The common rule for bare metal is that you do not allow it to be exposed for longer than three hours prior to coating it with a nonporous paint. Unfortunately there are many times when you are working on metal surfaces and need to leave them unpainted for extended periods of time before painting.
Three hours is not a magic number and if humidity levels in your area are high then this time might be shortened. Rust will begin forming on metal surfaces immediately. Any rust that begins to form could result in paint problems later.
To experience this in real life it is easy to see this happen when you are preparing large automotive panels for paint. If you sand the panel and let it sit overnight or even for the time it takes you to eat your lunch and then when you return to work you sand a stripe down the middle of the bare metal area you should notice that the panel has turned a slight yellow orange color. This is rust beginning to form.
Any rust that sits between your bare metal and your primer or paint is a problem and must be treated before painting.
How Factory Parts Are Protected From Rusting During Shipping And Storage
Sheet Metal Parts and other metal products that the supplier expects you to paint are normally shipped with a black primer sealer. Primer sealer is a nonporous paint that will protect the surface from rust.
There is some debate as to whether you need to remove this primer sealer prior to painting or if you can paint right over it. Most people will scrub the panel down prior to painting and most panels will have small dings and imperfections that need to be leveled with primer surfacer before painting.
Primer surfacer that is used for sanding panels straight before painting is porous so it will allow moisture to penetrate this means it should not be used for protecting panels from surface rust.
Stock sheet metal sheets and other metal parts that are not manufactured into a completed part are normally shipped coated with a rust inhibiting oil that must be removed before painting. This oil is thick enough that it will seal out moisture during shipping and normally during some of the manufacturing process but all of it must be removed before painting or you will have adhesion problems.
You can also purchase some metal parts that have been treated with galvanizing. Galvanizing is either a hot dip tank or zinc electroplating process where a small amount of zinc and other metals are applied to the surface of the iron / steel parts to protect it from rusting. It is really great in some instances to use galvanized metals but normally you would not use it for structural parts because the hot dip galvanizing process will alter the strength of the metal. In that case a zinc plate might be possible or just use standard uncoated metal. Welding of Galvanized metals can release toxic fumes and should be done carefully with proper ventilation.
How Can You Protect Bare Metal Before Painting?
We have talked about what the problem is and what manufactures do to protect metal parts during shipping and storage but what can you do to protect parts that you are working on?
Pretty much you have the two choices that manufacturers use to protect your parts. You can either spray a light coating of primer sealer on the part or you can spray a light oil on the part to protect it from rusting prior to paint.
The advantage that primer sealer has over oil is that oil will cause finish paint problems if you don’t remove it completely before you start the paint process. Oil is an option but it can also seep between panels at weld joints and if it is applied to interiors of frames, inside areas of doors or other parts then it can be extremely difficult to remove before painting.
If you are not mixing up your primer sealer a spray can should be sufficient.
Other products such as a cheap can of black spray paint might also be an option if you expect to remove it prior to painting by resanding the panel or part.
Another really good option is just resanding the part before painting. If you are working on some large areas such as stripping down a vehicle for total repaint this process can take days or even weeks. In this case if all you have is primer surfacer available to you or garbage paint in a can then sure you can apply it to reduce rust but you are doing so with the thought that you will be completely resanding the vehicle before you apply primer.
This is one method that I think is best when working on exterior parts that are easy to resand. If you are working on a total restoration that requires media blasting of parts and interiors and other areas that are difficult to get to then you will just need to work in a way that will allow you to seal these parts as you work.
This is the problem that many technicians get into when doing full restorations. They work too far ahead of themselves because they just want to get the work done.
If you were to send out a vehicle for blasting they will return it full of blast media that you need to vacuum out and blow out before you can begin painting. This is not a simple process and it might require more manpower than a single technician can deal with.
What To Do If Your Panels Have Already Rusted?
If you have missed the window of applying primer sealer or oil to the surface of bare metal then you need to reprep the parts.
Depending on how long the parts have sat you might be able to use a product like Metal Prep or Rust Converter. This is an acid solution normally Phosphoric Acid that will turn the rust to an Oxide that can be painted. However in most situations the use of metal prep to remove significant rust is not possible and although it is an important first step to neutralize the rust you will still need to resand the surface of the metal to remove the buildup of oxide.
Metal Conditioner will work if the part has been sitting over lunch and has a light orange tone to it but It will not work on its own if it has been sitting a week or more. In that case you need to resand.
Final Note
Rust is one of the biggest enemies of a long lasting restoration or paint job.
Whether you are painting metal fence parts or a 57 Chevy the problem is the same. You need to sand the part to remove old paint and rust but you also need to repaint it before it starts to rust again.
If I am working on a job I will normally use a primer sealer or cheap black spray paint to protect the metal from oxidation. This is always easier to deal with later.
Paints that say they can be used right over rust are really not the best choice. They are a lazy technicians option and I would never use them over rust. They might be a good sealer for inside of panels but no paint can convert rust to good metal or provide a lasting paint job.