I'm fixing up a yard sale bike (Specialized Crossroads). The non-stock Wiemann 700c rear rim wobbles about 1/2 inch side-to-side, in a couple of spots. Is this something I can true up myself or would this be considered a more-seriously bent rim? If it is managable by adjusting spokes, what would be the best way to go about it? Should I loosen them all first, or just work on the wobbly sections?
I'm not experienced in building wheels but have managed to keep my own pretty true over the years with minor spoke adjustments. This one does not seem out-of-round.

True it. Just focus on the out-of-round sections. Start with the worst spot, and work your way down to whatever your tolerance threshold is.

Agree. Then check the spoke tension by hand after the wheel is straight. If there are no seriously loose spokes, you should be ok.

Thanks for those.

This is a very good link for truing-
https://www.parktool.com/repair/readhowto.asp?id=81