Hi Ex-biker
IANE but FWIW, here's my take on this.
Chassis strength through panelling is only increased if the bond between panel and chassis is effectively continuous. So right off the top of my head, here goes...
- Rivnuts: need relatively large hole in chassis and may weaken it if placed close enough together to be effective in fastening the panel. Since rivnuts stand proud of the chassis member, the panel to chassis gap is large and will require a considerable thickness of adhesive/sealant which may or may not be OK. Never used a panel fixing adhesive on a car.
Pop Rivets: cheap and easy to get a close fit if holes deburred. A smear of sealant "under" the head prior to fixing plus a little dab in the hole ought to do the trick. IME they are cheap enough to use close enough to make a proper join but can work loose with vibration and other movement..
Self Tapper: As good as pop rivets if holes deburred. You already have steel chassis and aluminium panels. Are you proposing to use stainless self tappers or zinc plate or plain steel? There might be complications due to potential electrolytic corrosion but I don't know enough about this except that the fewer dissimilar metals in contact, the better. Suspect that the aluminium panel may be vulnerable especially in contact with stainless.
Panel Bond: I don't think this is a good idea if you were proposing to use this on its own with no mechanical fixing. The strength is then dependent upon the bond of the powder coating to the chassis as well as sealant to aluminium and sealant to powder coating.
I think I'd go for the pop rivet approach with the panel adhesive as a sealant.
If I remember correctly member
vojx has a background in finishes and might be able to shed more light on the adhesion of powder coating and panel bond.
Happy new year to all
Peter