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Some days ago I assembled the following joint using pipe dope (it is solvent based that semi hardens to a rubber like stuff).

PVC to metal

I waited 2 days to pressurise the system and after a few days it started leaking.

How can I fix that?

Edit: this is how it looks after disassembly:

disassembled joint

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You should try TFE pipe thread joint tape. This tape is a thin white film that you wind several times around the externally threaded part before screwing the parts together.

I would suggest cleaning away as much of that dope material as possible before re-assembling with the the TFE tape. Also it may be well to look at the possibility of using a new PVC part. That particular one looks a little bit low quality as the molding marks show that the two halves of the mold did not line up perfectly which leads to the slight misalignment of the threads at the mold line plus the mold line itself. This may be contributing to the leak problem.

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    Thanks, I went to pipe dope because I've seen on many forums and even here on stack exchange that it is better, and in case of straight threads (I've found out that male parts are tapered and female are straight when it comes to water parts) is the only solution. – Luiz Borges Feb 17 '16 at 13:17
  • Dope/tape is not designed to fill gaps to prevent leaks. It's designed to reduce friction, so you can get the joint tight enough to prevent leaks. – Tester101 Feb 17 '16 at 15:55
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    "The application of pipe dope prior to assembly will fill the minute voids between the threads, thus making the joint pressure tight. Pipe dope also acts as a lubricant and helps prevent seizing of the mating parts, which can later cause difficulty during disassembly." –Wiki, Pipe Dope It's called thread sealing compound for a reason. – Mazura Feb 27 '16 at 9:03
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You'll probably never get a good joint between metal and PVC, no matter what type of dope you use. Instead, you'll want to use an adapter. Something with metal threads on one end, and a PVC hub on the other.

enter image description here

  • Will try, BTW, considering metal threads to metal threads (the male being tapered and the female straight), how tight it should be?? I ask that because one of the joints I made (still haven't pressurized it) I couldn't go all the way because it would be misaligned if I tight it any more. – Luiz Borges Feb 17 '16 at 13:21
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    @LuizBorges - The male part being tapered allows it to get tighter and tighter into the threads as it is screwed in. How far is the question -- far enough that it does not leak. The problem is that if the threads are not fully seated and sealed the water leaks out through the spiral of the thread. – Michael Karas Feb 17 '16 at 13:27
  • @MichaelKaras, I understand that, but what is usually done when we can't fully tighten it (like an elbow or some other part that needs to be properly aligned)?? – Luiz Borges Feb 17 '16 at 13:40
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    @LuizBorges You'll likely never "fully tighten" a threaded joint, there will almost always be one to a few threads showing. – Tester101 Feb 17 '16 at 13:52
  • Yes, the fully tighten that I meant was the popular tight "snug that a little more". The joint I'm mentioning only gets to the snug part (using a small pipe wrench), if I make it "snugger" it gets misaligned by a quarter turn. – Luiz Borges Feb 17 '16 at 13:57
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You can fix this by removing any residue from this dope and re-joining with Whitlam's Industrial Grade 'Blue Magic' Pipe Thread Compound. I frequently make joints just like you have on hot or cold water pipe with pressures up to 100 psi and have never had a leak at the joint. This is quite expensive, 15-18$ for a 200 gm tin, but well worth it.

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