Double banger - Duragal removal and breathing mask for welding

I'm a hobby welder. Most of my welding is Duragal RHS, I want to remove the duragal coating in the area to be welded because of weldability (is that a word?) and toxic fume issues.

I know that I can flap wheel or grind the outside of the RHS to remove the duragal coating, but the duragal coating remains on the inside and will cause toxic fumes. I know that duragal vendors state 'duragal CAN be welded without removing the coating'. But health-wise, SHOULD it be welded without removing the coating?

I've tried soaking the RHS stubs in white vinegar, I couldn't tell if it helped or not. Any thoughts on other methods to remove the duragal coating?

What breathing mask (to wear under welding hood/mask) is recommended? I sometimes use a fan, but still concerned with breathing welding fumes from both duragal and non-duragal material. Obviously would need to fit under welding mask and be effective for welding.

Thanks for the help.