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Build with fiberglass cover with bondo?

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#1 ericp501


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Posted 21 September 2016 - 09:03 AM

I've done custom motorcycle tanks and seats for cafe style bikes in the past just using fiberglass and bondo and never seem to really have a problem..  I'm no pro by any means, just a fun hobby I've enjoyed.   But now that I got my hands on a Vela rossa I really want to be sure I'm not going to be hurting myself in the long run.


The kit is fully mounted and frame reinforced (convertible), so all the hard work is done.  Kit was done well and lines up for the most part.  Where the top of the rear fender and the door meet sat up quite a bit too high so I worked it down to get it more level with the door, looks great but the glass did get thin in few spots with some small holes.  I've figerglassed under the holes and was just going to do a few thin layers of bondo to smooth it out.


The hood is also slightly off near the doors.  It looks like I can level it all out, so I put a single layer of fiberglass down and again was just going to build with a 2-3 thin layers of bondo and sand it all smooth.


I've had success on motorcycle seats and such, but its only been about 2-3 years.  Will this all start cracking and falling apart on me in 5 years?  Is there a better way to build up these areas a bit other than fiberglass and thin layers of body filler? 

#2 wolfalp1592


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Posted 21 September 2016 - 10:27 AM

Lucky guy, I am working on a 280 that had some fiberglass/ bondo work done about three years ago. What I am seeing (I didn't do the work so cant say what his goals were) is that where the fiberglass was one sheet and then seam joined in bondo to the body panel it has cracked and did not hold shape well.It held up better when layers of fiberglass were used and bondo was used as you are describing.

Now when PO used both over rust - well that just makes me mad.


Good luck

#3 260DET


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Posted 06 October 2016 - 10:32 PM

With these sorts of jobs you need to work out where the stress concentrations are, keeping in mind that the whole body is a structural member to greater or lesser degree. Obviously the stressed areas need more fibreglassing and less filler. Around the door opening is one stressed area, the rear quarter panel areas are another.

#4 JMortensen


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Posted 08 October 2016 - 07:37 AM

Bondo will probably be fine, but if you want to use something a little better you can use epoxy and filler. I used West Systems low density filler with epoxy (did West System epoxy and others). So far so good. The low density filler is bondo equivalent. They have higher density stuff for joining different sections of fiberglass together with more strength.

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#5 justforkicks



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Posted 14 October 2016 - 11:32 AM

Having done considerable fiberglass work I offer these ideas to consider. Where steel panels can and will flex will cause cracking of bondo or non adhesive fillers such as low density. IF covered with fiberglass cracking flexing would be less. IF adhesive (high density and or fibers) filler panels will tend to be like one piece panel and less likely to crack. Use glass fiber or adhesive filler stronger option, least likely to crack. Welding of panels and adhesive filler and glass fiber over strongest I believe.

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