What is the best way to polish A-5 internals?

I'd suggest you do a search on here, there's multiple threads discussing the topic. Just type in "polish internals" and there's more than half a dozen threads. They're not long reads.

Here's the best link: How-To: Polish Your Internals Monitor's how-to.

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JackRock

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being new to this i whent the safest route, i just used a few SOS pads and whent to town. it took some time and alot of elbo grease but in the end it turned out well.

i cleaned all the recessed areas that the internal components slide on; the front and rear bolts, the brass rod connecting the two and the area that the cocking rod slides. i left the paint in the barrle adapter area because i heard that removing it could cause the adapter to be a little loose.

the real thick areas took some time but with a little oil everything slides real nice.

I just did Monitor's method (the link posted by JackRock) in the last hour, and it worked pretty well.