Restoration techniques questions

[Gary S.]

I was discussing restoration techniques with a Corvette buddy and we disagreed on a big point. When restoring cast iron items like differentials and such, does shot peening, that is done to clean the surface, affect its appearance or original look? I claim it does but my friend says that the original appearance is unalterered for these cast iron parts. Anyone know for certain?

Second question. The 70-72 Judging Manual mentions that the strut rods were either natural or black, the black possibly being part of the undercarriage blackout process. Wouldn't that indicate that the strut rods were only painted black on the lower and rear portion and not the front and top sections? Again, anyone know for certain how these looked?
Gary

[Joe L.]

Re: Restoration techniques questions

Gary------

Any process to restore and/or clean a cast iron surface will change its original appearance to one degree or another. Shot peening is a process that probably changes the surface coloration less than other processes, but it changes the surface texture, even if the finest gauge shot is used. For sand-cast parts (as most cast iron parts are), the effect on the surface texture is "muted" by the original rough finish of the part. The coloration is changed, too, compared to the out-of-the-mold appearance of cast iron, but it's not a drastic change. I've studied this inasmuch as a friend of mine has a large, industrial-type shot-peening machine, and I've shot-peened many parts.

Other methods for cleaning cast iron will change the surface finish and coloration to a drastic degree. Bead blasting or sand blasting are 2 that completely change the coloration and surface texture. After these processes are used, one almost has to use something like Cast Blast to restore any semblance of original appearance. But Cast Blast is not all that bad since it not only restores an original-like coloration and texture, but it PROTECTS and prevents further rusting----very important for a driven car.

Original strut rods are forged steel. They have an as-forged appearance which varies from a cast iron-like appearance to a slightly black appearance, but NOT like sway bars. I don't think that they were ever affected by chassis black-out, but they may have been.

Just about any cast part has a "skin" of one sort or another. This is not really a skin, per se, but rather the out-of-the-mold surface finish imparted to the piece as a natural part of its casting. It's VERY difficult, if not impossible, to restore this subtle finish. A VERY light mist coat of satin finish, clear urethane paint is the best thing that I've ever found. Of course, this method does not work for any part subject to high temperature.

[Joe L.]

Re: Restoration techniques questions

Gary------

Any process to restore and/or clean a cast iron surface will change its original appearance to one degree or another. Shot peening is a process that probably changes the surface coloration less than other processes, but it changes the surface texture, even if the finest gauge shot is used. For sand-cast parts (as most cast iron parts are), the effect on the surface texture is "muted" by the original rough finish of the part. The coloration is changed, too, compared to the out-of-the-mold appearance of cast iron, but it's not a drastic change. I've studied this inasmuch as a friend of mine has a large, industrial-type shot-peening machine, and I've shot-peened many parts.

Other methods for cleaning cast iron will change the surface finish and coloration to a drastic degree. Bead blasting or sand blasting are 2 that completely change the coloration and surface texture. After these processes are used, one almost has to use something like Cast Blast to restore any semblance of original appearance. But Cast Blast is not all that bad since it not only restores an original-like coloration and texture, but it PROTECTS and prevents further rusting----very important for a driven car.

Original strut rods are forged steel. They have an as-forged appearance which varies from a cast iron-like appearance to a slightly black appearance, but NOT like sway bars. I don't think that they were ever affected by chassis black-out, but they may have been.

Just about any cast part has a "skin" of one sort or another. This is not really a skin, per se, but rather the out-of-the-mold surface finish imparted to the piece as a natural part of its casting. It's VERY difficult, if not impossible, to restore this subtle finish. A VERY light mist coat of satin finish, clear urethane paint is the best thing that I've ever found. Of course, this method does not work for any part subject to high temperature.

[Gary S.]

Re: Restoration techniques questions

Good comments, Joe. Thank you. A follow up is in order though. If one were cleaning a differential or other cast iron part, how is light surface rust removed if you want to complete the process. Obviously, I know that any sort of blasting will alter the surface appearance, but what about a less invasive procedure? A light acid/water mixture wash perhaps? Scrubbing with 000 steel wool?

Gary

[Gary S.]

Re: Restoration techniques questions

Good comments, Joe. Thank you. A follow up is in order though. If one were cleaning a differential or other cast iron part, how is light surface rust removed if you want to complete the process. Obviously, I know that any sort of blasting will alter the surface appearance, but what about a less invasive procedure? A light acid/water mixture wash perhaps? Scrubbing with 000 steel wool?

