grazzledazzle
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| posted on 15/11/08 at 09:52 AM |
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Advice please - GRP
Hello,
I've got a new bonnet for my motor, but it is a lightweight 'race' bonnet which is literally like paper - i've cracked it just
moving it!
I want to lay up a good thickness of grp on the back to give it a bit more rigidity but i have never done anything with grp in my life. So, my
question is can anyone tell me what i need and how to do it? Can i do this in the garage integral to the house or am i going to turn the house into a
no go area? Anything to watch out for? the bonnet really is a one off and i cannot afford to balls it up!
Thanks
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dubstar_04
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| posted on 15/11/08 at 10:18 AM |
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If you just want to strengthen it up a bit I would recommend
adding 1 layer of 450g/m^2 Chopped strand matt (csm)
Equipment:
1 soft roller (house decorating roller)
1 hard roller
1 brush for dabbing (will help position difficult areas)
1 mixing tub for the resin
1 rolling board
1 bucket with thinners (if you want to keep your equipment)
Materials:
Glass matting (CSM) calculate the size of your bonnet (eg 4 m sq)
Matt (csm)
British standards for pressure vessels recommends approximately 30% glass and 70% resin by weight for 600g/m^2 csm however I believe this is too much
resin.
Directions:
1. Mix up the resin and using the roller apply a generous layer to the bonnet
2. Having precut your matt to suit, begin wetting the matt out on your rolling board
3. Apply the already wetted matt to the area
4. using the hard roller, roll in ONE direction to remove any air and excess resin.
note: any air trapped will be visible as small (usually white) marks under the surface
5. Explain to your wife why the house smells so much!!
This is basic guide which should leave you with a good finish and more strength without adding too much weight to your bonnet.
The smell should be ok, just make sure you work in an area that is as ventilated as possible, eg kitchen, lounge...
Cheers,
Dan
[Edited on 15/11/08 by dubstar_04]
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robinj66
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| posted on 15/11/08 at 10:20 AM |
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Get yourself plenty of disposable gloves and cheap paint brushes (smaller sizes). A few glass bottles/jam jars would also be useful.
You need to buy some glass fibre matting, some resin and some catalyst (nasty stuff). I went to local fabricators and came away with a gallon of resin
and lots of matting - the matting comes in different grades of coarseness .
I cut down the brush bristles to about half length - this helps with stippling the resin into the mating and for getting rid of air bubbles.
Cut the matting to usable size bits - will depend on your application.
Pour resin into a jar and add approx 5% catalyst. Quick mix and then apply the mixture to the bits of matting. It may also be a good idea to brush
some resin onto the area you want to strengthen. Place the wet matting and stipple as opposed to brushing (which just tends to rip your work).
You can add several layers but you need to be aware that the resin mix will start to "go off" (harden) - quite how quickly will depend on
the ratio used.
Leave the work to dry - may take a couple of days depending on conditions. I have left a 500 W halogen lamp nearby to aid the drying process.
Once dry, you can add further layers or paint etc.
I should point out that this is just the way I have developed for myself - there may be some glaring errors in my "process" but I
wasn't doing anything that was required to be "pretty". It was just glassing in inner wings etc. Someone with a bit more knowledge
will point out where I'm going wrong and how you can make a better job
Seems to work, though
[There you go - someone beat me to it - must learn to type faster]
[Edited on 15/11/08 by robinj66]
[Edited on 15/11/08 by robinj66]
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dubstar_04
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| posted on 15/11/08 at 10:31 AM |
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quote:
Originally posted by robinj66
Cut the matting to usable size bits - will depend on your application.
I would try to apply the glass matting in the largest sheets possible as with chopped strand matt the strands are randomly interleaved and by cutting
the matt you will reduce its strength and create areas more prone to cracking especially on bonnets as they have a large surface with a tenancy to
flex and grp doesn't like that.
In all honesty though if your just adding an extra skin you can't really go wrong.
Cheers,
Dan
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MikeR
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| posted on 15/11/08 at 10:38 AM |
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What about adding some ali channel under the coat of GRP. Would that not add extra strength as well for not much weight?
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dubstar_04
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| posted on 15/11/08 at 10:46 AM |
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quote:
Originally posted by MikeR
What about adding some ali channel under the coat of GRP. Would that not add extra strength as well for not much weight?
If this was something that you wanted to try I would be careful with the material selection as grp has a low peel strength (approx 5 N/mm) so you
would have to add additional matt to make sure the aluminum didn't pull free from the original skin.
However if you wanted to remove unwanted flex from a composite shell then this is the route I would take...
