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nice_guy

Patching a hole in the bonnet.

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nice_guy

As the weather gets more into resin-friendly temperatures, I start thinking about patching the hole for the air filters in the bonnet.

 

image.png.07c7c0f7d0bb24a33eea59374be9e1dd.png

 

Well, it's been cut open for a tad more than the air filters.

 

I have a few ideas, starting with bevelling the edge of the cut, but I wonder if someone had a go in a way that does not involve sanding until the end of times.

 

My main concern is how to have kind of a mold/patch that follows the shape of the bonnet.

 

My current stupid idea is :

 

  • fill the hole with foam board, with the foam standing proud this great big gaping opening.
  • shape it with either hot wire cutter following the bonnet rounded staircase profile or knife, then sand it flat and nice,
  • fill the edge gaps with plasticine, candle wax or whatever putty I have at hand
  • Once I have the hole filled with a correct profile, use a lot of brown packaging tape to seal the area
  • make a cast of the outer shape, either using armed plaster (fragile but harmless) or glassfiber (potential catastrophic mess risk) to have an outer mold. Have locating surfaces on the bbonnet to ensure nice position.
  • Remove and wax the mold,
  • then remove the foam and tape, clean with brake cleaner, hot glue the mold to the bonnet and start stratification from the inside with woven fiber layers, bonnet resting on the mold.

 

Then once cured, sanding with a long block will hopefully be minimal.

 

A full respray will happen down the road.

 

Would there be another (simpler?) technique ?

 

(I alas do not have the missing bit that was cut a long time ago, and CTRL + Z did not work either)

 

Thanks a lot.

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hammie

IMHO.

  1. Prep is the key. Bevel edge. Lightly grind under side to rough up where your going to apply new matting to give a good key, degrease.
  2. Cut a cardboard patch. This will give some depth to filler. If you can encapsulate in vinyl or cover in tape. Coat the underside in wax polish to aid releasing from Matt when finished. Fit in hole and use gaffer tape to secure from top side all around edges and over the top of the cardboard.
  3. Matt from underside maybe 2 layers. Second a bit bigger than the first.
  4. When resin has gone off remove cardboard. It should leave a lower level from the outside to fill. Iff slightly proud sand down first

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Lyonspride

This is just my opinion, but IMO your never going to hide that, I would look at how I could make a feature out of it, even if that involves. a low bonnet scoop. If you try to make it flat and level it will always be visible.

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nice_guy
15 hours ago, hammie said:

IMHO.

  1. Prep is the key. Bevel edge. Lightly grind under side to rough up where your going to apply new matting to give a good key, degrease.
  2. Cut a cardboard patch. This will give some depth to filler. If you can encapsulate in vinyl or cover in tape. Coat the underside in wax polish to aid releasing from Matt when finished. Fit in hole and use gaffer tape to secure from top side all around edges and over the top of the cardboard.
  3. Matt from underside maybe 2 layers. Second a bit bigger than the first.
  4. When resin has gone off remove cardboard. It should leave a lower level from the outside to fill. Iff slightly proud sand down first

 

I was somehow confident in the fact that cardboard would bend out of shape and bow out, given the reach and the profile, but you made me rethink : I may get away  with it as the profile change of the bonnet section may be linear enough...

 

image.png

(initial pic, just to show the hole)

 

(I don't know if that makes sense, but let's say carbdboard may only be bent on one axis at a time ?). If glued /duct taped enough it may just work, and be easier than trying to cast a plaster shape...

 

Maybe some scaffolding (I.E a stick of wood hot glued) would help the carbdoard  follow the  inwards shape (the  crease starting a bit under the catch)

 

What cardboard source would you recommend ?

 

 

 

6 hours ago, Lyonspride said:

This is just my opinion, but IMO your never going to hide that, I would look at how I could make a feature out of it, even if that involves. a low bonnet scoop. If you try to make it flat and level it will always be visible.

 

Oh, I'm confident I can make an invisible patch, once prepped and painted.

 

The questions are how long will it stay invisible (cracking at the seam may happen over time) but as I have epoxy resin and leftover carbon fiber fabric I may have a strong and rigid enough patch so that the body filler and paint may not feel like cracking;

And more important to me how many weeks of painful hand sanding will that involve.

 

Initially the front carb was sticking out, but I went along a nice process of relocating engine an gearbox to sink the weber under bonnet line :

 

 

...so that i would'nt need a scoop, and fitting a scoop on this shape may not be easier...

 

I'll probably add carbon fiber rod (kite supplies) in some places to enhance stiffness....

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hammie

You have lowered the engine now and the carb body is lower than the bonnet line? How much of the hole are your trying to fill

 

Card selection. I would try to find some the same or slightly thinner than the bonnet thickness. Scaffolding sounds a good idea.

 

Use P40 filler as it has fibreglass strands in it. Once the underside is done and your filling key the gel coat top and it shouldn't shrink or crack. Using power tools like belt sander or orbital sander save hours of sanding by hand.

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nice_guy

 

 

 

Thanks a lot for your advices !

 

On 24/05/2019 at 06:51, hammie said:

You have lowered the engine now and the carb body is lower than the bonnet line?

 

By a whisker. More by mustache. Got 7mils of clearance between carb and project inner skin.

 

 

On 24/05/2019 at 06:51, hammie said:

How much of the hole are your trying to fill

 

Well...

giphy.gif

 

All of it, really.

