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Thread: Best Bondo, or other product, to use on the hull of an aluminum boat?

  1. #1

    Default Best Bondo, or other product, to use on the hull of an aluminum boat?

    Hello. I need to smooth out some brackets I had to install on the bottom of my aluminum boat's hull. What version of Bondo, or some other product, should I use? The product will not need to bond anything together. I need it to create a smooth transition from the hull to the brackets that are a quarter inch and bolted to the bottom of the bolt. The aluminum flexes a little; I guess that needs to be factored in.

    Apologies if this is the wrong forum. I did not know which exactly would be appropriate.

  2. #2
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    Body filler and flex really don't go together very well.

    Perhaps you could post a picture so we can see what you are doing.

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    Default

    Quote Originally Posted by thegreatfisher View Post
    Hello. I need to smooth out some brackets I had to install on the bottom of my aluminum boat's hull. What version of Bondo, or some other product, should I use? The product will not need to bond anything together. I need it to create a smooth transition from the hull to the brackets that are a quarter inch and bolted to the bottom of the bolt. The aluminum flexes a little; I guess that needs to be factored in.

    Apologies if this is the wrong forum. I did not know which exactly would be appropriate.
    The problem is the water, most fillers need to be protected from moisture or they will either peel off or create blisters in your finish. As far as flexibility goes you can use a flexible filler like the one shown below BUT is still needs to be sealed so that moisture doesn't get to it.


  4. #4

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    Quote Originally Posted by Houndogforever View Post
    Body filler and flex really don't go together very well.

    Perhaps you could post a picture so we can see what you are doing.
    No pictures handy at the moment, but it is some 2" wide x 1/4" thick aluminum pieces I have bolted to the bottom of the boat for some bracing. It is amazing how much a flat edge, even 1/4", will affect the boat's speed. That is why I am wanting to smooth it out.

  5. #5
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    Could you smooth a beveled edge of sealant on the edge? Like a bead of caulk, but of some good stuff 2 part urethane.

  6. #6

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    Quote Originally Posted by Houndogforever View Post
    Could you smooth a beveled edge of sealant on the edge? Like a bead of caulk, but of some good stuff 2 part urethane.
    I definitely could smooth it either in application or post cure with either a razor blade, sandpaper, or a grinding wheel (depending on how hard it is). It just needs to stick to aluminum and be at least somewhat slightly flexible and water tolerant.

  7. #7
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    Two things come to mind that donít include any filler. 1. Grind a taper onto the edge of the piece you are attaching and countersink tapered head screws into the piece so they donít protrude. 2. Take the boat to a boat shop that has a good aluminum welder and have him tig weld the pieces and put enough weld on so that it can be ground to the shape you like.

    Bob K

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    bobs suggestion is a very good one and would be the method i would use. but if it's just not possible, look at a product called splash zone a788.
    https://www.pettitpaint.com/products...s/splash-zone/
    it's a little stiff to mix up but it's a fantastic product. i'm using it right now to seal the weld areas on the bottom of my steel hull boat. i had to weld on almost two full sheets of steel to repair some of the failing plating. that's a story for another time...
    it's versatile for other things too, i keep the two gallon kits on hand. you only need a small amount though, i don't know if they make anything smaller than the two quart kit.
    b marler

  9. #9
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    Default 105 West System

    Quote Originally Posted by bmarler View Post
    bobs suggestion is a very good one and would be the method i would use. but if it's just not possible, look at a product called splash zone a788.
    https://www.pettitpaint.com/products...s/splash-zone/
    it's a little stiff to mix up but it's a fantastic product. i'm using it right now to seal the weld areas on the bottom of my steel hull boat. i had to weld on almost two full sheets of steel to repair some of the failing plating. that's a story for another time...
    it's versatile for other things too, i keep the two gallon kits on hand. you only need a small amount though, i don't know if they make anything smaller than the two quart kit.
    Marler,

