Tank building question

i dont have experience with metalwork, so i cant answer that question. i will say this. tank sealer is a good idea. it will prevent rust, and if you do have pinholes it will cover those just fine. you will read bad things about any product out there. i use KREEM. i have a tank that i sealed with it two years ago, and it hasnt flaked yet. it works if you follow the directions. go to a motorcycle shop that sells kreem, read the directions, and decide if you can follow them. if you cant, dont buy it. for my stock tanks, i use about half a pint to seal one tank. so a $20 bottle will do two tanks, and you can store it until you need to use it again.

I'm with Big I used Kreem on my tank to seal a pinhole in the mount.

on the copper tanks i was soldering together, i was using kreem, and it worked well.

now, my skills are a lot better, and the last few tanks i've made aren't sealed, and haven't leaked after a lot of miles.

as far as liners preventing rust, that's basically only true if the tank's gonna sit unused for awhile. a tank with fuel in it, especially an oily two stroke mix won't rust.

you should definitely clean the excess flux and junk out of the tank.

i'd do the whole tank, then check for leaks. you could always braze from the outside and grind off the excess if you find a leak.

When you are finished welding do an air pressure test. Close the fuel petcock and get at a plumbing supply store an air test plug. It is a rubber plug with a bicycle schrader valve fitting on it. Put the air test plug in the gas cap hole. Pump a small amount of air into it and submerge in a tub of water and look for the air bubbles. It it's air tight it will be fuel tight. I would recommend an epoxy based tank sealant. It's insurance as vibration may down the line open a pinhole that was previously clogged with a piece of flux and you will need to drain tank of fuel, take tank off bike, find and fix leak, repaint, and install again. No fun. Tap Plastics has a epoxy based resin and hardener that is very economical and is very chemical resistant that I used. I used this. Caswell sealant leaked twice on me. The Tap Plastic Resin and Hardener did it's job well. They have a chart that shows what resin and what hardener works best for each application. Remember, the longer the cure time of epoxy chemical resistant resin the more chemical resistant it will be.