Project: making a permanent Metro Kammback extension

[Whoops]

This is great stuff. My hat's off to everyone for the information.

The stuff your using uses moisture in the air to cure. The thicker you try to put it on, the longer it will probably take to set up. My experience with it was that it took more like 3-5 days to set up hard enough, deep enough to not do the seep, sag and bubble routine.

There are commercial compounds which are polyurethane based, which consist of 2 parts that you mix together. They have a variety of expansion rates, hardness, spongeness and so forth. They typically have to be mixed in some ratio of weight, so unless you have an arrangement for it, it can become a exercise in frustration.

If you use the thinner cardboard, I have a suggestion, after you get the shape, but before you lay on the fiberglass. I would take a small amount of resin, mix it so that the catalyst is on the high side and lay down a coat on the cardboard first. The reason is that you might have a problem, otherwise, with the cardboard becoming moist enough to start to lose it's shape.

I had a problem with a replacement air deflector I tried to make for my 89 Honda Wagon. I had created what I felt was a pretty good shape for my back window air deflector, but when I came back to it, I hadn't mixed the resin hot enough and the cardboard got soft and changed it's shape, before the resin set up.

[MetroMPG]

Originally Posted by aerohead

Darin,was that the high or low expansion foam?

I'm not sure - it was just "Great Stuff" in a can. The label said it would expand up to twice the original sprayed depth depending on humidity.

Originally Posted by Whoops

The thicker you try to put it on, the longer it will probably take to set up. My experience with it was that it took more like 3-5 days to set up hard enough, deep enough to not do the seep, sag and bubble routine.

That's good info. I'd say it was up to 2.5 inches thick in places. Probably too much. But I also don't want to wait 3-5 days for it to cure, so I don't consider it time wasted!

Thanks for the suggestion about the skim coat of resin on the cardboard. I'm going to do that tonight.

I re-built the side portions of the Kammback in cardboard this evening. Simple curves only. It's not quite as lovely as the compound curvature of the foam (before it distorted), but it'll suffice. So I should have 2 layers of glass on it by the end of tomorrow.

Anyone with experience glassing cardboard want to take a guess on whether that will be adequate stiffness given the shape I'm working with? Glass the underside as well, or just resin it?

[homeworkhome53]

I had the same problem with "Great Stuff". I was going to do a mod but it stayed spongy. I used some expanding foam in a house mod a couple years ago and that got as hard as a rock and the expansion was from 1/4" at application to over 2" when cured. The problem I is don't remember the name. But Great Stuff is not that great for our application but there are others. I'm going to pick up some other brands until I find something suitable.

homeworkhome53

[MetroMPG]

hwh: thanks for that info. Live and learn. I also bought another can - different brand - yesterday, because I have other areas where I can use it. Please let me know what you learn, and I'll do the same.

[Cd]

At Lowes we have a lot of customers with 'Great Stuff' horror stories.
A lot of folks insulate around their doors, only to find that they can't open them the next day.
There are two types of GretStuff : One in a light blue label and one with a red label. The stuff in the red label is the slow expansion stuff ( or maybe it is the opposite )
It sounds like you had the slow expansion stuff.
There is also a similar product called DapTex. It is sold side by side the GreatStuff.
It goes out of stock pretty fast.
It comes in a black can with a dark blue label.
Besides the fact that it wont expand like mad, it is also waterbase and is easier to clean up.
It is also slightly cheaper.

[ebacherville]

yeah 12 hours it cures but not fully you need days to let it cure up bofre ot can make a decent base ... i found that for thing shapes like a kamback using sheet foam or fiberglass over a mold or shape is the way to go .. my rear wheel skirts are just fiberglass resin with cloth as the fiber.. but not fiber glass cloth.. works well and allows some flexability.. want rock hard and fairly thick.. use fleece fabric, or hard and thin..fiberglass cloth.

[MetroMPG]

Thanks for the info, guys. More living & learning.

This morning I glassed the first outside layer over the resin skim coat on the cardboard. I'll do the 2nd layer this evening & get some pics. It's apparent that I'm also going to add some structure to reinforce the trailing edge of the top portion so it holds the right shape.

The more I do, the more I realize there will be a lot of "finishing" work to make it look nice - ie. a skim coat of bondo, and lots of sanding to level out the surface.

[Whoops]

When you get to trying to finish the shape with the Bondo, I have something I have found works well.

If you take one of plastic spreaders, you can cut a collection of slight notches in the edge, similar to what you would see on a glue spreader for linoleum or tile. Then using that edge, take the Bondo and create your surface, using the notches. This will give you a surface you can sand fairly easily, using a sanding block and sandpaper. After you get this surface to the shape your looking for, then you can take a regular spreader and fill in the gaps.

This makes it vastly easier to work, compared to laying a full layer of Bondo down. It does not seem to make any difference in the final appearance or durability. This also makes it easier to do a thicker fill, like over 1/16 of an inch or so.

[lunarhighway]

i really like this thread and i can't wait to see the final result but if it was me constructing this i'd look into sheet aluminum
(used it once for my undertray an now i'm in love with the stuff, such a pleasure to work with...)
... since you've already build a working version out of carboard i"d just transfer the shapes to some very thin aluminum carefully bring it to shape and rivet (or maybe glue) it together.. if you can find a sheep big enough you could even make this a one piece affair. add perhaps a curved metal strip at the back for strength.

it would still be the lightest thing on the car, be relatively strong , and if you cared about looks it could be painted to match the rest of the car, although natural aluminum finish would be cool too. since this is how most WW2 era planes where constructed it's not the worst option either

anyway, i hope the foam thing works out, as i'd really love to see this thing come together

[i_am_socket]

The words "sheep" and "affair" should not be in the same sentence