Or maybe a panel of better plywood. Lauan is at the bottom rank as a material; not strong, glue of dubious quality, and can't take moisture. Decent birch plywood or better yet, MDO plywood will last and not break down.
Casey

Hi Casey, thanks for your response! I was away visiting my sister and am back at home trying to scope out what to do.

That's a great point about the lauan not being able to take moisture.

What's "MDO"? Is it anything like MDF (medium density fiberboard)?

Online, I saw something about only putting paneling over something fire retardant, like gypsum board. There's a cavity space that the lauan is currently covering--about 19" wide and 37" tall. Is it safe enough to simply put a sheet of plywood over that cavity without having a piece of drywall behind it?

Hi,
You may need fire-retardancy if you are afraid that the radiators will catch fire. Does the recess present any opening to above? Like opening into the spaces between the studs? An area like that would be a fire-spreading hazard, allowing flames to get in the walls and to the next floor above. Open stud bays like that should be sealed. 2x4 stock is acceptable, cut into well-fitted blocks.
MDO means "medium-density overlay"; it is a good quality fir plywood with a smooth resin/paper laminated on. It's also called signboard, because it is so well-suited for sign-painting. Unlike sheetrock, the edges will remain intact, and it can sort of float in the space without needing support at all 4 edges. You can screw or nail it to whatever framing is under it, and the edges will take care of themselves.
HTH
Casey

Hi Casey,

Thanks for the description of MDO. That part about the "intact edges" sounds pretty useful.

And you are right on target about the fire-spreading hazard. You will be amazed at what I found behind the lauan panel! Yesterday, the plumbers disconnected the radiator that was in front of the panel and moved it over so I could get in there. When I took out the lauan piece, behind it was an recessed space with two layers of wallpaper, and a 5" open hole leading into the chimney!

Long view, showing recessed space and hole:

Close-up of the hole:

Looking in:

Needless to say, I was a bit freaked out, because not only is this is the chimney that the oil boiler gases go up, there was all that combustible wallpaper. Sigh of relief today; the hole is no more! My favorite chimney sweep guy has sealed it up.

He suggested using "Hardy/Hardie Board" as a panel--that would protect from heat of the radiator and he says it's what his team uses in chimney-related situations. Maybe it would be overkill, but I find there are enough things to worry about in an old home without one more thing. :-)

Casey, thanks again for the info you provided. It was really helpful.

--Lee

This is serious, have someone brick up the stovepipe hole into that flue! Carbon monoxide alert! A sheet of anything is not going to make it airtight, which it needs to be.
Casey

Hi Casey,

Yes, that hole is bricked up now!! :-)

I'm glad this didn't happen during the winter heating season.

The chimney guy used fire brick and wrapped it with ceramic wool, and then patched it over with cement.

Pic with the hole patched up and the wallpaper off:

Now I have to figure out what to do with this niche. Ideally, I'd like to just have a flat surface all across, because the radiator is going to go back in place in front of it.

As this has gone beyond a "paint" question, I'm going to x-post to the Home Repair forum and see if there are any suggestions on what's best to do, what the pitfalls are to watch out for, etc.

--Lee

That's a beautiful plastering detail with the beads run so neat & all, it's a shame to cover it up. Why not patch and paint? No real need for a flat panel. How tall was the radiator in relation? could a small shelf be fitted even with the top of the rad? Anyway, even if you do cover it, the good workmanship, although hidden, will still be saved.
That mantel is killer, BTW :-)
Casey

Hi Casey, thanks for the compliment on the mantel! :-) I love all the woodwork in this house. It was one of the things that sold it for us.

Thanks for continuing the conversation on this. I am a bit stumped on how best to proceed. I agree with you about the neatness of the beading detail. My husband is concerned about losing radiator heat into that niche, and thinks that if we put a reflective panel in back of the radiator, that that would be a more practical way to go.

The radiator is 38" tall, and when in place the top was butted up against the outer row of those horizontal rounded beads on the underside of the mantel. In a perfect world where I could wave a magic wand and swap that radiator with one upstairs, I might do that. But no magic wand here. ;-)

If I wanted to keep that beading visible, how would I best put something flat across the niche?

The niche space is about 19" wide, 37.5" tall, and 3.5" deep. I'm imagining making a rectangular frame of 2x2's and inserting it into the space and securing it into the floor and sides. Then putting a 19x37 gypsum wallboard over that between the vertical beads and finishing it so that it would look like 3 vertical surfaces in that under-mantel space.

Then the radiator goes back in place and some future owner can find the space later when he/she renovates. I could leave a time capsule in the space with pics to let them know what we did. :-)

What do you think of the frame/wallboard between the beads idea to cover up just the niche portion?

--Lee

I don't think you'll lose all that much heat in that little niche...though it's possible I am misunderstanding the problem...