What Rivnuts For Arches?

I know this has been discussed before, but I just wanted to clarify some points....

I've removed the large arches from when the car had 13" wheels, and I'm going to fit the Mini Special arches.

I'm going to use M4 rivnuts as recommended by several people, but what material rivnut would you guys recommend? Steel, Stainless Steel or Aluminium? Flat head or Countersunk?

Is there a preferred fitting technique? I was going to mark out and drill the holes, then paint the bare metal. When the paint is dry, I was going to put a grease or sealant on the rivnut before it is fitted to try and keep out the moisture!

Thanks for your help!

I used stainless steel on mine, as my fixings don’t show on the arches I fitted (Zeemax) I just used stainless bolts but if yours are going to show I would use flat head unless the wholes on the arches you are using are counter sunk then obviously the countersunk would look better.

I would opt for zinc plated steel to avoid any dissimilar metals. Then also a smear of tiger seal or other seam sealer to the rivnut to seal it up. I expect only flat heads will work, and hopefully a low profile style as not to interfere with the edge of the arches. I plan to do this when the time comes.

I used aluminium rivnuts and stainless m4 screws,Not good as 10 years later none of them would undo even though I assembled them with Coppergrease.This time I have used plated steel nuts and stainless screws.Life permitting another 10 years will show if successful.Steve..

I wouldnt mix aluminium and stainless unless I wouldnt be concerned.

I've used zinc played steel rivnuts and stainless steel dome allen head bolts one mine for years in all weather, and they're still just as fresh as the day I fitted them. 

Mild steel in contact with stainless steel in a damp atmosphere will corrode.

Thanks all, I'm going to go with steel plated rivnuts!