I don't quite get what you mean - do you want to round off the welds or something? What needs to be flat?

I can however just chuck some ideas out there!

Smaller air grinder
Rat's tail file
One of those DIY belt sanders (sort of a pointy nosed belt sander designed to get into corners)
Putting in a hydraulic spreading device to physically make something larger...

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!