Most, radiator shops do oem tanks (not all) also, if they can't fix it with out sealant, I would not let them do my radiator either! I have had 100's of tanks & rads rebuilt, I find that the smaller the shop, the better the work I get back. (not garage shops), But then again I do my own tanks now at home, but not radiators.
I only seal to hold the crap inside the seams in the tank, stop it from going to the carbs only! never to stop a leak. I laided down my bike when I was younger because of it, and won't use a new steel tank again with out it.
If you are using a oem car tank, it should be soldered, not welded, it's alot easier do.
I fill them with water and let them sit, then look for wet spots or drips.

Pat