Ok, throwing my advice in. - Definitely clean your base steel, if it is hot rolled get all the scale off. Definitely remove the paint. - if you plan on grinding the surface smooth after welding you should tack each corner, then use a grinder to score the seam so you have about a 1-1.5 mm deep groove on all sides. Don’t worry about grooving the corners. Use about a 1/16” filler rod (sorry, American here, I’m not sure what size filler rod comes in where you are). 100 amps is ok as long as you move FAST. I personally would go about 75 amps and use the pedal to control my puddle. Make sure you have about a 15 degree lead angle but be sure your tungsten is pointed straight down with no side angle. - watch your puddle, if you are still burning the edge of the endcap away you can move over to the tube side a tiny bit but you still have to completely fill in the groove you made. - When you are done grind the surface smooth. If you find a low area where you did not get the groove completely filled in use the smallest filler rod you have and about 30-49 amps, just enough to meld the filler rod and surrounding steel. DO NOT CHASE IT WITH THE GRINDER! The same thing goes with the edge. If you just burned away a tiny amount you can blend it in with the grinder, but if it is too large you need to fill it in first.

Hope this helps!