Evaporative air conditioners can cause rust issues with roofing,guttering and down pipes. This is due to the reticulation of water increasing the salinity of the water run off.
Ideally PVC pipework should direct the water into a down pipe, not just a gutter and if the down pipe is PVC, no issues with rust can can develop. Unfortunately, this is not often done and hence rust issues, like in the photo above can happen.  This down pipe is extremely damaged but more commonly surface rust or rusted sections are seen around the down pipe near the air conditioner overflow. 
​ 

​Seaside living has many disadvantages, one being the rusting of steel items around the home. Staircase balustrades in particular, if steel, requires constant maintenance. Best practice is for hot dipped galvanising of all external rails, balustrades etc. Painting of red primed or mild steel that has not been hot dip galvanised as a finished manufactured product will undoubtedly rust.
Pre manufacturing of fully galvanised balustrade sections is common on higher end houses, but unfortunately more budget orientated houses are often produced cheaper. This results in the issues such as in the picture above. Total replacement is required, costing many thousands of dollars.

​Having been involved in the replacement of complete balustrades as a building work supervisor, I know first hand that each balustrade section can cost over $1,100. 
Each balustrade section requires measuring up, manufacturing, galvanising, powder coating then fitting. Most balconies have several pieces, so it's an expensive project.
If your'e looking at buying a coastal home and you notice a bit of rust, think of the consequences and expenses that may be involved. Or better yet, pay for an inspection from some one who knows what to look out for, so then you will know what you are really buying.