Here's a pic of the 1.5" drain he installed:

After installation last week, the drain kept leaking.
I posted numbers on my pic. corresponding to the order in which the drain leaked.

After the 1st leak, I contacted him, and he came over to adjust it.

But then it leaked in the 2nd & 3rd areas (see TWO/THREE labeled in the pic).

As of now, it seems to have stopped leaking for the most part.

But I'm still concerned, because some guys I spoke with (including a local hardware store) said that metal nuts should have been attached to the metal strainers (rather than the plastic ones depicted.

Yet another guy said that he would not only place metal nuts, but also lengths of metal pipe descending from the metal nuts (directly beneath the strainers.

The plumber is stating that he's never had such issues before, and also said that metal nuts tend to corrode, and that brass nuts might be a better solution.

He also tried rubber washers (versus the plastic ones which come with the kits). Maybe the rubber washers helped, who knows.... In any case, i'm not sure if merely rubber washers are the best fix.

Any expert advice on this matter would be appreciated.

P.S. actually, what i'd have preferred most, would have been good-quality PVC nuts/washers so that if there's clogs in future, I myself would be able to unscrew, apply Turbo Snake or Drain King, then re-screw. Whereas the nuts/washers that came in this kit, seem to be the type of non-durable junk found in dollar stores. I mean, how much of a beating can plastic washers & threads hold up to?

It would be helpful to me if I could understand this better.

To answer your question, yes, actually he is a plumber who lives across the street (taught by his father).

Granted he may be amateur, but he's been doing plumbing for years in this area, and many people have been satisfied.

He had installed a complete sink and faucets and drains at my sister's house, and it's been running perfectly for years.

In past years he also installed my new water boiler as well as kitchen faucet. And there were no problems with those either.

Had this been done, it should have been obvious that the left side was not correct. The horizontal pipes should be straight across, not at an angle.

And in regards to the guy at the hardware store who told you that you need metal nuts,,,,,Now you know why he is making minimum wage as a hardware store clerk instead of plumbing.

They are no glue joints in this setup, they are all slip joints.

Straight really matters for slip joints, and make sure there is a bevel washer under every nut with the flat side toward the nut.

Recently changed out my bathroom vanity, had to redo the drain pipes and nearly wept with frustration until I figured this out.

BTW, the surprised response of "I never had such issues before" is just "profession exaggeration" that makes everybody feel better. Leaks after repairs actually can happen to anyone on occasion, which is why one should tests/inspect every joint before they leave. Using a small flashlight is helpful.

A piece of TP works even better.

The slightest moisture shows.

Re: the profession-exaggeration, homebound, I kinda suspected it (since I get that from not only servicemen, but also society at large, all the time). Thanks for validating that, since misery loves company ;)

In any case, I had done exactly that - in-person-summons... And he came last week and redid it though I'm not sure he followed all the steps listed in lazypup's pic.

So far so good. The floor of the cabinet seems bone dry, and the finger-test tested dry as well. What's TP? Couldn't find it on google images.

Any particular strategic areas that should be tested?

He had used a locking pliers to tighten everything, that's aside from the rubber washers at the strainers.

If in future it gets clogged again, can I easily detach the U-shaped pipe so as to insert my Turbo Snake into the pipe beyond which is connected to the metal piping? (a/k/a "trap adaptor) He seems to have clamped two bands surrounding the aforesaid metal piping. Those aren't so visible in the pic.