Paint and wire brush. I have two new rear arms on my 2015 and VW were confused as its not normal with that age of vehicle. The price is not cheap at £640+VAT (parts only) each and typical VW. I've literally just thrown the old ones away. Very heavy items, so surface rust would be of no concern.
I changed because I had a siezed cross threaded bolt which I couldn't remove. This was installed by another mechanic following suspension work. As it was small and I've removed bigger, older, more rusted bolts in the past, I wasn't concerned. However, I tried everything and could not remove the bolt, I was just about to give up., drill and helicoil when on the final throw of the dice I snapped an extractor in there. Now impossible to re-drill out.
I also wasn't confident that I wouldn't encounter a similar issue on the other side and or other bolts as the previous mechanic had removed the arms (I had a issue on the front with his work which I fixed). Hence I lost confidence and had then replaced - cost me £2700 ish with new rear bearing and labour.
Found out that the bushes were also installed incorrectly when they were removed.