The Stovebolt Site Forums The Shop Area Paint and Body Shop Help sanding to bare metal

Hello everyone,
I was trying to spot sand some rust spots out of my 1950 cab roof and it has now become almost the entire roof. I sanded all the way to 36 grit but as you seen in the pictures this is what I am left with. My question is that black stuff I am seeing is it rust? I cant seem to get it to sand out. You can see the scratches from the sanding but I am not sure if this is just pitting in the original metal and it would be fine to leave or not. Any advice on how to remove or is it ok to leave?
Thank you!

My suggestion is this...Dupont or some paint supplier may have some type of metal perp which is a mild acid to kill rust...and/or put a thin coat of body filler over it. There are now new primers that are much better than original lacquer primer and you could check with a good auto paint supply and ask for advice. Some etching/activator primer might be a good choice.

Yeah, you've got rust pits in the roof. Not uncommon. You can try using a wire wheel if the pits aren't too deep. Phosphoric acid (aka Ospho) available at your local hardware store will convert the rust to iron phosphate, but most epoxy primers (which is what you should use) aren't compatible with phosphate treated metal. Check with your primer supplier. Evaporust makes a gel product which is different than Ospho, and will remove the rust, however, I'm not sure of the compatibility with epoxy primers. Again, check with the paint supplier. The REAL way to get rid of rust in pits is to sandblast, then immediately prime after a grease and wax remover wipe-down.

thx

I'll typically sand with a 40 or 60 grit and work my way up to a 180 grit. If, while sanding, I still see orange dust being made, I keep sanding. If I don't, I chalk the dark area to stain and move on. I haven't had any paint work bubble to date. And some of my epoxy primer jobs are going on 11 years!

I would go to town with a red Scotchbright pad by hand on those areas. It gets into recesses better than sand paper and you won't risk making the metal too thin. If you do use Ospho, make certain then you diligently scrub off the area to remove any excess dried on Ospho and the white powdery residue left behind. This powder is a byproduct of the chemical process. I use a red Scotchbright pad moistened with Ospho to do this cleanup. Again, the Scotchbright pad gets into the depression nicely.
This additional application of Ospho used for cleanup will not create more powder since there is to iron oxide present to make a reaction.
Clean up with your favorite degreaser before applying epoxy primer. I use acetone. Wipe it on with a wet lintless cloth and immediately wipe it with a dry lintless cloth. Do this until the dry cloth remains clean.

thx again..will try the scotchbright followed by the ospho

If you do end up sealing the bare metal with epoxy primer, just make sure to do a thorough clean up of the metal first.
Failure to do that will ruin any kind of primer.

I'm also an Ospho user. I scrub with red pad and after it drys hit it with 120 on the sander before epoxy. But I use an Epoxy that is intended to be used over acid based treatments. Some, such as Southern Poly do not allow that. Read the spec sheets to be sure of yours. I chose Kirker 'cause it works for me and is reasonably priced.

I was not aware that some Epoxy primers were acceptable for use over Ospho. I have some Kirker Epoxy here. Are you saying that it OK to use it on top of ospho. If so, that would simplify another little project that I'm working on.

Interestingly enough someone decided to do a "test" of Kirkers claim on the adhesion of their primer over ospho treated metal. Here's a 2-part video (not mine) that shows the results.

Ospho and Kirker Primer Test Video - Part 1

Ospho and Kirker Primer Test Video - Part 2

Appears that the adhesion claim is good. The testing methods aren't what I would call being done in the safest manner , but the results of no adhesion loss kinda gets the point across.

Thanks for sharing the Kirkers product knowledge on this topic KENC!

Dan

That info helped me a lot. I currently have the vertical pieces of my grille treated with ospho. These would be almost impossible to sand or even scuff completely. So I'm curious for an opinion. Do you think that using a stiff wire wheel on them would serve to accomplish the same thing, i.e. do an adequate job?

Mike,

Just MHO, but the video guy's 4 different tests showed no real difference in the adhesion on the Kirker's primer. It seems scuffing may not really be necessary.

I do think it may be fine to use a gently applied wire wheel to do the "scuff" work, if you have concerns about getting the best adhesion.

The "gentle" part may be the most important factor though from both a safety aspect and not doing any damage to your pieces.

As with any scuffing action, don't forget the follow on cleanup to remove any loose particles before applying your primer.

Dan