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I had a guy come in to do some drywall taping today and I had a bag of 20 min hot mud and a bucket of each green (regular) and blue (lightweight) joint compound. I always though that the green should be used for the first two coats cause it is stronger and blue for the final coat cause it is easier to sand.

Once the dude finished the bag of hot mud, he went straight for the blue to do a little bit of first coating that was left at the end of the day. I asked him why not use the green and he said they were the same. I was not really sure in my assumption stated above and was tired at the end of the day to argue and also I wanted to trust his experience so I just let him do a couple of panels and a joint with the blue.

  1. Who is correct, me or him?

  2. If I am right and he is wrong, why could have led him to fail at differentiating?

  3. If I am wrong and if they are indeed the same, why would they even have two different products (by the same brand)?

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2 Answers

up vote 2 down vote accepted

You can use either all the way through. I have personally used both all the way through more than a few times and have mixed. There are generally some differences.

  • the lightweight end mix is thicker, meaning that the regular can actually be better for a final coat since it goes on thinner
  • the lightweight is easier to sand but I hardly sand... so this hardly ever helps me
  • I personally think the lightweight dries out faster
  • lightweight does scratch and dent easier due to less density

Pros will use the regular since they can use a "less fluffy" mix that feathers easier (with little to no sanding). But I could definitely see why a person with less experience would do the first two coats with regular and last two with lightweight.

Answers to your questions - 1. You are both a little right/wrong. If I had to choose I would pick him. 2. If he knows what he is doing he doesn't sand much and doesn't need lightweight. 3. I answered these above. The marketing for the lightweight is for the novice DIYer that hates to spend hours sanding...

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I always use lightweight myself but never even bothered to learn the differences. Knowing 'regular' is easier to feather I think I'll definitely go that route next time! –  DA01 Jun 5 '13 at 7:35
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That is really the only benefit of using either I think. If I need a really thin final coat to feather I add a little warm water and a couple of drops of soap, mix in my pan, and feather. I spend a lot of time feathering/scraping because my expectation is no sanding. And I have found that 10 mins extra scraping is 30 mins less sanding. –  DMoore Jun 5 '13 at 14:17
    
This was a big topic at work today. My workplace was like 50/50 on what they preferred. I sided with the regular but I generally don't care. However if I were to be very anal and want a job to hold up "forever" I would probably do two coats light and end with two coats regular. The light has more flex so maybe it doesn't crack in year 150 where regular would... And the other topic we had was putting your drywall up vertical vs horizontal - also almost a 50/50 split. –  DMoore Jun 6 '13 at 3:34
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According to USG, makers of SHEETROCK® Brand All-Purpose Joint Compound, and SHEETROCK® Brand Plus 3™ Lightweight All-Purpose Joint Compound.

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SHEETROCK Brand PLUS 3 Lightweight All-Purpose Joint Compound, Ready-Mixed, offers all of the benefits of a conventional-weight, all-purpose joint compound with three key advantages: up to 30-percent less weight; less shrinkage; and exceptional ease of sanding. It offers good crack-resistance, excellent adhesion, and superior ease of handling.

Finally

Lightweight Joint Compounds including All Purpose with Dust Control are not recommended for skim coat applications.

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