Answer: From the advice you've received, I am confident that the radiator-cover industry is cartel-free.
I had radiators in my first two houses.
At someone's suggestion, I painted the radiators with Rust-Oleum, because they are metal and you need to have the coating adhere properly to the surface and stand up to heat.
The surface of the radiator needs to be prepped, just like anything else, and if there is rust and corrosion, a wire brush is quite useful. Just make sure that the radiator isn't rusted through.
Prime with a metal primer. Let it dry thoroughly, then topcoat it, maybe twice.
If there is little room between the radiator and the wall behind, paint as far as is visible and let it be. I was cautioned once by a plumber not to move a radiator because you never know how fragile the pipes are connecting below the surface of the floor or the ceiling above.
He also told my wife to sit on me and call him whenever she saw me get within five feet of any plumbing, so now you know.
That said, it takes a long time for the odor of paint to dissipate, and for 11 years after I painted the radiators, you'd think I'd just finished.
And it was very easy to compromise the paint job by drying towels and wet gloves on the radiators.
In my first house, I had a contractor make me a wooden radiator cover, which doubled as a bookshelf. It had a metal screen that is sold at home centers and hardware stores in several colors.
The cover forced the warm air through the metal screen and at the person sitting in front of it, instead of letting it rise to the ceiling, where it doesn't do much good.
In fact, it's recommended that people with radiators and high ceilings install fans to push the warm air down to where people can benefit.
I don't remember the wood showing any signs of warping. The warm air from radiators tends to be dry - hence the art of putting pans of water under them to try to introduce moisture to a room in the winter.
I imagine, however, that whatever moisture is in the warm air might collect on the inside of the radiator cover and cause some warping over time.
Still, from what I've seen online, there are more manufacturers producing wooden covers than metal ones these days. Metal covers get hot, of course, and one of the reasons for using them is to protect tiny fingers from touching the fins of the radiators.
I've never seen anything about covers reducing heat output by 30 percent.
If anyone has data on this, please send it along and I'll report on it at a later date.
There is another option if you have some money or are able to do this over time: Replace the radiators.
Modern radiators are much more attractive, smaller, and more efficient than the old clunkers that our ancestors came up with at the end of the 19th and in the early 20th century.
Rich Trethewey, the master plumber on This Old House, talks about them at http://www.thisoldhouse.com/toh/asktoh/question/0,,385604,00.html.
Most are European-made and are designed to reduce the temperature of the water in the boiler without compromising comfort. They are flat-panel, primarily, and powder-coat painted for a permanent finish.
The ones I've used in Ireland, France, England, and Italy have individual temperature controls and take up very little space.
Those I've seen at home shows over the years had paint finishes that let you dry your towels and wet gloves without reaching for a paintbrush afterward.
Questions? E-mail Alan J. Heavens at firstname.lastname@example.org or write him at The Inquirer, Box 8263, Philadelphia 19101. Volume prohibits individual replies.