Cheers mate, Matt

Check out the uk fizzy forum called fizzychat

Hi Matt,
welcome to the Yamablog. Send us in a photo of the fizzy when you can (or put it on the forum?).
Good luck with your shop. Please allow me a moment to get on my soapbox; If you give your customers “fair and square” treatment, with no BS, you will do OK. Build a good reputation and it will carry you through tough economic times. People will pay for expertise, so don’t give it away.

If you have any”shop view” comments regarding topics on our blog or general questions that come from your customers, I welcome them at any time.
Cheers.
DS

Unfortunately, you do not indicate what model of motorcycle you have. The answer would differ depending on the material the fuel tank is made from.
If I assume the bike is a road model, it will have a steel fuel tank that is primed and painted and (probably) clear coated. To have something coming through the tank, it would almost certainly be corrosion, but that would not “dissapear” after being in sunlight. So I would suggest that it would be a growth on the surface of the paint. that is discoloring the painted surface. It is not uncommon to have such discoloration “bleached out” by the UV rays of the sun.
If the bike has a plastic fuel tank, the fuel volatiles will permeate through the plastic (most common complaint being gas bubbles appearing under tank decals) and leave some discoloration which is easily cleaned, and again will be faded by UV light.
If it is definitely mold or mildew, it is caused by the cover not being able to breathe and trapping moisture on the surface. 2 things to do after cleaning and waxing the tank before storing: 1. use a proper breathable storage cover (if the bike is in a dry garage, an old cotton sheet is better than most covers as all you are needing to do is keep dust off the surfaces) and 2. block off any windows that let sunlight get onto the bike and raise the daytime surface temperatures.
Cheers,
DS

See, that’s what happens when you share! Dave