How smooth is smooth

Discussion area for builders of Pietenpol aircraft, both beginners and experienced folks. Share ideas, ask questions and help build the Pietenpol community.
Post Reply
LarryM
Posts: 15
Joined: Sat Aug 12, 2017 12:09 pm

How smooth is smooth

Post by LarryM »

A friend cut a piece of steel into strips for my center section fittings. Unfortunately, he left me with rust and tool marks to contend with. I've sanded with 80 and 120 grit sandpaper. How fine do I need to go?
Attachments
DSC08562.jpg
DSC08562.jpg (24.73 KiB) Viewed 3884 times
User avatar
Terry Hand
Posts: 87
Joined: Thu May 18, 2017 2:37 pm

Re: How smooth is smooth

Post by Terry Hand »

From Sportplane Construction Techniques by Tony Bingelis, Pages 50-51 -

"One chore remaining is to finish your fittings to a uniform external shape, making sure that all of their edges are smooth and free of saw and file marks. Surface nicks, scratches and gouges, If present, should be dressed out. All steel and unclad aluminum parts should be smooth-sanded. I use aluminum oxide sandpaper. Smooth finishing a fitting's edges is much easier with the part clamped in a vise. Use a smooth-cut file and follow that up with number 180 (or finer) wet-dry sandpaper on steel parts. Use it dry, as wet sanding 4130 encourages rust to form before you can shake up the spray can of zinc chromate primer.

Sometimes, we builders will find a piece or two of 4130 steel that has been laying around for years—and it shows in its surfaces which may have varying degrees of rust. scratches and even pit marks. There is no harm in using the material, although it should be cleaned first to determine its acceptability before cutting out parts. This precaution will serve to assure you that you will not have to discard a part after you have spent much time making it Rust will sand off easily enough provided that corrosion has not eaten into the metal to any degree. All rust must be removed, otherwise the residual will keep working away on the metal even after it has been primed and painted. Sanding the surfaces to a nice shiny appearance may or may not prove successful in removing all of the rust. Although the new shiny surfaces might look perfect, the grain of the metal will still have tiny little imbedded specks of rust invisible to the (if you will pardon the immodesty) naked eye! To be sure, treat steel surfaces with a metal conditioner to neutralize the rust. A product similar to Osphos, or any of those put out by paint companies may be used to take care of that problem. (Be sure to read and follow instructions)"

I hope that helps. In addition, I was always taught that if, after sanding and prepping, if you could not snag a fingernail in a scratch when dragging your fingernail across the piece of metal, it was acceptable.
Semper Fi,

Terry Hand
Athens GA
User avatar
Terry Hand
Posts: 87
Joined: Thu May 18, 2017 2:37 pm

Re: How smooth is smooth

Post by Terry Hand »

One More Thing -

A good place to go to get questions answered is the FAA Advisory Circular, AC43.13-1B, ACCEPTABLE METHODS, TECHNIQUES, AND PRACTICES - AIRCRAFT INSPECTION AND REPAIR. I have attached a link to the FAA online copy that you can download for reference in the shop -

https://www.faa.gov/documentLibrary/med ... w-chg1.pdf

To save you time I have attached a pdf file of the pages from the AC43.13-1B that discuss corrosion and metal preparation.

I hope that it helps.
AC43.13_1B (dragged).pdf
(401.24 KiB) Downloaded 265 times
Semper Fi,

Terry Hand
Athens GA
Post Reply