Shop Primer Coating & Rust on Structural Steel

[sbw]

This is a two part question:
1. My impression is that standard practice in the structural industry is to specify a primer coat to all structural steel, regardless of exposure. As I understand it, the typical primer is relatively porus and does not provide a good barrier against moisture. Section M3 of the LRFD (3rd edition)states 'Even in the presence of leakage, the shop coat is of minor influence.' On page 48 of chapter 2 (2-48), AISC says '..., shop primer or paint is not required unless specified in the contract documents,...' While I understand applying a primer coat for steel that will recieve additional coatings, what is the purpose if it's final application will be in a protected, indoor environment? Is it related to temporary protection during erection?

2. Are there guidelines for how much rust can be tolerated on new steel (such as that ocurring during field storage and erection/installation)?

Thank you in advance.

 

[HgTX]

Depends on your application.

I can answer one small piece of this.

For a slip-critical connection, the mill scale needs to have been blasted off, and then any loose rust that may have formed thereafter needs to come off before bolting. A good power-wash will do it. Tightly adhering rust that will survive a power-washing is actually beneficial for slip conditions.

Hg


Eng-Tips policies: faq731-376

 

[Redfish205]

Another question that comes up is, "Does the steel have some type of sprayed on fireproofing?" Most highrises I have seen have no coating at all and the steel is usually exposed for quite some time before fireproofing begins

 

[VTPE]

The AISC Code of Standard Practice (a free download at AISC.org) decribes the uses of shop primer. It specifically says that primer is not a rust inhibitor and should not be used as one. See section 6.5.1.