I am grinding rust spots off a very old car roof that the previous owner never took care of. This rust has been sitting for many years..
I am grinding and grinding with a 36 grit disc on an angle grinder, and just about every angle grinder attachment you can think of.
I CANNOT GET RID OF THESE LITTLE PITS!!!!

My question is, is this surface considered "bare metal" enough for body fill or epoxy sealer? I suppose the car won't live forever but I am intending to not only get some good practice, but keep the car as long as I can! I literally can hold a 10k rpm 28 grit disc on these spots until the thing catches fire and they don't budge.

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Thanks!

Media blast that metal and those spots will disappear.
Grinding can remove too much metal and then cause oil canning and other issues.

And after you get it clean you can treat it with phosphoric acid. It just makes it a little more rust resistant.

Blasting is not an option for me. Don't have a booth, nor a place to take the car.. they want insane amounts of money to blast the roof. More than the car is worth in some cases lol.

What is oil canning?

I am not sure how blasting would remove the spots if taking .5mm off with a grinder won't!

Only blaster I could actually use would be one of those cheaper air compressor gun style... but I didn't think those would have the power.

So I take it the surface is no good for epoxy then?

glytch5 wrote:What is oil canning?

Google is your friend.

glytch5 wrote:Only blaster I could actually use would be one of those cheaper air compressor gun style... but I didn't think those would have the power.

what are the specs on your compressor?

I stripped an entire engine compartment once with a $20 Harbor Freight Spot Blaster and a couple 50 pounds bags of Play Sand from Lowe's ($2 each and make sure it's dry and wear a dust mask so you do not breath the dust). The compressor was a tiny Craftsman 3 HP with a 20 gallon tank set at 90 PSI (set the PSI as low as you can that will still get the job done), all done in a day in my back yard. If you have to do it inside just take some old bed sheets and make a temporary barrier to contain the sand and have a shopvac handy. That's all you need to clean out those pits.

Oil canning is warping of the metal.

Bill911 wrote:And after you get it clean you can treat it with phosphoric acid. It just makes it a little more rust resistant.

While phosphoric acid does work to convert rust, it has to be properly neutralized before painting over it. If not you run the risk of paint failure due to adhesion problems. I found this out the hard way and had to strip and repaint the roof on my signature car.

glytch5 wrote:Blasting is not an option for me. Don't have a booth, nor a place to take the car.. they want insane amounts of money to blast the roof. More than the car is worth in some cases lol.

What is oil canning?

I am not sure how blasting would remove the spots if taking .5mm off with a grinder won't!

Only blaster I could actually use would be one of those cheaper air compressor gun style... but I didn't think those would have the power.

So I take it the surface is no good for epoxy then?

Media blasting when done correctly will remove the rust from the pits without removing the good metal surrounding the pits. Grinding removes all the metal down to the level of the pits - not a good idea if the rust has deeply pitted the metal.

Oil canning is when the metal flexes. They call it oil canning due to the old style oil cans that you pushed the bottom in to get the oil out and it would pop back out when you released pressure on it.

When asking for advice on this site, we only want to tell you how to do it correctly. Too many substandard methods out there today being pushed.

The bottom line is that it's your car so you can do whatever you want. However, we will not condone bad practices or methods.

Compressor is just a 30 gallon husky. Basically the biggest most powerful motored 120v compressor I could buy, with a full air drying system with copper plumbing. I do a little bit of spraying as well.

From what I have read on here for answers, the hardbor freight gun can't hurt considering how cheap it is, and I already have many grits of SiC powder from also being into knife sharpening, I use it for lapping natural stones.

Thank you all so much for the responses, I am going to try the gun and get back to you!