# fab techniques for custom hood scoop



## speederbike1 (Dec 5, 2007)

Try getting a summit racing catalog and look at the scoops they have. or look at http://www.eharwood.com/index.php for selections there. If you are doing just a small scoop, you could make a female mold of MDF board and seal it, pva it, lay up a couple layers of mat. you can look online for some tutorials and youtube has quite a few good vids. if it is something intricate, make a male plug, then make a female mold of that, then lay up inside the female mold. You could also use a vacuform machine and abs to make a scoop, though it is a little more involved, but not hard by any means, less messy too. as far as body panels, they would require a lot of time and effort and you would lose crash strength.
Will


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## piotrsko (Dec 9, 2007)

series of wierd comments from an old geezer:

carefully locate scoop. some of us learned that a backwards facing scoop worked better than a forward due to pressure build up from the window. I would poke a hole in the hood, run it naked for a while, and tuft it with yarn before I select the scoop of my dreams.

plain ol household foam, sanded really smooth and slathered with drywall compound sanded smooth makes a fine male pattern. Vacuum bagging the 'glass while curing causes less sanding. add either 1/4 oz MEK or gasoline very carefully to melt foam.

IMHO a really monster scoop would add that vital HUNH? effect to the car.


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## dtbaker (Jan 5, 2008)

piotrsko said:


> ...from an old geezer:
> ...a backwards facing scoop worked better sometimes...


geezer comments are often instructive, I don't discount them ,and I almost qualify as a geezer.  I do know from experience that with an ICE engine bay (open radiator) getting the air OUT of the bay is often the issue. However, i plan to seal my front end up and don't really have a choice for the 'ram controller cooling' as it is on the front side of a rack.



piotrsko said:


> plain ol household foam, sanded really smooth and slathered with drywall compound sanded smooth makes a fine male pattern. Vacuum bagging the 'glass while curing causes less sanding. add either 1/4 oz MEK or gasoline very carefully to melt foam.


I have no idea what you mean by vacuum bagging?!



piotrsko said:


> IMHO a really monster scoop would add that vital HUNH? effect to the car.


absolutely, especially if I rig the rope lights inside to go back and forth like KIT car.


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## toddshotrods (Feb 10, 2009)

If you use household foam (Great Stuff, etc.) be certain to use a barrier over it before you do the fiberglass lay-up, or you will have a mess! Polyester fiberglass resin will completely destroy that foam. You can use epoxy resin over it without a barrier, but it's much more expensive. I like to use duct tape for the barrier because you can simply pull the foam and tape away from the cured fiberglass, by hand.


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## dtbaker (Jan 5, 2008)

toddshotrods said:


> If you use household foam (Great Stuff, etc.) be certain to use a barrier over it before you do the fiberglass lay-up, or you will have a mess! Polyester fiberglass resin will completely destroy that foam. You can use epoxy resin over it without a barrier, but it's much more expensive. I like to use duct tape for the barrier because you can simply pull the foam and tape away from the cured fiberglass, by hand.


Great info.... having never done any fiberglassing....

I am halfway considering cutting out *most* of the hood and using some clear plastic for maximum visual exposure. Might end up looking bad once scratched or yellowed from sun though.


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## JRP3 (Mar 7, 2008)

I'd use some half inch or one inch thick pink or blue foam board insulation and epoxy. Make the scoop shape by cutting and gluing up the foam boards, then radius the edges and shape them as you like, then cover with fiberglass cloth and epoxy resin. It will be very strong and lightweight.
Or if you don't want the foam boards to be part of the final product you can use them to make a foam plug that you can shape as you want, then cover with plastic and laminate cloth and resin over that, pull it off when cured.


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## dtbaker (Jan 5, 2008)

This is all sounding very exciting....

Having not done body work or fiberglass before... are the 'boundaries' able to be sanded flush down to the surrounding metal, primed, and painted to appear seamless, or will it be easier/better to trim a defined edge and just leave it?

I am also wondering about easy techniques to make sure the scoop stays stuck to the hood surround, and doesn't crack around the edges. Do I do a layer, pop-rivet through, and the put another layer? run small studs or anchors up from underneath? How much hardware is reasonable or required?


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## toddshotrods (Feb 10, 2009)

dtbaker said:


> This is all sounding very exciting....
> ...I am also wondering about easy techniques to make sure the scoop stays stuck to the hood surround, and doesn't crack around the edges. Do I do a layer, pop-rivet through, and the put another layer? run small studs or anchors up from underneath? How much hardware is reasonable or required?


If you use polyester resin, it'll crack. Use a two-part epoxy made for bonding dissimilar materials. Actually if you take JRP3's suggestion to use the blue foam and epoxy resin, you may be able to get an epoxy resin that will work for bonding the scoop to the metal hood as well - but I like using the bonding pastes for that.

http://www.aircraftspruce.com/menus/cm/epoxy.html
http://www.fibreglast.com/showproducts-category-Epoxy+Resins-5.html


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## JRP3 (Mar 7, 2008)

Personally I'd not try to blend in to the hood, a lot of work and if it's not done properly it will probably crack. Not saying it can't be done, but if this is your fist glass project it might not be the best way to start. I'd make it a separate piece and either epoxy in nuts or studs right into the foam.
The other benefit of this technique is that it's easier to change it if it doesn't work out the first time. Space the bolts evenly and you can experiment with turning the scoop around from front to back to see how that works. You can always glass it in later once you have it the way you want it.


