I'd like to use galvanized metal conduit as deck balusters with pressure-treated lumber. Will the bare metal resist rust? If not, is there an inexpensive way to improve its rust resistance?
Typical EMT Type conduit will resist rust for a few years. It is however inevitable that it will rust. The most likely place for rust to start will be at the ends where it has been cut or along the surface where the finish has been scratched.
I've used EMT type conduit for various types of outdoor things and had some deployments last for many years without much rust at all. On the other hand there have been times when the conduit has rusted on parts of its surface having been exposed to the outdoor elements for one or two seasons.
I can think of one or two other things to think about for your proposed application.
If it was your intention to bore spaced holes in cross lumber pieces that the conduit would be inserted be aware that this makes for a possible water collection point. Especially in the lower section where collected water will speed the process of the conduit rusting.
EMT type conduit that is captive on two ends will have more of a tendancy to bend out of shape as opposed to wood that would have more of a tendancy to flex. Of course a whole lot depends on the length if the pieces and what the cross sectional dimensions are.
If you wanted to take some steps to seal the metal pieces to provide longer term resistance to rust you could paint them. Even better would be to prepare all your pieces and then send them out to a shop that could apply high temperature cured powder coat finish. This latter has the possibility of sealing the ends where the metal was cut as long as the ends were smoothed and had no burrs or sharp edges.
Rigid conduit (which is pretty much the same thing as galvanized steel pipe) is galvanized with a considerably thicker layer than EMT - it's also stronger (emt is a bit flimsy for a railing) and more expensive (such is life.) Since you didn't specify what type of galvanized metal conduit you were considering using, what happens will vary with the type you choose.
- EMT is very flimsy and prone to rust - inexpensive and easy to work with, but not a great railing material, IMHO.
- Rigid would work nicely, and probably not rust for a good long time if the galvanizing was not deeply scratched.
- I know there is a third product (IMC) but I don't have any direct experience with it. I believe it's "Intermediate" Metallic Conduit and heavier than EMT, lighter than rigid - and I don't know how its galvanizing compares.
I have built rails with 1/2" emt vertical balusters inserted into pressure treated 2x4 lumber stringers. I cut, de-burred, and cold galvanized the cut ends of the conduits. This installation was finished in 2001 and displays no rust or bent conduits after 18 years.
I am currently doing another installation and expect trouble-free service for many years.
I like the look of the galvanized conduit and it never peels like powder coated balusters will. Current cost is $3.52 for 10' of conduit which provides 4 or 5 rungs depending on rail spacing. Total cost of the materials (conduit, paint, stringers, and top rail), is about $4.50 per linear foot.
Use black pipe; it'd be better physically and aesthetically. I could tell from your neighbors yard if you used conduit but otherwise I'd have to walk up to it and then I'd say: Ooh, nice. They used black pipe. You would have to paint it though. At least consider 'sticking' with wood before you go with flimsy old EMT.
Go to The Rust Store and get a gallon of their rust converter. It's amazing. I live in near the ocean where salt air makes everything turn to rust overnight. I coat all raw steel with this before painting it. I've used it on totally rusted out pallet racks and was amazed how it turned the rust back into steel. I think Lowes carries a rust converter by Jaffeys.