 

 

On 24/05/2019 at 06:51, hammie said:

Card selection. I would try to find some the same or slightly thinner than the bonnet thickness. Scaffolding sounds a good idea.

 

Please forgive the liberal use of cad, I find it clumsy to put that into words :

 

image.png.eed225af49d72bd7b20c9df7dcb12d92.png

 

My idea is to chamfer the inner side of the bonnet, than lay cardboard on the outside, flush-ish with the outer skin.

(please not that the chamfer is way smaller in the picture than what it'll be in real life)

 

"Scaffolding" would be (at least) there :

 

image.png.34296662ab31ecb1a3da01e017c1b4ed.png

To beat cardboard into submission following the inwards angle.

 

I'll probably add a few clamps and a bit of wood to comform with the lower edge, too.

 

That means I'm going to lay fiber from the inside, and I do not intend to put gel coat first. (It's going to be resprayed, so UV not that of a concern)

I intend to start with a bit of fiber cloth that is bigger than the hole and overlaps the chamfer, and then each layer (crossing patterns) will be a tad smaller to cope with the chamfer.

 

4 to 6 layers probably.

 

No gel coat, no mat, little resin, most of it in carbon cloth.

And i'll mostly be glued 'from the inside"

 

I may, once cured, grind/dremel/sand a bit on the outer seam (a bit the actual bonnet, a bit the added patch) and lay a single ribbon of fiber across the seam in order to have mecanichal purchase from both sides and not relying on "glueing" from the underside. (don't know if it makes sense, but a bit like a rivet, expanded on both sides of the hole)

 

Like this, if we cut at seam level, with outer skin on top of the picture :

 

 image.png.acf49eca3be578bad360da8e4b2fb876.png

 

And the ribbon (CF/epoxy too) would be in green :

image.png.4dfb0530885b542ac13d8ac2970f1f02.png

 

So that it "locks".

Not too sure about it, but I have the vision of a random friend trying to bend over the bonnet, resting a hand on the patched area, and the broken matchsticks sound of ripped fiber...

 

 

On 24/05/2019 at 06:51, hammie said:

 

Use P40 filler as it has fibreglass strands in it. Once the underside is done and your filling key the gel coat top and it shouldn't shrink or crack.

 

I was looking for a somehow bendy filler, as the bonnet is wobbly, to prevent cracks...

 

 

On 24/05/2019 at 06:51, hammie said:

 

Using power tools like belt sander or orbital sander save hours of sanding by hand.

 

Well, belt sander would work on the top part of the hole, the orbital will be useful for roughing the thing, but I'll have a hard time preserving the "straightness" of the lines, and I fear that reflections on these would be pretty unforgiving. Given the size of the patch it would be pretty easy to mess up and sand a dimple given how far the reference surfaces (I.e : original bonnet surface) are far apart.

 

So I planned to use a sanding beam (basically a one meter long construction aluminium straight edge with sandpaper glued to it, maybe with a smidge of foam backing). That's the kind of tool you use to level the frets on a guitar neck, and it bears similarity to the surface I want to get (straight along, curved across). Guidecoat, rattle can filler, dust, dust, dust, up a grit, rince and repeat. (up to 400 wet)

 

Hence given CF/epoxy is not the easiest material to sand through, I'd like to have the repair sticking out the least possible amount

 

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hammie

You seam to have all bases covered however with you CAD ability can you not plastic print the template to fill the hole prior to using carbon cloth. 

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the stoat

A tip from me, if you cover your cardboard template with parcel tape the resin will not stick to it.

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robhume

Easiest solution is if you have access to a similar bonnet.

Take a mould from that and position on your own bonnet, then back mould.

Release outer mould. If you don't have access to another bonnet and if the main section of bonnet

is single curvature, what I have done in the past is use a piece of very thin aluminium sheet, self tapped

to the bonnet and then pulled over the curve, then self tap at the next bend and pull the ally slightly out

to follow that part of the bonnet, self tap again to hold in place, then bend again to follow the final section and

self tap again. 

That's probably as clear as mud but has worked for me on many occasions

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nice_guy
10 hours ago, hammie said:

You seam to have all bases covered however with you CAD ability can you not plastic print the template to fill the hole prior to using carbon cloth. 

 

Cad is good when you start with measurements, or a definition. In that very case, there is a lot of... alterations ? I briefly considered patterning the profile of the opposite side, but going digital on this would be very very very time consuming....

 

 

2 hours ago, robhume said:

Easiest solution is if you have access to a similar bonnet.

Take a mould from that and position on your own bonnet, then back mould.

 

Would have been the dream plan (finding a narrow lowline bonnet without a hole might have been slightly challenging). Alas, reality kicked in...

 

2 hours ago, robhume said:

 if the main section of bonnet is single curvature, what I have done in the past is use a piece of very thin aluminium sheet, self tapped

to the bonnet and then pulled over the curve, then self tap at the next bend and pull the ally slightly out

to follow that part of the bonnet, self tap again to hold in place, then bend again to follow the final section and

self tap again. 

That's probably as clear as mud but has worked for me on many occasions

 

That's perfectly clear, and more or less what cardboard will attempt to do, but with great gobs of hot glue instead of self tapping and some length of dowel to persuade carboard to follow the profile.

Thin sheet of metal (0.8mm is the thinnest I can source) would have worked well too, but maybe more prone to an accident... (It's a bendy part, and aluminium gets dented soooo fast)

 

And my friend the brown tape will join the party with his friend wax, too.

 

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