    I know you have a lot of boat experience, so what would be your take on grinding down the edges of newly installed plate and sealing/smoothing over with the 105 West System? 20 years ago I owned a 1956 Chris Craft, 36 foot with a 7' beam and 24" draft, twin screw with carvel siding. I pulled that boat out of the water onto a trailer I made for it as to do a bottom job with copper based sloughing paint. On several places aft where the carvel siding met the teak I had dry rot. I repaired the dry rot and sealed it in with the 105 West System prior to rolling on the copper based paint. This repair lasted for at least 10 years when I sold the money pit. Would the West System hold up for the OP's intent? Not sure if there would be a difference in using this product on wood vs aluminum.

  10. #10
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    Quote Originally Posted by Bob K View Post
    Two things come to mind that don’t include any filler. 1. Grind a taper onto the edge of the piece you are attaching and countersink tapered head screws into the piece so they don’t protrude. 2. Take the boat to a boat shop that has a good aluminum welder and have him tig weld the pieces and put enough weld on so that it can be ground to the shape you like.

    Bob K
    I'm with Bob. Get someone with a heliarc (aluminum) welder and fill in the space you planned on using bondo by welding in aluminum. then grind it smooth. The problem with using a polyester filler (generic term -"bondo") is that you can't have a filler that is both waterproof and flexible (at least from the auto body supply). Water proof filler gets very hard and is brittle (won't work for flexible surface) and the flexible filler will suck up water like a sponge and will delaminate if water is introduced. Flexible filler is also very soft so any scrapes on the bottom of the boat will open it up to water absorption.

    So heliarc aluminum welding is what I would do.

    ORRR you could put hydrofoil ski's on it which would lift the hull out of the water. LOL ! (obviously joking)

  11. #11
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    Default

    Quote Originally Posted by Ronf View Post
    Marler,

    I know you have a lot of boat experience, so what would be your take on grinding down the edges of newly installed plate and sealing/smoothing over with the 105 West System? 20 years ago I owned a 1956 Chris Craft, 36 foot with a 7' beam and 24" draft, twin screw with carvel siding. I pulled that boat out of the water onto a trailer I made for it as to do a bottom job with copper based sloughing paint. On several places aft where the carvel siding met the teak I had dry rot. I repaired the dry rot and sealed it in with the 105 West System prior to rolling on the copper based paint. This repair lasted for at least 10 years when I sold the money pit. Would the West System hold up for the OP's intent? Not sure if there would be a difference in using this product on wood vs aluminum.
    west system 105 is an amazing product and is another thing i always have on hand. there are a ton of fillers you can add to it to make great fairing compounds and a host of other compounds for fixing almost anything marine. the problem with it in this application is the flexing. i think it's just a bit too brittle and might crack over time. west system also makes a semi flexible thickened epoxy called g-flex that might perform a little better. when i install something on the boats bottom i bed it with 3m 5200 adhesive/sealant so it doesn't leak. if it needs fairing i look for something that fills the void and will accommodate whatever the situation calls for. with a wood or fiberglass boat that's pretty rigid there are quite a few fairing putty's that fit the bill, and west system would be high on the list. with the aluminum hull i think i'd use the splash zone if welding was out of the equation. the biggest problem with the splash zone is that it's a very hard product to apply smoothly and will need to be sanded or ground to an acceptable finish if you want good high speed performance.
    b marler

  12. #12
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    Default

    Quote Originally Posted by Bob K View Post
    Take the boat to a boat shop that has a good aluminum welder and have him tig weld the pieces and put enough weld on so that it can be ground to the shape you like.

    Bob K
    Quote Originally Posted by Phil V View Post
    Get someone with a heliarc (aluminum) welder and fill in the space you planned on using bondo by welding in aluminum. then grind it smooth.
    Not even that, find anyone with a simple mig welder/spool gun can weld that up for you in minutes.

    Like Bob K & Phil V say, a heavy bead around the area then contour the edge as you see fit.

    Using bolts & bondo is not a good way to go.

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