Is body filler necessary for fiberglass repair - Fiberglass RV


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Old 08-30-2017, 10:41 AM   #1
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Is body filler necessary for fiberglass repair

Hey ya'll,

So I have been deep in repairing fiberglass holes and cracks on my boler and I am about to fill in the opening for the old built in heater. (Quite a large square hole)

I've been using fiberglass and resin and it is working just greeeat.

After the repair, I sand down around an 1/8th" on the repair and fill it in with Bondo body filler, later sanding it down to smooth. Sometimes the filler leaves 'ruts' I believe caused by air bubbles behind even with my efforts to overfill the repairs.

Can anyone tell me if it is actually necessary to use Body filler after a fiberglass repair? Does paint / primer adhere to it better than raw fiberglass? Any downside of just avoiding it?

Attached is a photo of a recent repair on inside door where latch gets mounted.
Thanks!
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Old 08-30-2017, 05:06 PM   #2
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You can get actual gel coat from a boat supplier - mix it up to the color you want and spread it in. Then feather it out - better job.
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Old 08-31-2017, 10:04 AM   #3
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The fiberglass resin can cure to a very smooth finish that paint may not stick to permanently. When you sand it you put tiny scratches in the surface. The body filler will penetrate these tiny scratches and adhere well. When you mix the catalyst into the body filler you can incorporate tiny air bubbles which will remain in the body filler when it sets. Sanding the filler exposes these tiny voids. Sometimes, if you use a primer to promote adhesion of the paint to the filler, the primer will fill these tiny voids. You can also use a "spot putty" which you don't have to blend with a catalyst. to fill these tiny voids. Sanding this putty leaves a very smooth surface, without voids, that holds paint well.
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Old 08-31-2017, 10:23 AM   #4
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Awesome, thanks for the great replies.. I appreciate your explanation Mike.
As my body filler cures, it becomes pink like bubblegum and it appears others cure to a green colour. Is this just a product thing?
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Old 08-31-2017, 11:00 AM   #5
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You don't need the body filler except for appearance. Very important for a fine smooth interior wall area, for instance, but not so much where the area won't be seen. By the time you're ready for Bondo, the structural work is done, so you can decide if appearance is important in that area. Use resin that cures to a hard finish and scuff it up for painting.
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Old 08-31-2017, 11:07 AM   #6
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Although I've used polyester putty for years I always felt it had several disadvantages. The amount of hardener (usually red but can be various colors) is very critical for setting time. Too little and it never sets up and to much provides inadequate working time. Even after it has set it will have a somewhat sticky surface for a day or two during which it will easily clog sandpaper. In the last few years I've switched to using two part epoxy resin with the addition of glass microspheres in varying quantity. The resulting viscosity can be made appropriate for various job conditions. Epoxy will bond extremely well to almost any material with the exception of some plastics such as polyethylene. Just prepare the surface with fine sandpaper prior to application. The glass microspheres produce a material which is very easy to sand and promotes excellent bonding of paint. After epoxy dries overnight at appropriate temperatures it can be easily sanded with no clogging.
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Old 08-31-2017, 11:19 AM   #7
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I like epoxy better too and have used a lot of it building a boat.
Just remember that epoxy sticks to polyester, but polyester doesn't stick to epoxy. Once you use epoxy for a repair on polyester, you have to use epoxy later in the same area. That's fine because epoxy is a great material. You can even use Marine-Tex which is close to the original gel coat color.
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Old 08-31-2017, 02:55 PM   #8
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Body filler.

Epoxy cures in the absence of air. It has a wax that rises to the top and seals it. This wax must be removed before anything, paint for one will adhere to it. Buy TSP-PF in the green box at Lowe's, mix with water per instructions and wash the wax off. West System sells additives for epoxy, I used the fairing filler mixed to peanut butter consistency. It can be sanded. You can buy this at a boating supply store. Be sure to get the fairing addative and it can beven sanded. The strength additive cannot be sanded. You can try a West Marine store or online. West Marine is a store that sells the West System supplies they do not produce it.

PS--Gelcoat can be added after fairing it is a good way to do it.
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Old 08-31-2017, 09:58 PM   #9
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Glass is hard, resin is soft. If the surface is sanded the glass stays raised and the resin sands lower than the glass. When painted the glass fabric may show after its painted. This is called print through. Adding a surface filler sands smooth so it shows a smooth painted consistant surface.
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Old 08-31-2017, 10:30 PM   #10
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Mark, Bondo (auto body filler) and fiberglass are very similar products. They both use a "styrene' resin as their base. Auto body filler has talcum power as a filler. Fiberglass uses glass fibers as a filler.
I have used fiberglass resin to "thin" auto body filler to make it easier to apply or when I need a stronger product.
Auto body filler can be shaped. Fiberglass is much stronger. The styrene resin is cured with MEK (methyl ethyl ketone) peroxide which generates heat to cause the styrene to harden.
The hardener is colored so you can be sure you have mixed it through the product you are using evenly. This is what gives the bubblegum, or other, color.
I use a clear hardener.
Once mixed the product should set up in about 5 minutes. Too much hardener and the product will set up before you can apply it. There is no such thing as too little. The product will set up eventually however it may take longer, sometimes over night, but it will set up.
Fiberglass for strength, auto body filler for texture.
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Old 09-01-2017, 06:26 AM   #11
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Fiberglass repair.

When we say styrene I assume we mean poiyester resin, styrene is added to resin to reduce it's viscosity. Polyester resin does not adhere well to cured polyester. That is the reason for using epoxy resin over cured polyester. In case you may not know, polyester cure time can be adjusted by adjusting the amount of hardener used. This cannot be done with epoxy. The larger hole you want to fill, put some backing over the opening on the inside, thin plywood like laun. Then cover with fiberglass and epoxy. Fiberglass mat or previously cured sheets of FRP would be my choice. Overlap the opening with thin pre cured FRP. You could also grind or sand the edges of the opening to produce a taper after backing is installed. This will help have a smoother transition. Prewet the fiberglass with resin before applying. To make a smooth finish over epoxy fairing I cover with plastic wrap and gently smooth over with a hard roller of some kind. The plastic will peel off when cured and sanding time is reduced. Good luck, not nearly as bad a job as you may think.
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Old 09-01-2017, 10:17 AM   #12
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I hear you J Ron. I spent a lot of years in school and paid for it by working in auto body shops. I stopped doing that a long time ago but still did my own and still do. I'm doing one of my vehicles today.
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Old 09-02-2017, 09:51 PM   #13
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if your looking for the ultimate surface to apply paint to after body filler, spot putty,fiberglass resin, get some high build primer. It comes in a spray can, follow directions on can. I spent many years in the auto body repair buisness now only do it for a hobby
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Old 09-02-2017, 11:13 PM   #14
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Yes Jan, I like that stuff too. It's like a thin coat of filler, sands easily and gives a great finish for painting.
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Old 09-02-2017, 11:54 PM   #15
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Thanks everyone, I am overwhelmed with the amount of participation in this discussion and the thoughts being thrown around. I will definitely consider using epoxy as a filler and will look into high build primer as I will be painting my trailer soon.
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