How to prevent the fender stays from rotating?

On my XL chassis, I've noticed that the fender stays want to rotate backwards, and if I let them do that, they will eventually contact the brake lines.  It seems that no matter how many times I loosen the bolts, then rotate the fender stay forwards, and tighten the bolts to 37 lb ft, the fender stays still rotate backwards on their own.  Since the bolts are 3/8" and mate into the 1" thick aluminum spindle, I don't really want to over tighten the bolts and risk damaging the aluminum threads.

I'm wondering if the powdercoat on both the spindle & fender stay creates a mating surface that is too slippery to keep the fender stays in place.  Perhaps I should use some sort of toothed lockwasher, and/or grind off the powdercoat where the two parts meet?  Or, maybe use the grinder to make some tiny "teeth" on the fender stay so that it digs into the aluminum spindle when its tightened?

Thoughts?

It looks like I solved this by just adding a regular steel washer between the fender stay & the spindle, on both attachment points.

Sorry I just saw this. Yeah I have noticed that powder to powder surfaces are quite slick.

star washer might be better.... but sounds like u got it anyway.

Well, I tried some outside tooth lock washers, which helped a little bit, but still not quite good enough for me.

Then, I started to wonder if part of the issue is caused by the gap between the 1/2" ID of the steel tube and 3/8" OD of the bolt.  At the local HW store, I found some 1/2" OD x 3/8" ID x 1" long nylon sleeves; I bought 4 of 'em and got 'em installed now.  Will report back on how it goes.

I think the best way to address it would be to weld a short tube between the two mounting points and use a steel sleeve inside the mounting points.  That would eliminate both the bending & the rotation.