Using bondo to modify female mold chine angle?

Discussion in 'Materials' started by Bullshipper, Apr 20, 2024.

  1. Bullshipper
    Joined: May 2008
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    Location: Mexico

    Bullshipper Bullshipper

    I have a 28' female mold surfaced with polyester tooling gel. The side chines are tilted upwards at 5 degrees and I would like to fill them with a sanding filler that can get 3/8 -0" thick by 3" wide after dewaxing and sanding the underbelly with 80 grit.

    I would like to router the sharp edges round using a 1/8" R, 1/4R, 3/8, 1/2, 5.8 and 3/4" round off router bit to get the rolled edge to look better than I could by sanding.

    I know there are epoxy fillers like Total Boat, but it is not available here in Mexico.

    So does any one know if I can fill the 2 28' long chines with automotive bondo or some other common product then gelcoat and wet sand over the bondo to get a hard finished surface to continue laminating in the same open female mold that only has 4 pulls on it so far?

    Thank you
  2. Blueknarr
    Joined: Aug 2017
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    Location: Colorado

    Blueknarr Senior Member

    Should work
    Laying the bondo evenly over 28 feet will be very difficult.
    Especially if you need to use a router to achieve a smooth round over
  3. C. Dog
    Joined: May 2022
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    Location: Coffs Harbour NSW Australia

    C. Dog Senior Member

    Is "bondo" automotive body filler? It is probably better to avoid non international brand names in an international forum for maximising replies. It is difficult to shape and finish if so, and clogs sanding abrasives. Useful for small hull repairs in my opinion.
  4. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Don't use anything containing any talc. Bondo brand has a lot of talc in it. It will soak up water, expand, and pop off. High end auto body fillers might be okay, but you would need $1000 worth of it by the sound of things. You need to fill 90% with fiberglass and just use bog for 1/16" or so to smooth it out. That way you'd only need about gallon of stuff like this -
    3M Marine Premium Filler, 46006, 1 gal
  5. wet feet
    Joined: Nov 2004
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    Location: East Anglia,England

    wet feet Senior Member

    I agree that brand names may be unhelpful because I have used good polyester filler to modify moulds,but not that brand,as I've never seen in in the UK.When the surface is correctly shaped for the new version it is quite possible to spray gelcoat over it and work up a very high quality surface finish.How many components you are able to make will be largely dependent on the standard of preparation before adding the filler.One negative is that you are highly unlikely to be able to use a router unless you have a 90 degree corner.So you may have to make a series of bevelled templates that feature the angles you have and the radius at the specific points.
  6. gonzo
    Joined: Aug 2002
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    Location: Milwaukee, WI

    gonzo Senior Member

    The styrene in gelcoat will soften the bondo. It is better to thicken the gelcoat and use it as a filler. I am not quite understanding the description of what you want to do. A router bit will only give you an even edge if it is, for example, on a 90 degree edge of a board. Why are there sharp edges? That is not ideal on a mold. Creating a fillet would save you sanding and shaping.
    Last edited: Apr 21, 2024
  7. fallguy
    Joined: Dec 2016
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    Location: usa

    fallguy Senior Member

    I recommend pictures. There are experts here, but they may not be experts at deciphering.

  8. ondarvr
    Joined: Dec 2005
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    Location: Monroe WA

    ondarvr Senior Member

    To be blunt....automotive body filler is a terrible product for tooling applications. Every physical aspect of it is substandard compared to even the low end, sort of similar products used for tooling.

    The HDT is terrible, the water resistance is terrible, the chemical restance it terrible, the physical properties are terrible, it's crack resistance is terrible. The only positive aspects are that it is relatively low in cost, and somewhat easy to work with.

    If you plan to make more than one part out of a mold that's been modified like this with boby filler, you'll likely be severely disappointed.

    It does work OK in building limited use plugs for building a mold though. But it needs to be covered completely by a better product to protect it.
    Last edited: Apr 22, 2024
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