Problem with blister/lumps appearing in paintwork

[stuwindsurf]

Hi all
I'm looking for some professional advice ref problems with the paintwork on my custom motorcycle tank.

The tank was modified by myself which included some seam welds. The tank was then sealed inside by a professional company here in the UK (Tank repair) who then did the filler work and paint.

The tank has been repainted twice now, after the first time and after a few months I noticed some small lumps appear on the tank where a couple of the weld seams were. The tank sealing guy ground out the filler, re-filled and repainted. He also put some more sealer inside the tank around that area in case there were any pin holes that fuel may have been penetrating.

Anyway, after approx 6 months the same lumps have appeared in roughly the same place. I don't understand what could be causing them so I though I'd post a picture to see if anyone has any ideas what else could be causing this?

I'd appreciate any input as I'm just about to take the tank back to the guy again but don't want to go through the whole process again to find more lumps appear after a few months.....

Many thanks
Stu

 

[TAZ]

those might be air pockets in the bodywork, but since the bodywork has been done twice, I would suspect they are pinholes in the welds thus letting fuel vapors escape
I noticed you did not say the tank was pressure checked. I would have suggested doing this before sealing.

If you put fuel in the tank and the bubbles get worse on a hot day, this would pretty confirm it.
You can just slice one of those bubbles with a razor blade and see if you smell any fuel.
If you do this on a hot day with fuel in the tank, but makes the smell stronger.

I hope you only find it's air pockets in the bodywork though, but I highly doubt it

 

[stuwindsurf]

Thanks for your reply, that's pretty much what the guy who painted the tank has suggested.

I did ask the guy to pressure test the tank before sealing but I don't think he did.

He is going to take it back to the metal and try soldering along the top of the weld so he doesn't destroy the sealer inside the tank with excess heat from re-welding.

I appreciate your thoughts, thanks again! ?

those might be air pockets in the bodywork, but since the bodywork has been done twice, I would suspect they are pinholes in the welds thus letting fuel vapors escape
I noticed you did not say the tank was pressure checked. I would have suggested doing this before sealing.

If you put fuel in the tank and the bubbles get worse on a hot day, this would pretty confirm it.
You can just slice one of those bubbles with a razor blade and see if you smell any fuel.
If you do this on a hot day with fuel in the tank, but makes the smell stronger.

I hope you only find it's air pockets in the bodywork though, but I highly doubt it