i seen the insides of wrought iron steel gates and fencing cladded in sheets of aluminium and mild steel plates on a recent visit to Paris.
the outside of the plates were painted the same colour as the fencing so blended in rather nicely.
I can guess, but since i didn't do it i dont know if i am right.
I also wonder about your gates, or more to the point what holds them up. Taking the picture you posted as an example that looks like timber, it must weigh some. So if what ever your gates are fixed to can not take too much weight solid timber is out of the question.
self tapping screws or rivets would probably be the method of fixing the plates.
fair points from breezer as well, though as your existing gates are 6 feet high , i was working on the assumption that the pillars are already substantial.
Kool - unless you live in a period property with original gates they are unlikely to be made from wrought iron (anyway most of these were stripped-out during the last bun fight with Jerry and melted down ... unless you were a nob who owned a grand house). 'Wrought Iron' gates is a generic term used to describe gates made from steel (or 'black iron') strip welded (or sometimes riveted) together. The problem with drilling into these strips is that the rear edges will be narrow but carefully done will be possible - once drilled fixing can be machine screw (thread the hole), self-taping screw or pop-rivet. Another common way of attaching modesty screens to steel gates is to use thick iron binding wire loops. Clearly the best method is to weld the screens on but this will tend to be beyond the skill-base of most DIYer (even those with welding tackle) - thin sheet to thickish strip will result in 'blow holes' through the sheet.
The picture you posted shows gates made from tubular steel; being hollow it's easier to drill so will allow self-tappers or pop rivets for fixing.
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