants - GoLite Storm - 7.1 - - - Applied McNett Tent Sure to stop breathability - VBL's critical in cold temps if can't dry out things".

Does anybody know if this was effective or not? It seems like it would be a better choice because you actually brush it on.

How about silicone thinned out with a solvent (like what you would use on tent seams). I think I remember somebody coating an entire coat to make a non breathable waterproof shell before.

I get that this is in the MYOG thread but it seems like you are doing extra work of applying something to already made gear when you can very cheaply buy the raingear that works as vbl. If you want this to be a MYOG project, good for you I love MYOG. If that is the case though take it all the way and sew or tape the gear out of fabric that has already been treated or is made vapor proof. Then you get the real benefit of MYOG, something 100% custom and the satisfaction of making it.

Maybe you can try this method:

http://www.youtube.com/watch?v=-UTZPllgqSc

It seems workable to me -- just use a nylon shell instead of uncut fabric. If you do try it, I'd love to hear how it turns out.

That looks really interesting. Might have to get together a bunch of stuff and treat it all. I also saw this video: http://www.youtube.com/watch?v=RzR9-HASSIA&feature=related

I am not sure if using gasoline as a solvent is a good idea, but it definitely is cheaper than mineral spirits.