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I'm looking at some datasheets for interior plaster by Saint-Gobin (which unfortunately are not available in English as far as I can tell) and I'm miffed why some interior (white) cement-based plasters are advised to be sanded as the final smoothing step and for others the advice is to wet float them. As far as I can tell the maximum aggregate (sand) size is the determining factor, those that they advise to float have max particle size of 1mm, the other ones are in the hundreds of micrometers range. Is this the determining factor, or are there other considerations?

To add some details, these are all skim coats. The one with 1mm max particles allows for maximum 5mm layer, while the other ones (hundreds of micrometers particles) only 2mm layers.

  • Any true white coat plaster I seen has been wet trowel finish, never sanded. Drywall mud get sanded, there is no aggregate in either. The brown coat has plenty of sand, I could be mistaken about the plaster white coat. – Jack Jul 13 '17 at 3:58
  • @Jack: I guess you've never had property (or worked) in Germany, Italy or other continental EU countries where skim coats are mostly sanded. – Fizz Jul 13 '17 at 4:05
  • No I can't say I have. I tried to find your location on your profile, but the info was not there. Many people not in the trade consider drywall joint compound plaster so I wanted to get that little bit out there just in case you may have been referring to that. – Jack Jul 13 '17 at 4:19
  • Also, I find it strange that you say that plaster in the US does not contain aggregate (sand, perlite etc.). The only material that doesn't have that is 5-minute gypsum, and that's far too fast settling to use as plaster. The retarded (2 hour) gypsum has 3:1 sand:plaster typically. The quicker (but not 5-minutes), e.g. 30-minute gypsum has hydrated lime added to the 2-hour composition. – Fizz Jul 13 '17 at 4:44
  • I am not an expert on plastering but I have overseen many applications over the years as a residential site supervisor. Perlite is used quite a lot as brown coat, still topped off with a lime based plaster where a build up coat is called for, and there is a "gray board" that is in 4X8 sheets that only need the white coat over that after the joints are treated. Still all wet trowel finish. This has been years ago (10+) maybe the states has the same thing as the EU nowadays. – Jack Jul 13 '17 at 4:53
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Yeah, after trying this in practice, this floating 101: you can't bring out aggregate that's much finer than your float's sponge. The max 200um stuff turned out to be some lawyerly spec (as in them not getting sued if someone finds a spec of sand in it); in practice it's as fine a flour or even finer (perhaps close to toner dust). There isn't any float in the typical hardware store that has a sponge that fine. You can "float" it of sorts, but you're not really bringing out the aggregate, just leaving impressions that match the sponge's pattern. You can surely do interesting decoration this way, but it ain't floating.

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