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## piotrsko (Dec 9, 2007)

sorry, worked too long in Aerospace:

vacuum bagging takes a plastic material formed like a sandwich baggie all around the part. seal it and apply a car AC vacuum source. let sit under vacuum until cured. Vacuum forces out all the excess resin /air/ bubbles and the plastic acts like a smooth mold face.

down sides: mold has to be a bit stronger because of vacuum pressure, bit of a pain to make baggie.


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## 1clue (Jul 21, 2008)

Observations:

Do you _REALLY _want to make a hood scoop yourself, or do you just want a hood scoop?

If you just want a scoop, then don't even consider trying to make one of your own. If the idea of learning fiberglass part manufacture turns you on, then continue down this path. It's a useful skill to know, but speaking from personal experience it's a huge pain in the rear if you're learning how just for one part.

My experience is this: I've made an 18 foot long hovercraft using polystyrene foam, vacuum bagging and fiberglass. And wood. I've also repaired hovercrafts and helped others repair theirs. The building part includes using molds.

DO NOT USE GREAT STUFF! It is not strong, it changes shape especially the first two weeks after cure, and then afterward depending on humidity and temperature. It doesn't sand worth a darn, and it soaks up water like a sponge.

IMO, it would be easier to cut a hole in your hood and then have the scoop come up from inside as a separate piece. Fold the edges of the hood in and smooth it out.

Vacuum bagging is a good thing. Vacuum bagging "the real way" with an AC pump or other hard vacuum source might not be so great. The foam you use will crush in a hard vacuum, and your shape will be deformed. Been there, done that.

Instead, use a vacuum cleaner with a good seal. Test it first: Get a piece of clear plastic tubing at least 16 feet long. Any diameter, use thin stuff since you need less water. Seal it to your vacuum cleaner. Throw the other end over the rafter in your garage. Stick that end in some sort of water. Colored water works best, tea is good or kool-aid, but if you put something sugary in there you will get sticky crap all over next time you use that tube. Bubbly stuff throws your reading off. Anyway, once you have the end in your water source, and the water source is on the floor, then turn on your vacuum. If the water in the tube gets to be at least 6 feet above the level in your bucket, you're good to go. Otherwise you have a weak vacuum and need to fix that before you try.

The vacuum cleaner will run for hours, so it better be in a place where that doesn't matter, and you better have a fan handy to cool off the motor. Realistically speaking, if your garage is anywhere between 60 and 80 degrees you will not need the vacuum more than 5 hours or so.

Now, for the project and the plug.

Making a mold is about 3 times the work of making the part. First you make a perfect example of the part, then you seal it, then you wax it with mold release wax, then you glass over the top of it with more glass than you would use in the final part. Once that cures (hard cure, figure 2 weeks) you pop it off there and clean the wax off. Now you go over the mold with successively higher grit sand paper, ending up with wet-sanded 1000 grit or so. Once you get it perfect, that's your mold. Then you wax THAT 3 or 4 times, and then glass the inside of it. Once that cures it's your part. The good thing about that is you can make a hundred of the exact same part after that. But you don't want that, you just want your hood scoop, right?

Before somebody pounces on me, you CAN make a mold directly. You won't be exactly sure what it will look like, though.

Another way to go about this is to make the scoop out of a big piece of white bead board, OR pink/blue/yellow polystyrene foam from Menards or Home Depot. You can glue the layers together if necessary. This is much more intuitive:


Plunk it down on your hood.
Look under to see where the foam touches the hood.
Gouge that part out.
Repeat 1-3 until the outside edge of your block touches the hood all the way around. It helps to have marks on the hood to line it up perfectly each time.
Find a way to stick the foam down to your hood.
Go at the foam with saws, sanders, whatever until it starts looking like your scoop should look.
As you get closer, start just using a sander with fairly small grit.
Once you get the proper shape, clean the foam off.
Wax the foam several times with either a mold release wax (best) or several layers of high-carnauba car wax.
Wax your hood several times with the same stuff. Go at least 4" into (under) the foam, and a food back. Speaking from experience, you should wax the whole hood.
Cover the windshield and air vents with plastic.
Glass the foam plug. go out onto the hood 3-4 inches too. Start with several layers of 3-4 oz glass, and then for a final layer use 1-oz glass for a nicer finish.
tape a vacuum bag over it if you want to go that way. Probably for this approach it won't help much.
Wait for a cure, say 10 hours or so. Don't want a hard cure here!
Pop the whole works off the hood.
Wash the hood, get all the nasty stuff off.
Trim the edges of the scoop that landed on the hood.
Cut the opening on the front/back of the scoop.
Lay some plastic down and lay the scoop back on it.
Touch the surface up, but don't sand it.
Wait for a hard cure, 2 weeks.
Sand it if necessary, touch it up more.
Dig the foam out of the scoop.
Attach it to your hood.
Sorry this turned into such a long thing.


