Hi guys. I've not had to clean a washer tub, however I routinely clean porcelain-over-steel refrigerator cabinets. I would assume the porcelain coatings are similar in nature.

What I am saying here, is from my own firsthand experience. This is not stuff I read on the Internet or even from the back of a product package. I have actually done this and am about to describe the results.

I have never used lye based oven cleaner on a porcelain surface.  I have, however, used Zep Industrial Purple degreaser, at full strength, on porcelain fridge exteriors. It is based on NaOH lye. It did not visibly affect the surface. I left it in place for about a minute and then pressure washed it off with a very stout pressure washer. The cleaning effect was not so good. I would say it didn't hurt the finish but it wasn't worth the effort either.

As a side note, it will absolutely etch clear glass. I was preparing a car for painting and washed it with this degreaser. There were what appeared to be water spots on the windows after this. They were not profound, but they were there and have persisted for 15 years.

I would not want this in a washing machine because it can strip paint and other coatings. It could damage the seals in the washer if it went through the tub holes and got into the  bottom of the machine, where the drain line connects etc.

I have tried acid type cleaners on porcelain and caused IMMEDIATE DAMAGE. The acid instantly etched the gloss from the surface, leaving a satin, chalkboard-like surface. The acid was aluminum brightener, which is a miz of HF and HCl acids. I was trying to remove rust stains with it. I also tried CLR brand rust remover, and it didn't cause damage but it also didn't work to clean the surface.

What did work?  Well, a similar approach to what Good Shepherd used - a mechanical, abrasive process. I didn't use a powdered cleaner, however. I used Scotch-Brite pads with Windex or Rain-X glass cleaner. The porcelain is so much harder than the abrasive in the Scotch-Brite pad, that it will NOT polish or scratch the porcelain. The pad glides over the surface more easily then it does when it is scrubbing bare metal or anything it can cut into. The result is pretty dramatic, with it taking off rust stains, paint overspray, etc. with medium scrubbing. The best part is, there is no harsh chemical residue to get in the cracks and crevices of your appliance and contaminate it or cause hidden damage.

Sorry that got long!  Hope it is helpful. :)