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Thread: Best body filler for fiberglass?

  1. #1
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    Best body filler for fiberglass?

    I am looking for opinions on the best filler for fixing larger imperfections in fiberglass body work, and also for a good filler for a skim coat just before block sanding.

    Here is the problem, on my last kit car I used bondo short strand fiberglass filler for major body work, and top coated with regular bondo, after 1-2 years you could see everyplace there was filler on the car as it created a very slight depression in the paint.

    Obviously I want to avoid this on my current build and wanted to see what has worked well for other members.

    Thanks for any help.
    Last edited by mkman; 03-18-2014 at 09:39 PM.

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    Senior Member murcie-me's Avatar
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    I've never understood why anybody would "skim coat" the body in bondo before block sanding. Bondo and fiberglass dont get along well, so you should use as little as possible and only when completely necessary. Since they have different expansion/contraction rates, using it as a skim coat will eventually result in spider cracks across the entire panel its applied to.
    If you need to skim coat an entire car with something for sanding, invest in a few gallons of high build polyester primer (PCL). It builds fast, has the same characteristics and chemical structure as gelcoat, therefore bonding to fiberglass permanently without any problems. It will never crack, lift, or discolor. Its seals the surface its applied to and creates a uniform surface for paint with unsurpassed bonding. It can be applied as thick as you like, but would require multiple sprayed applications to get a buildup of 1/4". There is no sanding required between coats, so low areas do not need to be scuffed as long as the surface is clean and free of oil, wax, dirt, salsa or coffee stains. It can be wiped down lightly with acetone prior to painting without loosening or dissolving, and is available in multiple colors.
    Once you use this, you'll never need bondo again for filling small imperfections over small or large areas.
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    Last edited by murcie-me; 03-19-2014 at 09:05 PM.
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    Hi
    As murcie-me has expertly explained use a filler only in small places and I usually polyester as described above.but when block flatting polyester do it dry.never wet flat as its like a sponge and absorbs moisture which can pop after you have applied 2k primer.the moisture try's to escape when it gets warm.

    I apply 3-4 coats the first cost being a mist type coat so it grips and doesn't go on too wet.then 2-3 wet coats depending on what thickness required.flash off around 10 mins between coats and as murcie-me says no need to flat in between coats.polyester is a great product.

    Hope all the above info helps

  4. #4
    Senior Member murcie-me's Avatar
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    The PCL primer can be sanded wet or dry though, it does not absorb water or moisture of any kind. I usually sand it dry, but for the final sanding before paint its best to wet sand with 320. This primer uses the same MEKP catalyst to cure as fiberglass resin (mix about 1 teaspoon per quart for a 3 hour cure), it can be speed cured with heat if you're in a hurry without any problems. The polyester primer you are talking about is a non-catalyst type, and it does absorb some moisture when its being wet sanded, but the PCL Poly primer does not.
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    Hi guys I went searching for the product above to see if it was available in NZ , still not sure if its available here, but I did find these posts that I thought were interesting to say the least. Comments like this "I don't use Poly as it sucks up moisture like a sponge, not good on glass bodys and even worse on steel."
    Now leaves me confused as I once painted a car and 5yrs later all the bondo started to blister and you could see everywhere it had been applied on a steel car, and my Countach polyester shell will need some filling here and there and I really don't need a repeat of the last car, any thoughts on what these guys are saying.
    Spray on bondo pros and cons - THE H.A.M.B.
    Last edited by Lambolex; 03-20-2014 at 04:02 AM.

  6. #6
    Senior Member murcie-me's Avatar
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    I dont know what they are talking about. There are several types of polyester primer, they are probably using the non catalyst type. This polyprimer has been around for many years, and is the number one choice of bodyshops for steel and fiberglass bodies. I've used it on both also, and have never has a problem with it absorbing moisture. I dont see how it can, its basically polyester resin with MEKP catalyst, just like fiberglass resin. Does fiberglass resin absorb moisture? If so, I think the boating industry is in alot of trouble lol.
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    Can you spray the PCL over the entire car if you've already put (fiberglass stranded) bondo on ? Can the PCL be used to seal the car's surface ?

  8. #8
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    yes totally right the polyester in the uk we use is chalky and if it gets wet then it will absorb moisture,its then sealed using 2k primer.the polyseter in other countries for other types of jobs will be differant so maybe are ok to wetsand.we use standox products.

    So murcie-me is totally correct there are various products

    Here's the data sheet for our sprayable polyester sprayfiller
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    Last edited by beemaman; 03-20-2014 at 10:46 AM.

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    Here's a 430 replica with 3-4 coats of standox polyester sprayfiller

    Guide coated and blocked then 2-k primed to seal in polyester.
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  10. #10
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    Thanks for all the information, I will check the availability of the PCL primer in my area. One more question though, is it ok to spray the PCL primer over bare fiberglass, or must there be a layer of gel coat before spraying?

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