Any metallurgists? advice needed

Have taken my window and lamp guards off to have them shotblasted and re-coated. At all the fastening points, the paintwork has 'bubbled' and the aluminium body panels are pitted and bubbling. The guards were fixed with stainless screws or rivets.Do these react with the aluminium? and what should I do to this surface area before refixing the guards? Any help/advice would be appreciated.
Tony

You will always have a reaction when 2 different materials are put together.

You may be getting Galvanic reaction. Its basically where both metals oxidise. Its these oxides that react. It worsens when wet and even more if salt is present. A great thing to have in our climate. Oh and if there is electrical current in the area it can worsen.

If memory serves, its an electrical process that occurs with the lowest corrosive material coming off worst (ali). Thats probably why you have bubbling. The aluminium is being eaten away. How old is your truck? With small screws and large ali area it takes time to react.

I may be wrong as its been a while since i done this (about 12 years ago).

I believe insulating joints is best. Prob best to rub down and paint up. Maybe even grease up the threads or holes before putting SS screws back in.

Hope this helps.

As I said, been a while since I done this. Maybe someone more knowledgeable will be along shortly to update and advise more.

Ta, Will certainly rub down and repaint affected areas, before re-fixing. Vehicle is 7 years old and guards have been fitted from new- (they allow me to keep side windows open so dogs can get fresh air,) - but have recently become very rusty. Also live on a boat on a tidal river so plenty of salt about !!. Would it be worth putting fibre/nylon washers between guard fixing points and bodywork or will the fact that ss screws will still make contact negate such action?

As you live in a salty damp area i would very much use washers. Any form will suffice. Cutting some washers from a plastic bottle would suffice. The bigger the aluminium surface area and the smaller the SS area reduces the corrosive effect, that is of course if i am right.

But it is always worth isolating 2 surfaces of different materials. Nylon would be ideal.

Also make sure you rub down the ally then treat with a strong acid etch primer before repainting.

Insulation and isolation is needed. Ali and stainless are dissimilar enough for the ali to become a sacrificial anode.

Problem is, with self tapers, for example, you just won't be able to isolate the two. Can you change to a nut and bolt? Slightly oversize hole and sleeve with a nylon/rubber sleeve/bush?