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Auto body repair & detailing/Fiberglass pinholes

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Expert: Cameron Orsich - 9/9/2006


Question
I sometimes get bubbling of a fiberglass repair if I don't use metal under the repair.  I am guessing it is due to pinholes  in the cured fiberglass that allow water to pass through and eventually cause a bubble.  It may take many months or a year.  I do not have this problem if I cover the spot to be repaired with metal. Is there anything I could do to prevent this or some other material?

Answer
You must make an indent then make an "all around" weld around the affected hole.

If the welder really makes an "all the round" weld, then its really sealed off; then there's no need to use fiberglass, as you should only be using when you believe you may not have the affected area completely sealed off, as it alone is no match for moisture/rust...thats causing your problem now.

However, if you not certain you have a sealed "all around' weld, then use a layer of fiberglass over your the welded area as an "added measure".  Myself...I weld...then bondo..no problems.

The only time you should be using fiberglass, is when you're not certain you have sealed off what will lies under your weld (i.e., your not certain you really have an "all around" weld, or you have no weld at all, and are making a repair with just fiberglass out of necessary).

If making a repair, just using fiberglass, the repair can't be where moister can further cause the substrate to hold additional moisture and rust, which will cause your repair to fail...as is the case now with your work.

If you're going to make a non-weld repair with fiberglass, it must be done only where the back side of the panel will NEVER be exposed to the elements--such as a fender with an inner well, or a sealed door. Stay away from trunks--a quarter, or the deck lid; as moister loves it in there.  Also, stay away from hoods...as they are open to moister from the underside.

In short, always weld a hole shut when you can, especially if the hole was caused by rust--as there's a reason it rusted out in the first place--moister; and fiberglass, by itself, will only slow the rust down...it won't stop it.  

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Cameron Orsich

Expertise

NOTE: MY EXPERTISE IS AUTO BODY...PLEASE limit your questions to the subject of GENERAL AUTO BODY PANEL QUESTIONS...you know: OlD Fashioned, Do It Yourself, By Hand, Back Yard, Under Shop Lights at 3:00 AM AUTO BODY QUESTIONS. I can answer auto body questions for any make, model, or year regarding replacing valances, door skins, fenders, etc... I can also answer questions on how to grind, weld, fiberglass, fill, paste and spray top coat, feather, mask, prime, paint (enamel, lacquer, base-coat/clear-coat), touch-up, melt-in, wet sand, wool polish, sponge polish, finesse, and much, much, more...to much to list here. I love air tools, but if you don't have any...no problem...as I can answer all your questions on how to achieve superb results doing it yourself...the old fashioned way...by hand. If you want a shop answer, go to a shop and pay someone to do it for you. If you want to take pride in doing it yourself, just ask someone who's been there and done that--me, Big CO. I'm available to answer general Auto Body questions for those whom want to do it themselves. Please...no collision, basket case, or train wreck questions. If you have something special that you want to fix, paint, or restore yourself; but don't know how, you've come to the right place...ask me...Big CO, the neighborhood know it all with the shop lights on all night

Experience

Having put myself and my brother through college in the late 80's and early 90's buying and selling over 250 cars. I restored my first car, a '72 350 Chevelle Malibu 4 barrel, in 1978 at just 16 years old. From there, I never looked back...usually restoring classics or sport cars: 340 Cuda's, 396 SS Chevelle's, Novas, Dusters, late 60's Mustang Convertibles, GTO Judges, 455 Buick Grand Sports, about a dozen 280ZX's ('79-'83)--the most recent, in July of '06, I sold a '83 280ZX on cars.com to a gentleman that flew in from Colorado to my state of Ohio just to purchase it from me. As for experience: Have replaced valances, door skins, fenders. Have grounded, welded, fiberglassed, filled, pasted and sprayed top coat, feathered, masked, primed, painted (enamel, lacquer, base-coat/clear-coat), touched-up, melted-in, wet sanded, wool polished, sponge polished, finessed, and much, much, more...to much to list here.

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