Whats the best way to take a panel thats half original paint, half surface rust (desert car), and prep it for paint/ primer? When the surface rust is sanded off it still looks like theres very minor pits with rust still in them. What do I coat that with to be sure its gone and does't come back as a bubble later? Can you primer on top of an over the counter aerosol rust converter spray? I'm wanting to do my own body work and want make sure I take the right steps. Also do I sand off the factory primer on the non surface rusted part of the panel?

The primer and finish coat manufacturer that you are using usually has recommended surface conditioner preference. Check with your coatings distributor to see what is recommended.

I've seen some rust converting products that actually leave a significant residue on parts. If you can get a hold of some Ospho, that stuff converts the rust and does not add any additional coating and was compatible with a primer I used, but that was decades ago.

These days, I sand everything down with 80 grit, straighten the panel as much as I can and then seal it with catalyzed high build primer in multiple thin coats allowing plenty of dry time between them. Thick coats shrink too much, and once-invisible sand scratches will show up a few weeks/months later.

Cool, thanks for the reply. I'll give that a try

Here is an example of the type of panel I'm talking about. I got the truck pulled out and ready to start on it now.

Dipped or blasted otherwise you will never get the rust out.

And if you blast it ... you will still have pits of rust that were too small for the blast media. Blasting is NOT a solution to rust unless you're refurbing a bridge. Get rid of the majority however you choose BUT the surface absolutely has to have a final treatment with a liquid that will either remove or convert the minute rust pits.

I would wire wheel then hit it with a rust converter, then rewash, dry and epoxy primer. I forget the name of the rust converter I use but like most of them it's phosphoric acid based.

i used OSPHO on something similar 5 yrs later its still holdin up.but i did lightly blast it first.

you seem to know lots about blasting sheet metal. whats your experience in blasting cars other than what you read on the internet?

If you blast those panels with silica sand or Black Magic, or aluminum oxide, you will warp the hell out of the sheetmetal from the heat. You have to be VERY careful when blasting stuff like that to not create too much heat.

Old school stuff in the junkyards here look like that. Sun just bakes it right off. Usually the wind blows after it rains so the rust they do get is light even surface rust and not pitting. Paint shops here would sand that with 80 grit DA and then maybe clean it with PPG DX metal prep or similar before primer coat.

No dipping tanks here that I know of.

A few sandblast shops, but they do steel stairs and such for a living, and not old cars. You would need to make sure they can handle it first.

that fender is super easy to blast. moparts is comprised by a ton of guys who just hang out behind the keyboard expressing opinions that they read from other guys who read the same thing on a different forum.

i stand 100% behind everything i blast. If i ever warped anything i would replace it or pay to have it fixed. In 10 yrs i have had one hood damaged and that was my first month and being lazy/inexperienced. I regularly blast $100,000+ cars with no damage, i know i am not like 99% of the blasters but "warp" and blasting have gone together because any idiot with $200 can buy a crappy pot and some 30 grit garbage to blast with and then go into business. I get customers from 500 miles away. those guys probably pass 25 blasters to see me. wonder why?

i try to defend the blasting world that all blasters are not the same but hard to get through to people. see samples of their work and ask questions.
grit, pressure and technique are huge.

I just went through this with my Power Wagon. The whole top was baked right off but it worth it to buy a desert truck. I DA'd with 80 until it was shiny metal. Then treated with OSPHO and let it sit for a week, cleaned it all off with more OSPHO and steel wool wet and wiped dry. I DA'd one more time with 80 and two coats of SPI epoxy. It came out well. It may look like shiny steel but there will be pits that need to be treated.

I did the exact same thing as T2R9 with awesome results 5 years later on my 92 w250. Sanded the big stuff off. ospho overnight. Wash with wet cloth. Dry. Sand with finer grit. 2nd coat of ospho. Wash. Sand with primer prep grit (320?) The epoxy primer. Then whatever, in my case basecoat.

Lots of surface prep. it sucks but looks good 5 years later durability wise. Gun was off so i got orange peel
http://ramchargercentral.com/mopar-trucks/1999-w250-project/

Agreed, hacks, junk equipment and crap media, or the wrong media gives blasting a bad rep, I've been blasting auto bodies/panels for decades, I blast/bead/shot just about every automotive component with ZERO issues, often times I'll use glass bead or Black Magic/Skat to remove a stubborn "oil canning" in a panel, or purposely crown a fabricated panel rather than hit the English wheel...

For the Op's issue I'd hit that desert metal with Black Magic/Skat in the 80 grit range (although I generally prefer crushed glass in 40 grit range) to get into the pits and put an aggressive tooth to the metal, then follow up with 2 coats of Dupont Vari-Primer etch primer, or PPG Epoxy primer/sealer if that's preferred, then a heavy build 2K primer

When blasting sheetmetal I blast the panel at a 45 or greater degree angle changing directions of blasting as well, straight on is a recipe for creating warping/oil canning, and move around a lot, focusing in one area too long generates too much heat and raises the sheetmetal surface

I used Metal Prep and Scotch-Brite. Some of the deep pits I used a pic. Even if there is any small amount of rust left it will be converted and inert. 5 years no issues.
Doug

Thank you for all of the replies, I really appreciate it! I'm looking forward to getting this truck one color again.

Its actually my wifes truck and it has been in her family since new. Her mom always wanted it yellow (so she could find it in a parking lot), so we agreed on a yellow Macho paint scheme in her honor.

A few thoughts.

Rust is a chemical reaction, and can be chemically neutralized. While the chemicals won't replace the missing metal, they can neutralize existing rust and prevent it from re-occurring.

I kind of see two schools of thought here. One of them involves aggressive mechanical grinding/sanding methods to remove both the surface and pitting. This thins the whole panel down to the depth of the pitting. The other method involves sanding the surface rust, and using blasting, chemicals, or both to remove the rust pits. Personally, I prefer the second method - I never understood thinning the whole panel which IMO reduces the integrity and strength of the panel. So I prefer to lightly sand the surface rust away, media blast the pitting if necessary, and chemically treat the area to neutralize existing rust and prevent further corrosion.

I wonder how many times the reason that rust that comes back is mis-diagnosed as an improper repair, and not seen as "new" rust. Reason I say this is that many times I see rust repair or bodywork done, then the car sits in primer for weeks or even months before topcoat. People don't realize that many primers are porous and allow moisture to contaminate the metal under the primer. Many primers are to level imperfections and promote topcoat adhesion, but are not designed or intended for corrosion protection.

maybe use something like this? looks like it uses a pressure washer with lots of sand

https://youtu.be/K8vyeMwRm0o

I use Ospho and it works great. You still have to manually sand, its not a miracle product. Once you get it as clean as you can by sanding I spray on Ospho and scrub with it. Wipe it off before it drys. I had my brothers challenger completely bare metal and treated with Ospho. It stayed 100% rust free in my garage for over 5 years. It will rust if the oil from your hand gets on it or water.
I usually sand the panel right before primering but supposedly you can paint right over it. I would rather not take the chance.