how would I use a metal part as a 'buck'?

[wulffy]

Please be gentle with me..
I am making a fiberglass hood for a car (none is available for it) and I have the original flat hood. I have worked with fiberglass quite a bit (made some fiberglass cold air induction, etc. I also made somw smaller molds before.
I have a large hoodscoop (fiberglass) that will be in the center. I was going to bond this to the steel hood (I did this on my Monte Carlo a few years ago and it never did crack so I think that would work okay) but I wanted to cut some weight down, hench the glass.
At first I was going to do half the hood at a time (and leave out the center where the scoop will go) and make a plaster cast of the hood. I know this would be heavy but I was going to keep it thin and re-enforce it with wood.
I also thought about using fiberglass as a mold but wondered if I would have a hard time releasing it from the metal? Lastly I thought about covering the exhisting hood with something like aluminum foil so is will come loose (I made a spoiler like that once, after it was hard I used a brass wire brush to remove the foil) and putting the hoodscoop in the proper location put a thing coat of fiberglass and cloth enough to be strong enough to retain the shape. and remove the whole thing and on the 'backside' re-enforce it with extra fiberglass and cloth. Once it was strong enough I would use a filler to smoooth the surface of the hood. On the spoiler I used a poly filler and mixed it with resin it made the surface smooth and was real tough but was also hard to sand.
I have heard of people making molds using car body parts but how do they do it without it becoming 'part' of the metal body???
Any thoughts?

[Len]

I did a little studying on mold making a while back but ended up never making one. The process that I was planning on used a "mold release agent" that is sprayed on the plug so that the sprayed-in gel coat would not adhere to the surface. The resin and mat was then laid on top of the gel coat. You can get more detailed info from a fiberglass supplier, there are probably plenty on the web as well as scattered thoughout the country. We have a supplier about 20 miles from me that is used by several locals for making molds and glass parts so I'm sure you can probably find one that you can talk to.

[Restoguy]

To duplicate your hood in fiberglass, finish the hood youhave as perfect as you can. Every imperfection that is in the hood when you take a splash off it will show up in the new part. Once your hood is finished, examine it for undercuts. If it ain't tapered the right way, you will never get the splash off it.

There are many commercial release agents on the market, but you will want to use a good paste wax like Johnsons floor wax, and then spray it with a thin coat of poly vinyl acetate, PVA. You should be able to get this stuff at any resin supply house.

You are now ready for resin and glass. First, apply a gel coat to your hood. Brush, spray or roll it on. This gel coat is critical.It becomes the surface of your mold. You do not want any imperfections or air bubbles in it. When it is set up enough so light pressure will not penetrate it, apply resin and glass. It is a bit of an art. Use a squeegee to work out the air. Build up a good thickness. Take your time. Do a layer, let it setup, but apply the next coat before it cures completely so it all bonds together as one piece. You will probably want it to be about a half inch thick. Also, this material shrinks, so do not overdo it with the hardener, and do not try to do it all at once.

When the splash if fully hardened, you pull it off the hood. The PVA is water soluable. And it raises havoc with the aluminum in spray gins, so clean them quick. There are spray guns with all stainless steel fluid pasages, but no need for one or two jobs. Just get a cheapie conventional for the PVA.

Now that you have a splash, or mold, do it all over again. The part you now make from the splash will be your new fiberglass hood. Simple as that. Maybe you want to practice on a small thing first.

[wulffy]

Thanks guys, that is great information! Right now the hood has the old paint (many layers!) with chips in some areas. Should I strip all the old paint off and paint it with auto primer? I read about someone somewhere saying the primer was good for holding 'onto' the wax. I bought some carnuba "mold release wax" would this be as good as the floor wax?
As you mentioned the hood should have a draft (no reverse angles) and I looked at the edges and it wraps around at 90 deg for about a 1/4 to 1/2 inch, I was going to just make the mold that "deep" and build the sides up after it's finished would a better way be to use modeling clay to fill in the sides so they are smooth?
One more thing, if I was to brush on the PVA with a very fine bristle brush would this smooth out okay?

