So the armor is 

Armor (breastplate, half plate, or plate), rare (requires attunement)

This magical armor appears as a jug of molten bronze. When you attune to it, the bronze adheres and contours to your skin. The armor can be worn under normal clothes, but it doesn't impede bodily functions. Once you put it on, it can't be removed unless you choose to do so.

While wearing the armor, you have resistance to fire damage.

Does this mean it would negate heat metal completely since it is not technically metal being worn (in the conventional sense) or would just halve the damage due to the resistance? How would you all handle this at your table?

You're absolutely wearing that armor, making it a valid target for Heat Metal.

The answer is in the item’s description:

Armor (breastplate, half plate, or plate), rare (requires attunement)

This magical armor appears as a jug of molten bronze. When you attune to it, the bronze adheres and contours to your skin. The armor can be worn under normal clothes, but it doesn't impede bodily functions. Once you put it on, it can't be removed unless you choose to do so.

While wearing the armor, you have resistance to fire damage.

 
That being said, from the wording the item is obviously homebrewed, so you may want to ask the item’s creator what the RAI is on that, and ultimately it’s up to your DM to decide.    

Quote from IamSposta >>

The answer is in the item’s description:

Armor (breastplate, half plate, or plate), rare (requires attunement)

This magical armor appears as a jug of molten bronze. When you attune to it, the bronze adheres and contours to your skin. The armor can be worn under normal clothes, but it doesn't impede bodily functions. Once you put it on, it can't be removed unless you choose to do so.

While wearing the armor, you have resistance to fire damage.

 
That being said, from the wording the item is obviously homebrewed, so you may want to ask the item’s creator what the RAI is on that, and ultimately it’s up to your DM to decide.    

 It's not a homebrew item, it's from Mystic Odysseys of Theros

I would house rule that the armor itself is immune to fire and cannot be affected by heat metal or similar spells.  But it is clearly a house rule

Quote from 6thLyranGuard >>

Quote from IamSposta >>

The answer is in the item’s description:

Armor (breastplate, half plate, or plate), rare (requires attunement)

This magical armor appears as a jug of molten bronze. When you attune to it, the bronze adheres and contours to your skin. The armor can be worn under normal clothes, but it doesn't impede bodily functions. Once you put it on, it can't be removed unless you choose to do so.

While wearing the armor, you have resistance to fire damage.

 
That being said, from the wording the item is obviously homebrewed, so you may want to ask the item’s creator what the RAI is on that, and ultimately it’s up to your DM to decide.    

 It's not a homebrew item, it's from Mystic Odysseys of Theros

 
And they worded it like that?!? What the 💩?!? They must have had a junior writer on that one or something. Holy cow.

Yeah, there was a lot of that book that had me thinking it was stuffed off to a junior team.

Quote from CaptGladMane >>

So the armor is 

Armor (breastplate, half plate, or plate), rare (requires attunement)

This magical armor appears as a jug of molten bronze. When you attune to it, the bronze adheres and contours to your skin. The armor can be worn under normal clothes, but it doesn't impede bodily functions. Once you put it on, it can't be removed unless you choose to do so.

While wearing the armor, you have resistance to fire damage.

 

Does this mean it would negate heat metal completely since it is not technically metal being worn (in the conventional sense) or would just halve the damage due to the resistance? How would you all handle this at your table?

 I can be targeted because it is worn. Heat Metal will do half damage.

Quote from 6thLyranGuard >>

Yeah, there was a lot of that book that had me thinking it was stuffed off to a junior team.

 
I wasn’t interested, so I never bought it and thought it is shared with me I haven’t looked through it, and never noticed before. For something like that they normally would write something more along the lines of:

This magic armor normally appears as a jug of molten bronze. When you attune to this magic armor, the bronze adheres to your skin and contours to your body. It does not impede your movement or interfere with other functions, and you can wear clothing over top of it. Once you don this magic armor, it can't be removed against your will. When you remove it, the armor returns to its normal state.

You have resistance to fire damage while wearing the magic armor.

Quote from 6thLyranGuard >>

Yeah, there was a lot of that book that had me thinking it was stuffed off to a junior team.

 You don't have a copy, do you?

Lead Designers: F. Wesley Schneider, James Wyatt
Designers: Ari Levitch, Jeremy Crawford, Kate Baker, Bill Benham, Orion D. Black, John Compton, Kelly Digges, James lntrocaso, Adam Lee, Ben Petrisor, Jessica Price, Morrigan Robbins, Amber Scott, Greg A. Vaughn
Rules Developers: Jeremy Crawford, Dan Dillon, Christopher Perkins, Ben Petrisor

 The magic item isn't that difficult to figure out.

It uses Breastplate statistics to account for weight and AC, and it doesn't impose disadvantage on Dexterity (Stealth) checks. It's skin, covering you completely. Some part of it is always visible unless you're invisible or otherwise obscured. A generous DM could allow you to wear enough clothing to completely cover it, but there's no rule for that. It also grants resistance to fire damage, so even if you did target it with Heat Metal you're still taking half damage.

You get to look like a living statue and have resistance to the most common damage type. That's dope.

Quote from Jounichi1983 >>

Quote from 6thLyranGuard >>

Yeah, there was a lot of that book that had me thinking it was stuffed off to a junior team.

 You don't have a copy, do you?

Lead Designers: F. Wesley Schneider, James Wyatt
Designers: Ari Levitch, Jeremy Crawford, Kate Baker, Bill Benham, Orion D. Black, John Compton, Kelly Digges, James lntrocaso, Adam Lee, Ben Petrisor, Jessica Price, Morrigan Robbins, Amber Scott, Greg A. Vaughn
Rules Developers: Jeremy Crawford, Dan Dillon, Christopher Perkins, Ben Petrisor

 The magic item isn't that difficult to figure out.

