Tank blistering

[Bartman]

I have been slowly going through my bike. It cranks and runs very well. Getting 40~44 mpg. Wanted to paint next.
So I red coated the extra tank and then a buddy helped me pull a dent on the right edge. He had to bronze fill the area where holes where made. For two months we worked with it because the bondo kept blistering up. We thought that we had finally fixed it so we painted.....
Now the bike looks good but that area has a large blister come up over the filled area!
I'll show a picture when I figure that out.
Any ideas?

[Bartman]

The good:


The ugly:

[rdmcguy]

Mr. Blister

That's a drag,
I have been lead to believe by way of professional painters that moisture can be trapped within the bondo (or in primer) if not kept COMPLETLY dry during the prep .....Only to rear it's ugly blistering head after paint...

[Cobia]

RDMCGUY is spot on regarding the moisture. Yet, another thing to consider is the incompatibility among the layers you may have there, lacquers and enamels, epoxies and resins. You have to layer them according to their compatibility for a trouble free finish. JAT...

[bikerphil]

Looks like a good place to put a racing sticker, cover it up, BTDT

Looks like gonna have to have a redo.

[jetmechmarty]

I am not a painter, but I believe the brazing is the issue. I have a friend with that happening to his car where the quarter panel is brazed on. Hopefully, some one will come on and give you the details, but I believe that's the problem.

[Bartman]

Yes I think it is the braising too. Before it was painted we had it all ready and would leave it in truck at work and as it heated up that same area would bubble up the etching primer.
I have to change projects for a little while before I get back on it. My daughter is getting married and I have to build a walnut chest of drawers before Oct.

Then my plan is to grind off the braising and either use some kind of epoxy puddy or cut it out and have steel tiged back in.

I also found another tank in the barn that I can use. Just more practicing with the painting.

[fredintoon]

Hi Bart,
it ain't the brazing. If dissimilar metals affected paint HTF would they have made the paint stick onto those hearth-brazed Velocette frames or the soft-soldered gas tanks on vintage Nortons?
What I can say is if you don't thoroughly remove the brazing flux (scrub with wire brush and very hot water) afterwards, that might do it.

[jetmechmarty]

I'm out of town for the holiday, but I know a couple of paint professionals I can get an a definitive answer from. I wouldn't care to mess with that spot again unless I was positive I could get it right. Doing it over once is enough. As Fred suggests, I believe it's more than disimilar metals, but I'm just a dummy with a keyboard who happenned to see the problem before.