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Anything I've done that I can't paint or primer right away, I've used Eastwood's Fast Etch (used to have a different name, but this is their improved stuff). http://www.eastwood.com/eastwood-s-fast-etch.html It leaves a zinc phosphate coating that won't rust. All you have to do is spray it on when you're done grinding/sanding. When it comes time to paint, you just prep like normal (thinner, pre-paint prep recommended by your paint manufacturer, etc).