Remove rust or convert it?

[cancow]

I have been reading about rust prep on various forums and it seems there are 2 schools about rust (I am talking about large areas of surface rust). Does one sand down the rust to bare metal then prime and paint? Or does one lightly sand the loose rust, spray on a converter or use an acid, and then prime and paint?

[70 ranger]

when it comes to rust the best thing to do is remove it. If that is to hard due to the location,pitting,cost or whatever the reason is, using a neutralizer is the next best method.

[mrollings53]

You can always do both. Sand the rust off until you reach clean metal, then apply a chemical conversion coating. That way any microscopic rust or new rust will be converted and the rest of the metal will be treated to prevent further rust. Then prime and paint like normal.

[cancow]

Do you have to use an etching primer over the converted rust or is an epoxy primer the way to go?

You can always do both. Sand the rust off until you reach clean metal, then apply a chemical conversion coating. That way any microscopic rust or new rust will be converted and the rest of the metal will be treated to prevent further rust. Then prime and paint like normal.

[mrollings53]

An epoxy primer should be fine if you have converted the rust or used a chemical conversion coating. Other wise you will need a self etching primer on bare metal.

[Canadian Ford Eh]

I usually use a DA sander to bring down surface rust, and that is enough to get something like the door on this down to bare metal:



So it looks something like this:



I know its a Chebby, but be nice... I'm allowed to make one mistake, right?

If its underneath (bottom of box,cab or inner fenders or frame) save yourself the time, lightly brush off the rust and use POR 15. That stuff is tough as nails, looks great underneath and LOVES rust.

if theres a lot of pitting, you can use a wire brush on a high-speed air grinder, or get it blasted

[cancow]

Nice sanding job! So, what are you going to use over that bare metal? This is the quandry I keep coming across. You take off all the rust to bare metal, but there still might be pits of rust you dont see so wouldn't it be a good idea to use some rust converter or something like vinegar anyway? But if you use a spray on rust converter you no longer have bare metal so you go with an epoxy primer? Seems etching primer only would be used if you have parts that have never had rust, like a new bare metal part or something rustfree that you stripped good paint off of. Sorry about all the questions, but I find so much contradictory info on painting rusted body parts.

[Canadian Ford Eh]

I use the DA to this point, then wire brush where there are pits. Then a thorough wipe down with wax& grease remover and 2-3 coats of epoxy. If you are using epoxy (at least, the stuff I am using) you don't need self-etch underneath. I dont know if that is a universal quality of epoxy or just this particular brand. As for how to tell if all the rust is gone, yes you can do a phosphoric acid wash, but I've found that leaves a surface coating that needs to be sanded off again.

If it is large surface rust like the door in my pic above, the DA will take it down to bare metal no problem, and the rust should be all gone. Particularly when using epoxy, a tiny bit of rust (like, what you can't see) won't develope enough to bubble or do any damage -- the epoxy is tough stuff. But be sure to wire brush in each pit, in 2 directions (up-down and left-right) to get rid of rust in pits.

Also - dont sand through your panels

[cancow]

So when you see light coming through you should stop sanding?

[Canadian Ford Eh]

That's the general rule of thumb --> then check 2 things: 1. has the metal rusted through and 2. are you using 36-grit (DON'T). Remember you are not trying to sand out the pitting. You can use epoxy over the clean metal, then use high build 'primer' (which is pretty much liquid plastic) to fill small pits. Enough coats of high build can do wonderful things!

[70_F100]

As for how to tell if all the rust is gone, yes you can do a phosphoric acid wash, but I've found that leaves a surface coating that needs to be sanded off again.

I've always been told that the phosphoric coating should be left on the metal.

I painted a lot of cars/trucks/equipment in a previous life, used the acid wash, and the paint still looks good on them. In fact, Sherwin-Williams used to make a two-part metal prep, and the phosphor coating was actually applied as the second step.

Maybe I'm wrong, as I have been many times...

[robroy]

Good morning,

I think this topic's really interesting, and I'd love to do an experiment some time to see which works better (removal, or chemical treating, or both). The only trouble is that the experiment would probably take years to do well, and would need to be done carefully for the results to be worth much.

Like 70_F100's saying, I've also allowed the discolored finish left by phosphoric acid to remain and painted over it. It has been a while now, but I believe that I read on a bottle of this acid solution that I was supposed to let the discolored finish remain.

In 1972 F250 12"x2.5" Bendix rear drum brake rebuild: advice?, there are photos of a Dana 60 cover that I did in this way.

Robroy

[Canadian Ford Eh]

You're probably right about that, when I did it I was just working off info from a friend
I still prefer DA sander, maybe its just because I like the shiny metal...