Hello,
no idea what I’m doing so thought I had better run this past the STW hive mind before I start.
Is it a good idea to scrape/wire brush/otherwise remove the rust from underneath my van and then brush with the primer in the title? If so, would anyone like to recomend one?
I assume 2 or 3 coats will be in order. Then what? Paint (if so just normal car paint?) and underseal or underseal straight onto the primer? Is brushing better than spraying? Or brush most of it but spray inside the hard to reach bits? Anything else you think I should know?
cheers guys.

old engine oil mixed with wax oil

red oxide primer these days is pretty much just red paint.

Is the new lead-free stuff not as good then?
As it happens I’m due an oil change in about 1,000m so will have some old oil going spare.

I always thought Zinc Oxide paint was a good anti-rust primer…

I’ll start with what I know and guess from there.

Yes to your first bit. Surface preparation is key to a coatings effectiveness. Many coatings will say they can be painted over rust but they will still work better if the worst is removed. What you are looking for is as stable a surface as you can manage so no dust, dirt, grease etc. So scrape and wire brush then use a softer brush to remove the dirt.

I don’t like the term “red oxide primer”. The red oxide part is just iron oxide and is used because it is cheap rather than it has any magic properties. Red paint hides new rust and staining. If I remember correctly the anti-corrosive pigment will be zinc phosphate.

The manufacture should provide recommended thicknesses and number of coats. Too much is as bad as too little in many cases as it creates stress when drying and can cause the whole lot to de-laminate or fail in other ways.. I guess that consumer targeted paints will be quite high VOC and tolerant to over application as I guess you don’t have either a wet film gauge or dry film gauge to hand.

You’ll need something to seal it. Not sure what would be recommended. But if it is on the underside of your car I’d suggest something pretty tough as it will need to cope with spray, stones and salt.

Brush is going to be as good as spraying. Spraying a small area would be a pain in my opinion.

you could try POR 15 from Frost.

tbh when I was prepping the OHs car for the mot (been laid up for 3 years) I attacked the underside with a wire brush on a drill and just mixed up some underseal with some white spirit and gave it a good coat with a paintbrush. Not sure how long it will last but it looks a lot better.