NEED ADVICE ON BUBBLING PAINT

JC Daniel

Active Member
I painted a wrecker for a friend last year, He talked to me early last spring about the bubbling finish so I redid the front fender which is fiberglass. I used feather fill G2 over the fiberglass and sanded it then shot SPI regular build 2k over that then used SPI black base and Euro for clear. I saw the wrecker yesterday and it has another bubble on the same fender so this will be the third time I will have to fix it, I need to fix it right and need your help and advice.
 

Jim C

Oldtimer
how big are these bubbles? just pinhead size or are they bigger? when you redid the fender was all the old paint stripped off first and taken down to glass/gel? also, is it the paint bubbling off the g2 or is the g2 bubbling off the glass?
 

EddieF

Top Banana
I know i've read epoxy sealer here 100x for adhesion before base.
Sorry it happened but you'll learn now.
 

dhutton01

Active Member
Corvettes are notorious for bubbling paint on the hood due to contamination from AC compressor oil on the hood underside. No amount of cleaning etc will prevent it once the fiberglass has absorbed the oil. Is it possible oil has contaminated the back side of the fender?

Don
 

texasking

Active Member
As Jim said, you have to figure out what is bubbling to know how far to take the finish down. Take an xacto knife or corner of a razor blade and pop the bubble off. See what is on the bottom where the bubble was. Sand that area with 180 and see if there is a layer that will not feather out because of poor adhesion, to make sure it is isolated to that one area, or a bigger problem. Fiberglass can be a real pain sometimes, because of the porous nature of the material absorbing oils, as Don mentioned, and moisture. Epoxy as a first step seems to really help, especially on raw glass and gelcoat.
 

JC Daniel

Active Member
The bubble is about the size of a nickel, The fender had been repaired 2 times before I touched it it had been cracked by a heavy gate swinging into it. When I repaired it last spring I cut out a lot of that area and started from scratch building the repair up. I will have to see when I can test the bubble to see where it is lifting from.
 

EddieF

Top Banana
I learned something today about working with fiberglass.
JC, i owe it to you. I never would've considered oil seeping thru from backside.

Edit- hey JC, take a good close up photo if you can.
 

Chris_Hamilton

Trying to be the best me, I can be
The bubble is about the size of a nickel, The fender had been repaired 2 times before I touched it it had been cracked by a heavy gate swinging into it. When I repaired it last spring I cut out a lot of that area and started from scratch building the repair up. I will have to see when I can test the bubble to see where it is lifting from.
90% likely that's your issue right there. Shame you have to eat it because of someone else's crappy repair. Most likely you are working on SMC not fiberglass. Remove all traces of the old repair. Take it all down to glass. Epoxy on the glass (it's probably SMC though). SEM 2 part SMC repair would be what I'd use to fix any cracking or for bonding new material. Polyester primer for build over the epoxy.

SEM has a good tutorial on working with SMC here:https://www.semproducts.com/blog/how-to-complete-an-smc-repair

In the future if you encounter a repaired area on Fiberglass or SMC assume it was done incorrectly and remove it all. Only way to assure you don;t have these types of multiple comebacks.
 

Jim C

Oldtimer
i had a late 50's vette i did body and paint on awhile back that had oil contamination on both front fenders from the underside. i had the whole car stripped and sanded smooth. within a couple hours oil spots would start to form on the outside. i would wipe with w&g remover, they would go away then a couple hours later they were back. i ended up cooking each fender with a curing lamp for a week on each side. wiping off the spots every time i walked by. prob would have been easier to just replace the fenders. i got it to a point where it would take all day for a sign of them coming back. at that point i put about 5 heavy coats of spi epoxy on it. its been about 8 years now. car is black and they have not come back.

if your seeing the bubble only where you did the glass repair then its possible its an air pocket.
 

JC Daniel

Active Member
The bubble is right where I did the repair, I used stranded fiberglass mat with resin the do all the build work making every layer bigger than before. I just can not believe it has come back again, If I could be an abortionist I would abort that fender!
 

Barry

Paint Fanatic
Staff member
The bubble is right where I did the repair, I used stranded fiberglass mat with resin the do all the build work making every layer bigger than before. I just can not believe it has come back again, If I could be an abortionist I would abort that fender!
JC,
The bottom line is something was not dry enough for the next product to be applied.
Rule of thumb is the layer that is left is the cause 90% of the time and with rare exceptions the next product trapped solvents.
 

metalman

Oldtimer
An lot of the fiberglass resins you buy at a box store or maybe the auto parts are finish resins. They contain wax which is supposed to float to the surface to promote full cure of the resin. I buy "laminating" resin from a boat orientated supplier, no wax. The last coat can be sealed with water based PVA mold release to avoid the sticky surface or it can be sanded off.
That said epoxy resin is slower but better for fiberglass repair work. SMC parts, better to use a dedicated SMC product like Chris mentioned.
 
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