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## dtbaker (Jan 5, 2008)

wow....

BUYING a scoop is sounding better all the time. 
Preliminary measuring indicates a subaru wrx impreza scoop may fit, or an ABS scoop from jcwhitney.

I do appreciate the details, but it may have scared me off!


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## 1clue (Jul 21, 2008)

dtbaker,

I don't mean to scare you off if you're really interested in learning how to do it. It's really not that difficult if you know how, but LEARNING how is the problem. You're not talking about just learning how to make a mold, you're learning:


How to make a mold.
How to shape foam.
How to design an air scoop that keeps its shape at speed.
How to use fiberglass.
How to mix epoxy.
How to use a vacuum bag.
Doing all that at once is going to be tough. As you have seen, lots of people here have done it. Some of them (not me) have done it professionally. I've done it as a hobby but on a large scale (if an 18-foot long project is large, there are definitely bigger projects) and you can too, but the question is, what do you really want out of this?

Most of us who learned on our own learned one thing at a time. For me it was two at first: How to use fiberglass and how to mix epoxy. I sucked horribly at both. Actually, it was how to make a propeller from plans too. I sucked at that too.


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## etischer (Jun 16, 2008)

I've done quite a bit of fiberglass work. 
Here is an air dam I made back in the day, I used a card board box and bent it to shape. 










I laminated both sides of the card board with fiberglass. This turned out to be a mistake because the cardboard rotted out and I was constantly having to glue the two laminated pieces together. Its been on there for about 6 years, and never broken off even with all the curbs I've hit. I put a subaru flat six in that 914, and had to build a cooling system in the front trunk. 









It was a little lumpy, but not bad for a first project. 










Make your mold out of wood and bondo, use mold release wax and make your mold break away. I router my edges cause you cannot get fiberglass to lay flat around sharp corners. 









For complicated shapes, sometimes it's easier to make a few pieces and glass them together. If you are making a box, it is often easier to make a thin shell, cut it in half to remove your mold, then glass the two halves together. You can buy microspheres and mix it with the resin for a strong lightweight filler.














I like using the triple layer fiberglass. It has two woven layers with a random direction layer all sewn together. It's much faster to do 3 layers at a time. Use a single layer of woven for the last layer, and for areas with alot of corners. 











Those are the materials I use. Note, that resin is not waterproof, it gets soggy and soft when wet. You can add parafin wax to the last layer so it is not tacky, or just spray it with clear paint.


Spray can foam needs air to cure. I tried using it, but I always ended up with a puddle of glue at the bottom of my mold. Never had much luck with vacuum bagging either.


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## JRP3 (Mar 7, 2008)

etischer said:


> Those are the materials I use. Note, that resin is not waterproof, it gets soggy and soft when wet. You can add parafin wax to the last layer so it is not tacky, or just spray it with clear paint.


The polyester resin isn't waterproof? You mean before it's fully cured? Instead of adding parafin you can also spray a thin layer of PVA, (polyvinyl alcohol), which will seal the surface from air but can be washed off with water if you want to paint over it later.


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## dtbaker (Jan 5, 2008)

the buy versus build decision on the scoop is purely whether or not I can find a pre-fab one the right size for a price less than the time to would take to build one.... From the sound of it, fiberglass work would be time consuming even if I did a simple build-in-place over foam without vacuum bag and all that.

so... thanx for details, because it gives me a much better idea what would be involved to go that route.

There are some WRX scoops available for about $100 as new aftermarket units, or I may try for a couple hours to find one in salvage yards. There are also some ABS scoops from places like jcwhitney that may work fine once I have the exact dimensions I need.


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## etischer (Jun 16, 2008)

JRP3 said:


> The polyester resin isn't waterproof? You mean before it's fully cured? Instead of adding parafin you can also spray a thin layer of PVA, (polyvinyl alcohol), which will seal the surface from air but can be washed off with water if you want to paint over it later.


My experience is that it turns opaque and gets soft. Once it dries seems like it gets hard again. Doesn't make sense that a two part cured epoxy would be like that, but thats it worked for me.


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## JRP3 (Mar 7, 2008)

Polyester resin is not the same as epoxy. Polyester is a catalyzed resin while epoxy is a 2 part cross linked resin. Since the surface of the polyester isn't fully cured in contact with air I can see how water might affect it unless you block off the air with wax or PVA to let it cure. There are also polyester finishing resins that do cure when exposed to air. Epoxy will air cure fully, though there will be an "amine blush" on the surface which would be wiped off before painting.


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## 1clue (Jul 21, 2008)

JRP3,

If you look on all the boat forums, they generally mention that polyester is not waterproof. I don't know about getting soft, but I do know that in the few places I looked with regards to boat building they try to get a waterproof layer. What I thought happened was it just seeped through. If it were to get soft, I would call that a strong argument against using it in boat building.

AFAIK, you could contact the manufacturer of your particular brand of polyester resin and find out if it's waterproof. It might be on the data sheet as well.

I've only ever used epoxy. It's superior anyway, even if it's a bit more expensive.


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