Fiberglass sanding and finishing

Anything goes in the world of fiberglass and plastic



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PostPosted: Sun Sep 21, 2014 10:16 pm
I had to extend the lip on this frame (about 1/4" see red arrows) to accommodate a replacement lexan window which is smaller that the original.

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I taped sheets of plastic (a garage sale sign) to the existing lip and applied fiberglass mat (1.5 oz) and poly resin.

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Inside view where I started applying the fiberglass:

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There are a lot of imperfections in the fiberglass (there was probably a better way to do this) and now I need to start sanding and shaping it so I can "glue" the lexan window in place. I have a compressor (33 Gallon 150 Sears) and I've found that 40 grit (2") sanding discs on my angle die grinder seem to work best. It's very difficult o get a smooth level surface with this type of sander, at least it is for me.

Can anyone recommend a better tool and/or technique?

What filler should I use for the final finish before primer? Would Duraglass work?



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PostPosted: Sun Sep 21, 2014 10:43 pm
I can't think of any better method. Really just have to use anything with a rough grit and knock the stuff down. Fiberglass gets pretty hard and isn't very easy to sand. Seems like to me you just need it smooth enough to get a good seal for your lexan to adhere to. You can probably use duraglass but not sure you need to. Regular body filler would work to get it smooth. Unless you need extra strength or are worried about it flexing and cracking.



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PostPosted: Mon Sep 22, 2014 2:17 am
What is the maximum thickness for filler?

I was thinking about Duraglass because some areas need more than just a skim coat. The sign material that I used for a "mold" sagged here and there.

One other thing I should have mentioned, when I bolt the frame to the car it might flex a little bit

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PostPosted: Mon Sep 22, 2014 6:11 pm
You might want to consider doing a combo of fillers here. Duraglas would be better where you need some thickness and strength and one of the poly putties used in bumper repair would be flexible plus fill in and be a lot easier to smooth up over the fiberlgas. I use USC's Body Icing over fiberglas quite often. They even make an extreme flex version of it called Poly-Flex. That stuff can be used on parts that flex in normal service.
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PostPosted: Wed Sep 24, 2014 7:17 pm
Why not put more fiberglass on you sags?

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PostPosted: Wed Sep 24, 2014 10:30 pm
Sure, you can do that as well. You just have to watch about making the area too "resin rich." Guys that don't work a lot with fiberglas will sometimes just try to fill in with a lot of resin and little added mat. It ends up being a weak area that can shrink and twist in strong heat/sunlight.
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PostPosted: Sun Sep 28, 2014 3:58 pm
I started block sanding..............the low spots are the dark areas, would it be OK to use Duraglass or should I just keep sanding?

I will trim away at least 1/2" off the new lip once I get it leveled.

Any other suggestions are always welcome

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PostPosted: Sun Sep 28, 2014 5:37 pm
If those are low spots I would be inclined to stop with the sanding and add the duraglas to those areas now. I was always taught that if you grind too much fiberglas off you'll spend a lot of extra time replacing it. By adding the duraglas now you don't take a chance on breaking through your profile. Trust me, just looking at the sunlight shining through on the left side there it doesn't take much of a, whoops! I cut through.
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PostPosted: Sun Sep 28, 2014 10:14 pm
Would there be any benefit of adding small sections of mat and resin to build up the low spots in place of the Duraglass? Would the Duraglass shrink more than mat and resin?

If the Duraglass does shrink how long does it take for the shrinkage to stop?

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PostPosted: Sun Sep 28, 2014 10:36 pm
Yes, there is usually always going to be somewhat of an advantage when bonding "like to like" materials as opposed to using even a filler like Duraglas. Resin and mat will strengthen the entire frame. Duraglas is faster and can be done more easily however it does have some shrinkage. That shrinkage is pretty much done with in a couple of days IF you get it in some sunlight which will accelerate the cure period.
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