And one more thing I just remembered (sorry) can I use the common fiberglass resin as a gel coat or is there a special gel coat that must be used?
Thanks!!!

[Len]

Gel coat is gel coat not resin. I doubt that you could substutute one for the other.

If you brush on any materials I believe you will get the brush marks in the finished product. This may be ok but it could make the release a little more difficult and you will need to remove the brush marks from the hood when you prep for paint.

[MARTINSR]

Len, I was under the impression that "gel coat" was simply resin, talc and pigment. This is not right?

It is a moot point in this discussion because I would be using the gel coat and nothing else for the mold.

Wulffy, here is a little fun trivia question, "What is painted, before it exists?" A fiberglass boat! Or any other fiberglass piece really but for asking the wife the riddle, a fiberglass boat will work.

There is another option you could use that I have done with a motorhome fender once, don't make a mold at all!

You only really need a mold if you plan on making more of them. You could make it the way kit airplanes or surfboards are made. They shave foam down to the shape they want and then lay a few layers of resin and cloth over it, then sand that smooth and there you have it, a fiberglass wing or board.

I did this on the motorhome fender and ended up with a slightly larger fender (an 1/8" or so) which didn't matter at all on it. This wouldn't be the case on your hood, but you could pull it off.

You prep the hood just as you would for a mold. But instead you only lay one or two layers of mat and resin. You pull it off carefully and then lay it on something to support it well. Prepare the inside and lay mat and resin in there. Now you can shave the outside down flat and you have your part.

Polyester prime it, and surface it to perfection.

I have made a large mold, it takes a LOT of glass, a LOT of time, I think I would give this a try the next time.

If you want it REALLY light, use fiberglass cloth and then don't put anything on the inside after. Just apply the cloth to the hood, sand it smooth right there,then remove it. Now, make the side edges of the hood by trimming what you have to the correct width and making a support and applying cloth over the edge. Sand that to shape and then add some thin inner struture made the same way out of an inner hood structure from your metal hood. Bond them together and you will have about the lightest hood you can get on a budget. You can keep the film thickness to 3/32" or so!

Brian

[wulffy]

Brian, that was some good info.. I recently built a cold air induction for my Monte Carlo SS the way you discribe. I built it of foam and when I got it just like I wanted I covered it with aluminum foil and then covered it with resin, after it was dry I layed on the mat when that was dry I used body filler and smoothed it out. I used laquer thinner to disolve the foam when it was finished. it came out really nice but that was fairly small (about 4 foot long and 8" across). I did think about triming my hoodscoop untill it lay on the exhisting hood flatl then building a layer od glass cloth to the edge. I then was going to use the filler to fill like you suggested. And although as you said it would be a bit larger, if I only used enough thickness to allow me to remove it and keep it's shape in a relaxed state then reinforce the back.
The only thing stopping me was I was afraid that laying down that much filler it would crack.. now I doubt that as I did a full-feathering around a scoop a few years ago and it had quite a bit of filler and it never cracked. I suppose I could use my flat sander to flatten it out.
My problem is I always overbuild with my glass.. the air ducting I built is really heavy, I used about 4 layers of mat and/or cloth if I could remind myself to keep it to several layers maybe that would be better.
As for the gel-coat, on the ducting I bought a bottle of coloring (black) and mixed it with the resin and it came out nice.. since I have never found "gel-coat" locally I would try to use plain resin for my hood, did you do your fender that way? Did the glass fender you built flex okay without cracking?
Do you have any fotos of the fender?
Sorry for all the questions.. but you are the only person that I know that does 'moldless' parts like myself.

[Len]

Len, I was under the impression that "gel coat" was simply resin, talc and pigment. This is not right?

I'm not real sure but gel coat is sold as a different product. I know it comes in clear, white or colored but I don't know if the basic chemistry is any different than resin.