Auto body repair & detailing: Fiberglass pinholes, added measure, pinholes

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.