It uses Breastplate statistics to account for weight and AC, and it doesn't impose disadvantage on Dexterity (Stealth) checks. It's skin, covering you completely. Some part of it is always visible unless you're invisible or otherwise obscured. A generous DM could allow you to wear enough clothing to completely cover it, but there's no rule for that. It also grants resistance to fire damage, so even if you did target it with Heat Metal you're still taking half damage.

You get to look like a living statue and have resistance to the most common damage type. That's dope.

To your first point about not having read the book followed by the list of designers. Some of those names are well known, some less so, and I have absolutely no idea who some of them are. And there’s no telling who worked on what, so some of those unknown/lesser known folks were probably the people who wrote the parts that gave 6thLyranGuard their impression. Like that item for example. ‘Cause that ain’t Crawford’s writing, I can sure as shootin’ tell ya that.

And ass to your second point, for as easy as you say it is to understand, you did miss a thing. The armor could count as either a Breastplate equivalent, or half plate or full plate equivalent. So while it may not impose disadvantage on stealth checks, for either of the heavier types it looks like it still has a Str requirement which you either didn’t realize or at least failed to mention. So it’s still not as clear as it could be. Just sayin.’

 Going through each of the relevent phrases in heat metal:

Choose a manufactured metal object, such as a metal weapon or a suit of heavy or medium metal armor.
It is clearly manufactured and is discribed as being bronze therefor it is metal

that you can see within range.
Whether it can be seen depends on whether the "normal colthes" cover it completely if it does that also raises the question of whether you can avoid heat metal on any metal armor by completely covering it with a robe this is a DM decision

Any creature in physical contact with the object takes 2d8 fire damage
The wearer is in contact with the armor so providing the DM rules the armor can be see by the caster the PC takes the damage (halved due ot resistance)

You cause the object to glow red-hot. Any creature in physical contact with the object takes 2d8 fire damage when you cast the spell. Until the spell ends, you can use a bonus action on each of your subsequent turns to cause this damage again.

Yeah it makes more sense that Heat Metal would still affect it, but halved due to resistance. I have only ever seen heat metal used from PCs and not from the DM in games I have played, but a paladin forum brought this up so in reference to finding ways to negate DMs using Heat Metal against them (being the players). I figured I would also ask before making a decision on selecting this armor or a different rare item for my paladin. 

I think there is a reasonable debate to be made on whether you can sufficiently cover half plate or plate armor with a robe of stars or something similar, to the point where it cannot be targeted by heat metal.

I also think that RAW is not going to give you a satisfactory answer to that debate.

The armor says it contours to your skin. I always got the impression that it basically makes you look like a bronze statue, head to toe. So how are you going to cover all of it? Wear gloves a turtle neck and a hat with a veil or something? Even if you mummy wrapped your entire bode except your mouth and eyes they would still be able to target it because your lips and eyelids would be visible.

Quote from AWierdPotato >>

The armor says it contours to your skin. I always got the impression that it basically makes you look like a bronze statue, head to toe. So how are you going to cover all of it? Wear gloves a turtle neck and a hat with a veil or something? Even if you mummy wrapped your entire bode except your mouth and eyes they would still be able to target it because your lips and eyelids would be visible.

 "contours to your skin" isn't the same as "covers every square inch of your skin". It just means that, where it's worn (appropriately to the armor type), it's formed to your body rather than being some clunky form outside of you.

Quote from ArntItheBest >>

Quote from Jegpeg >>

 

that you can see within range.
Whether it can be seen depends on whether the "normal colthes" cover it completely if it does that also raises the question of whether you can avoid heat metal on any metal armor by completely covering it with a robe this is a DM decision

 

 But normal armor does not say that it can be worn under normal clothing. Saying you wrap yourself in a robe over the top of your armor should impose some sort of disadvantage (DM depending).

But this, and Mithral Chain Shirt/Breastplate specifically say that it can be worn under clothing. If you have clothing over it, it is not visible, and it cannot be a target of HM.

 I am not saying the shower to both questions is the same, it is perfectly reasonable for a dm to rule it is possible to wear normal clothes over molten bronze skin in a way that make it impossible to see the armor but putting a robe over armor can not completely conceal it. However if I was in a game where another player got molten bronze skin and the dm ruled it could not be targeted with heat metall I would at least ask the question if I would cover my armor with a robe.

Quote from Athanar90 >>

Quote from AWierdPotato >>

The armor says it contours to your skin. I always got the impression that it basically makes you look like a bronze statue, head to toe. So how are you going to cover all of it? Wear gloves a turtle neck and a hat with a veil or something? Even if you mummy wrapped your entire bode except your mouth and eyes they would still be able to target it because your lips and eyelids would be visible.

 "contours to your skin" isn't the same as "covers every square inch of your skin". It just means that, where it's worn (appropriately to the armor type), it's formed to your body rather than being some clunky form outside of you.

 
See what I mean an it being poorly worded?

Quote from Athanar90 >>

Quote from AWierdPotato >>

The armor says it contours to your skin. I always got the impression that it basically makes you look like a bronze statue, head to toe. So how are you going to cover all of it? Wear gloves a turtle neck and a hat with a veil or something? Even if you mummy wrapped your entire bode except your mouth and eyes they would still be able to target it because your lips and eyelids would be visible.

 "contours to your skin" isn't the same as "covers every square inch of your skin". It just means that, where it's worn (appropriately to the armor type), it's formed to your body rather than being some clunky form outside of you.

 My way’s cooler.