Gary

[Joe L.]

Re: Restoration techniques questions

Gary------

Just about any method will remove the original as-cast surface appearance; there's no getting around that. However, non-abrasive methods are better than abrasive ones, in my opinion. Non-abrasive is just about limited to chemical treatments. These include the following:

1) find a machine shop with an old-fashioned "hot tank". These will do a great job of cleaning cast iron. They're hard to find anymore, though, due to environmental regulations. Many machine shops use a high temperature bake process for cleaning now and this will not remove rust;

2) dip-type carburetor cleaner works well, but it's not too good for rust removal. Very light rust might be adequately removed, though;

3) Jasco Metal Etch will remove rust fairly well; the stronger you use it, the better it will remove the rust. The problem is that Jasco and most other acid-type rust removers like Naval Jelly, etc. are phosphoric-acid based. Phosphoric acid will often blacken the substrate iron due to the formation of iron phosphate. Muriatic acid (which is an industrial grade of dilute hydrochloric acid) will avoid the phosphating problem. However, it is very fast-acting on cast iron or steel and must first be further diluted to the lowest effective concentration. Also, unless steps are taken to prevent it, re-rusting will occur almost immediately after the part is removed from the acid and dried.

[Joe L.]

Re: Restoration techniques questions

Gary------

Just about any method will remove the original as-cast surface appearance; there's no getting around that. However, non-abrasive methods are better than abrasive ones, in my opinion. Non-abrasive is just about limited to chemical treatments. These include the following:

1) find a machine shop with an old-fashioned "hot tank". These will do a great job of cleaning cast iron. They're hard to find anymore, though, due to environmental regulations. Many machine shops use a high temperature bake process for cleaning now and this will not remove rust;

2) dip-type carburetor cleaner works well, but it's not too good for rust removal. Very light rust might be adequately removed, though;

3) Jasco Metal Etch will remove rust fairly well; the stronger you use it, the better it will remove the rust. The problem is that Jasco and most other acid-type rust removers like Naval Jelly, etc. are phosphoric-acid based. Phosphoric acid will often blacken the substrate iron due to the formation of iron phosphate. Muriatic acid (which is an industrial grade of dilute hydrochloric acid) will avoid the phosphating problem. However, it is very fast-acting on cast iron or steel and must first be further diluted to the lowest effective concentration. Also, unless steps are taken to prevent it, re-rusting will occur almost immediately after the part is removed from the acid and dried.

[Gary S.]

Re: Restoration techniques questions

OK, thanks again. I am not a real fan of acid cleaning because of the serious nature of the stuff and your comments just helped clarify that opinion. In addition, with children around, who wants to keep acid available. I had very nice results witha a soft brass bristle brush when I was cleaning up light surface rust on an exhaust manifold.

Gary

[Gary S.]

Re: Restoration techniques questions

OK, thanks again. I am not a real fan of acid cleaning because of the serious nature of the stuff and your comments just helped clarify that opinion. In addition, with children around, who wants to keep acid available. I had very nice results witha a soft brass bristle brush when I was cleaning up light surface rust on an exhaust manifold.

Gary

[Robert C.]

Re: Restoration techniques questions

Gary, the original strut rods have a "blue/surface scale" look that forms from the heat tempered process.They are the only items on the car with that look to them. I would not expect to see much black-out overspray on them.
If you have your rear end out of the car, just bite the bullet and wire brush the metal. These pumpkins have a darker look to the metal when clean. My buddy kept trying to get the black surface look clean, when all along it was clean.There are clear/ dull sprays available to keep it from rusting.I have also used acid. If you use muratic acid(swimming pool stuff) and don't neutralize it quickly after use, it will flash rust within 5 minutes.

[Robert C.]

Re: Restoration techniques questions

Gary, the original strut rods have a "blue/surface scale" look that forms from the heat tempered process.They are the only items on the car with that look to them. I would not expect to see much black-out overspray on them.
If you have your rear end out of the car, just bite the bullet and wire brush the metal. These pumpkins have a darker look to the metal when clean. My buddy kept trying to get the black surface look clean, when all along it was clean.There are clear/ dull sprays available to keep it from rusting.I have also used acid. If you use muratic acid(swimming pool stuff) and don't neutralize it quickly after use, it will flash rust within 5 minutes.