Any reason I can't use plumbers putty to seal an outdoor light fixture?

I just replaced an outdoor fixture, and as is frequently the case, the backing frame us not precisely the same size as the backing that it sits on (it’s slightly larger on top and bottom) which leaves a gap where rain and such can come in. Now the instructions say to use a silicone sealant to seal the frame to the wall but I think that’s anticipating a flush mount and filling up a hairline crack, but it would be tough to fill a decent-sized gap like this with silicone. It would seem to me a lot simpler to just stuff it up with a hunk of plumbers putty. Is there any reason not to do this?

Truth is that plumbers putty is probably more effective in its intended usage situation, where the main seal is being made by compression and the putty is just to fill in unevenness in the surfaces. But I’m thinking how tightly sealed do I need this to be anyway, and I’m inclined to go with it. But looking for dissenting opinions.

I’d go to the auto parts store and look for Dum dum sealer

It’s used to seal and fill gaps around body panels and fill holes.

Better to use a foam gasket, and silicone.

The heat generated by the light will degrade or even melt the putty enough to make it useless.

Silicone caulk is the more flexible version of duct tape. It’ll fill and last forever.

To add to the consensus: silicone stays flexible and adheres for years. I don’t think the plumbers putty will stay flexible and adhere through a winter or two, especially after a couple freeze / thaw cycles with water on it or behind it.

I second beowulf on the foam gasket (you can also use the tubular foam weather stripping) then seal it in with the caulking or alternatively try to build the caulking up in layers.

you can use a backer rod to fill any gaps before you caulk. backer rod is like a foam noodle that comes in different sizes and since it’s foam it compresses. stuff it in the cracks up high and on the sides to fill gaps and then caulk on top of it.

get a brand of caulk made by GE or G&E (General Electric basically). Home Depot sells it, others I’m sure. it’s awesome. don’t get DAP, it sucks. and if there’s some DAP that doesn’t suck, then it’s too little too late.

don’t caulk all the way around the fixture. leave an area on the bottom caulk free, so any water that gets in has a way out.

Another thing to consider, if any part of the putty is exposed to sunlight, it needs to be outdoor (UV) rated or the sun will probably degrade it fairly quickly.

Outdoors the putty will crack and degrade over time potentially allowing moisture into the fixture. Plumbers putty is used on horizontal surfaces spread over a wide area and heavily compressed. You need to use foam and/or silicone that will flex with changes in temperature.