Rusting Fuel Tank after leave it 4 days

[Nmore]

Hi all, anyone got this problem?
It is 100% new RC390 but stored from 2018. I bought it 10 days ago with 0 km on it and some small scratches on the bike.
4 days ago I got some work, and I need to go apart from home, then I leave the bike here with 2/3 fuel in tank. I don't know how it rusted like this just after 4 days. I asked the mechanic, and he said all the metal fuel tanks could be like this, just run till 7500 km then come to maintenance and he will clean it.
Is he lying to me, or is there a problem? Although I have owned motorcycles with metal fuel tanks in the past, I have never experienced this kind of issue.
Now what should I do? Will the gasoline pump rust as well? I intend to apply epoxy sealer after take it to the mechanic for him to maintain and clean when 7500 kilometers on odo.
Its just 500km on the bike now...

 

[Keep The Manual]

If you put 500kms on the bike and the fuel pump is still working, then I would say the fuel pump is fine. I would use sand paper to sand off the rust around the cap and hole and wipe it off with a wet rag so that the rust doesn't go inside the tank. I would also siphon out the fuel from the tank and see how it looks sitting at the bottom of the tank. You can get a rubber hose or a big syringe to suck up the bottom of the fuel tank. It's tricky buying an old motorcycle that has been sitting for 6 years, they are not designed to sit around without use. The seals dry up, water gets in the fuel tank from condensation, rubber hoses dry out, bearings dry out, etc.

 

[Nmore]

If you put 500kms on the bike and the fuel pump is still working, then I would say the fuel pump is fine. I would use sand paper to sand off the rust around the cap and hole and wipe it off with a wet rag so that the rust doesn't go inside the tank. I would also siphon out the fuel from the tank and see how it looks sitting at the bottom of the tank. You can get a rubber hose or a big syringe to suck up the bottom of the fuel tank. It's tricky buying an old motorcycle that has been sitting for 6 years, they are not designed to sit around without use. The seals dry up, water gets in the fuel tank from condensation, rubber hoses dry out, bearings dry out, etc.

Thanks, I did clean the gas cap myself but it keep coming back, I will take it to the mechanic and clean the gas tank when bike reach 1000km.
I bought this by its price, thought $3000 for brand-new is a great deal, but after getting it home and researching, I realize a lot