What to do about this rust. I think

[78-911SC]

As I'm stripping the paint off the car I noticed some bubbles on the surface of the paint. When I got down to bare metal I noticed black spots in the metal. Took Ospho and put it over the entire fender and let it set an hour the first time and it didn't do anything to the black spots so I did it again and left it on over night. Still no success. My question is do I need to remove the black spots from the metal and what do I use if the answer is yes.
Grind it off? Some look a little deep.

[962porsche]

personally I do not believe in using a product like Ospho and using it on outer surfaces .
in fact I don't really believe in using products like this much at all .
most products like this will put a clause in some where that will state that the finish could peel from it's use .
I would 1st read the products TDS and other info to see if it states that if so then remove every bit of it and then media blast the spots .
grinding or sanding will not remove all of it so forget that idea as just one little pin size rust spot will come back on you .
if the area is not to large then you could go buy a little hand held spot blaster the cost for one is like 35 dollars . then for about 10 bucks you can get a 100LB bag of #000 fine sand from your area masonry store .
by blasting it you will be removing it and not trying to cover it with hopes that it will not came back and screw up all your hard work .

[78-911SC]

962 I was hoping you would chime in on this. I will resand the entire area again to remove all ospho. I have a spot blaster that I have never used so I'll see if Home Depot or Lowes have the #000 fine sand. If not I'll locate a masonry store. I'm assuming that if I still see any part of the black spots remaining that it's still there and I need to keep blasting. I took a picture of the label and it seems that you can apply primer after using but as you say it's not worth the chance. It's been a ***** to get it to this point and I don't want to waste all this effort. Thanks again there will probably be more questions as I progress on the project.

[962porsche]

I just went to ospho.com site under directions .

there was a clause at the very end .
Dulling or peeling may occur in some finishes. the manufacturer assumes no liability for damage to finished surfaces .
most all products like this do have that clause or one stating some thing like it .

don't get me wrong products like this do have there uses .
we have a old 1940 hot rod coupe in the shop some one boxes the frame back in the 1980's .
when we are finished with it we will spray a coating of a product like this down the inner rails .
there is some rust in the inner rails that we could not remove from when we blasted it .
we use a old schutz gun that has a 6 FT wand on it . we slide the end all the way down the inner rails and coat the in sides of them .
we will also do the same things when needed to the inner rockers of cars that have rust but are not getting replaced .

[sturmundrang]

962porsche
What exactly do you spray inside those unreachable areas? I'm restoring a 914 and the longitudenals are in pretty good shape, but I can see surface rust up inside. Do you use phosphoric acid?

[962porsche]

we don't really have one exact product we use .
because all the jobs and areas we have to get to are different there are a number of products and ways we go about getting into the hard to reach areas .

this 1940 coupe we are doing the work to is getting a Heidts IRS rear so some of the back half has been removed and is getting a new back half .
the chassis is a numbers match to the body so we are only doing the back half because of this . if it was not a numbers matching car we would just build another chassis .
so the front half has rust down the rails when the chassis was boxed years ago no one put any thing down the inner rails and it's got a good covering of rust .
so for the front half we plastic media blasted the chassis got in the rails as best as we could after it was cut .
we then run chimney cleaning brushes down the rails to get any lose rust off .
stood the open ends of the rails face down and got all the sand and rust out .it was a big pile of crap that came out .
we will coat the inner front parts of the rails with a converter I buy from theruststore.com .
we also use SEM rust-mort for things like this but because some of the rust is a little heavy I'm going with a product that is more aggressive .
converters will only work over rust they tend not to stack to clean metals .
after we coat the inner rails with the converter we will coat it with SEM's rust-shield along with the new and blasted metal . as SEM rust-shield is a all in one product (primer and top coat) we will pick a color from the SEM color deck (posted) that is close to the out side color of the chassis (Rot) .

if you have just some little rust spots down the inner rockers with the rest of the metal still in good shape and maybe even areas where there are still some primer left on them then doing it how we are doing the 40 coupe is not the best way .
for some thing like this you would only have to coat the inner rockers with some thing like rust shield .
we use rust-shield all the time for things like this . we do this so often we also have the SEM rust-shield mixing system .
things like when you replace the outer rocker and the inner or center rocker is rusted this way we will spot blast with a hand held blaster the areas we can get to and then coat every thing with the rust shield (new and old metal) after we weld every thing together .


the guns we use for stuff like this are the real cheap ones (photo) .
we make up our own tubes to get to where we need to . you want to use a more ridged type tubing so it just does not bend up on you as your pushing down the rails .
don't try a sprayable converter with this set up or reduce the SEM rust-shield . thicker is better !
when ever you coat the inner rockers always poke a piece of wire back up thru the drain holes to clean them out .

[sturmundrang]

962, it is a real pleasure hearing from someone who knows what they are talking about. Your information is greatly appreciated.