Does that make sense?
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Bigheppy
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| posted on 15/11/08 at 01:03 PM |
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One important step is to lightly sand the surface then wipe over with acetone. If you dont use acetone there is a chance that the new lay up will not
stick. To make the lay up stronger add core mat between the new layers. Or use rolled paper to create strengthening ribs, if you do use this method
drill small holes in any sealed ribs to allow for expansion of the air inside. 
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grazzledazzle
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| posted on 15/11/08 at 01:04 PM |
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All making sense so far. I guess i could sikaflex the channel to the grp as is and then glass over. That way the glue is holding the channel on and
the glass covering it to give it some extra.
Sounds fairly straight forward - will probably do it on the drive if i can get a good day and then bring it inside to dry?
Next question - anywhere online i can buy the stuff?
Thanks all, much appreciated.
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RK
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| posted on 15/11/08 at 02:32 PM |
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If you use epoxy rather than regular polythene resin, your nostrils will thank you for it. But use a mask as well. The stuff is really nasty and
toxic. You will be needing a lot of it for such a large area. Chopped strand will suck it up like no tomorrow. I wouldn't use it just for that -
mat is better (and more flexible, which is what you want).
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dubstar_04
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| posted on 15/11/08 at 04:01 PM |
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quote:
Originally posted by RK
Chopped strand will suck it up like no tomorrow. I wouldn't use it just for that - mat is better (and more flexible, which is what you want).
I don't understand what you mean by this.
The glass is classified as:
1. Tissue
2. Chopped Strand Matting (csm)
3. Woven roving
4. Unidirectional roving
5. Unidirectional filament
As a diy car builder you would only encounter tissue and csm as the other types are for heavy duty industrial uses, and some need special machines to
wind on to circular shaped products.
On the contrary to what RK has said i would choose a styrene based resin as these are cheaper and require no special equipment, where as epoxy resins
usually have to be heated to create full bonds between the resin particles.
However thats just my opinion and different people like to use different materials.
As for adding the aluminum you really shouldn't need it. Using a styrene resin and csm usually gives a modulus in the region of 200 N/mm/kg/m^2
of glass content.
meaning that if you use 1 layer of 450g CSM you will result in an extra 90 N/mm in strength and add approx 5.5 kg to a bonnet that is 4m^2 so
don't go overboard.
I think what I have written here is correct but don't quote me  !!
I hope this isn't too much information!!
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HAL 1
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| posted on 15/11/08 at 05:25 PM |
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Another way to help strengthen panels is to use 'paper rope' your supplier should stock this, it's bonded in by laying mat over it,
it acts like a support beam if you lay it X shaped ( if possible of course ).
you can get it in different sizes to suit.
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Syd Bridge
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| posted on 15/11/08 at 05:26 PM |
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Firstly, as much as the chaps whose advice has been put previously is worthwhile, I'd put a layer of Coremat (as much as I detest the stuff) on
first, then a layer of 300gsm csm over that. Job done and a lot stiffer for not a lot of weight.
Epoxy... Avoid it unless you have a deal of experience. Although the makers don't like to publicise the fact, one of the many components in the
hardener/part'b' will be one of the nastiest carcinogens known to man. They have yet to devise an alternative. How do I know? SP/Gurit is
but a mile from me, and there are some strange illnesses come out of that place. Skin allergies are very common, followed by respiratory problems.
Same with Plessey radar/Bae systems, just a few hundred yards the other direction, with odd and uncommon cancers. And strange brain diseases. And low
level ( and sometimes not so low) microwaves and uhf radio don't harm you?? Riiiiight!
After all of that,
Cheers,
Syd 
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stevebubs
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| posted on 15/11/08 at 07:54 PM |
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www.cfsnet.co.uk
quite a few hints on how to glass on there, too...
[Edited on 15/11/08 by stevebubs]
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stevebubs
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| posted on 15/11/08 at 07:55 PM |
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definitely sand the bonnet and clean it with acetone beforehand - really does help with the adhesion...
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stevebubs
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| posted on 15/11/08 at 07:59 PM |
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quote:
Originally posted by grazzledazzle
All making sense so far. I guess i could sikaflex the channel to the grp as is and then glass over. That way the glue is holding the channel on and
the glass covering it to give it some extra.
Sounds fairly straight forward - will probably do it on the drive if i can get a good day and then bring it inside to dry?
Next question - anywhere online i can buy the stuff?
Thanks all, much appreciated.
If your garage is "in" the house then I would do it in the back garden and just cover it....the smell *will* permeate the house....
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