I have welded up countless numbers of motorcycle fuel tanks and never have a problem with them, you just have to ensure you have cleaned them properly with liquid soap and flush at least 3 times and they are not an issue. I have found over the years that I have to cut the rusted section out, make up a repair patch, bronze that in and then a wipe of silver solder over the top because I have no idea why but I will always end up with a pinhole in the bronze no matter how smooth it flows on. Then put petrol in it and blow in the filler cap to pressurize the tank to check for any weeps. If I end up with a weep it is go through the whole flushing/cleaning process again.
