Painting inside of differential

I have noticed some of you paint the inside of your differential and the HP44 I am rebuilding has the inside painted. I have built numerous axles in the last 40 years and have never painted one. Why do you do it? What do you use?

The paint in the diff I am working on, is peeling. Should I just leave it, strip it, or repaint it?

Me being me, I'd strip as much of it as I can. I see no reason to paint the INSIDE of the differential case. Maybe the insides of the tubes outboard of any seals (like on the D30 front - but nothing where the seals are just inboard of the wheels,) and the flaking/peeling paint would seem to really want to get somewhere it shouldn't be. I can't feature paint being that high on the scale of lubricity, and I don't think it would do your bearings any good...

Seriously, I'd like to hear the whys and wherefores as well - because I don't see any reason. It's certainly not for rust prevention - inside the case, it's an oil bath. I'm thiking there are some coatings for engine internals that would help with oil shedding, but I don't think oil shedding is that much of a problem with axles either - so why, pray tell?

5-90

I have no idea why someone would paint the inside, other than to help seal up the inside if the casting was bad maybe? I have seen automatic transmisions leak though a bad case before...but never a diff.

And 5-90 is correct alot of people paint the oil galley in an engine to help the oil return to the pan as quickly as possible. Its a gamble though because you will have no idea if the paint is coming off etc...plus its also tough to paint something that has been oily before as well especially when its cast?

There are specialised coatings for internal engine work - they're impervious to oil and solvents, and lower friction than painted surfaces, so it makes sense. I'd still prefer to just clean up the "sand-cast" surface, which has much the same net effect.

If the problem is a porous casting, just replace the damn thing. Nothing is going to keep it sealed up forever - so why try? If I can't get oil to stay in a casting (much less with NO pressure involved!) I'll just replace the thing, since it's cheaper to do so in the long run...

5-90