Bicycle Mechanics - Un-used threaded holes in frame: plug or leave alone?

I have several theaded holes in my steel-framed Lemond and since there is no paint in there on the threads, this seems like a place rust could start. These holes appear to be for fender mounts (not the vent/drain holes). My other bikes also have rack mounting threaded holes in addition to the fender mounts.

Do other people put a screw in the hole even if it is not being used?

I have put some grease in the holes on my Lemond for now, I was thinking plastic plugs are also a possibility.

What do you do?

Don't bother. Putting a screw in there will probably make it rust faster. Grease good idea though.

You can also put plastic plugs in them. My new tandem came with black plastic plugs where canti brakes would normally go. They look good and keep things clean.

You're right, unpainted threads are a great place for rust to get started. I put greased stainless button head screws in the unused holes. I get them cheap from boltdepot.com.

If you are able to find the same color paint at an automotive or hardware store, you could paint the inside of those holes. I was able to do this on one bike when I saw rust starting. If you decide to use a plastic or metal screw, put grease on the threads, that will keep it from binding and double as a rust preventative.

Nylon M5 bolts work well as thread protectors and gap pluggers.

I cover them with colored electrical tape to match the frame. (lighter than bolts)

Enjoy

button head screws with a libral amount of loctite. loctite will seal out the water and of course, prevent the screw from loosening which they have a tendancy to do.

If you're a weight wienie, you can use titanium screws. I did that on one of my bikes. I don't know what got into me.

Grant Peterson would tell you to use beeswax on the threads. He says that it keeps then from getting stuck and it keeps them from loosening. I've tried it and it seems to work.