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Old 12-10-2011, 09:57 AM   #1
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can sand be added to perlite plaster like Structo-Lite


I've been calling around for weeks to supply houses within an hour's drive of Philadelphia PA -- it is impossible to find a few bags of sanded plaster such as USG Red-Top Gypsum Plaster. Only full skids can be ordered.

Structo-Lite perlite is available by the bag but it is not recommended for a smooth finish. USG tech support says it dries hard and dense, and a veneer coat may not even adhere to it, even if their bond-enhancing liquid is applied to the base-coat beforehand.

Are there any tricks of the trade that could make it easier for the 1/16" veneer plaster to adhere? Can sand be added to the Structo-Lite? Will roughing the surface up very slightly with a brush improve the veneer bond?

I have to patch about 50 sq ft. The metal lath is already in place.

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Old 12-12-2011, 07:10 PM   #2
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can sand be added to perlite plaster like Structo-Lite


I don't install plaster, I only specify and inspect the work so take the following for what it is worth. I was trained by old school tradesmen who didn't always agree with current trends. I guess that this makes me old school, so don't take offense that I don't believe in chemical adhesion and bonding agents in plaster systems.

It sounds as if you are installing a two coat system and though I am most familiar with three coat systems, the principles are the same. The finish coat will more reliably adhere to the base coat when providing a "keyed" surface. The base coat can be scratched with a nail embedded darby to create keying for the finish coat. This will create grooves with accompanying ridges to which the finish coat can adhere. Dry brushing the surface will not create as reliable a keyed surface but is better than no keying at all. Because the scratch keyed surface has small ridges, you may want to consider a slightly thicker 3/32" finish coat. The base coat needs to be set and hard before applying the finish coat but not totally dry, otherwise the water will be sucked out of the finish coat and create shrinkage cracks.

If more available, USG Structo-Base gypsum plaster can substitute for Red-Top. Structo-Base is higher strength but costs more than Red-Top.

Sand in the Red-Top scratch coat isn't always a required additive. Sand is used to extend the volume (reduce cost) and reduce shrinkage. If you are repairing a small area, shrinkage may not be an issue. The sand does not contribute to adhesion.

The Structo-Lite plaster comes with pre-mixed pearlite aggregate, thus adding sand is redundant and likely detrimental. In any case, Structo-Lite it isn't recommended with smooth plaster finish coats.

Rick

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Old 12-13-2011, 08:29 AM   #3
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can sand be added to perlite plaster like Structo-Lite


Quote:
Originally Posted by Arkitexas View Post
I don't install plaster, I only specify and inspect the work so take the following for what it is worth.
I consider it worth quite a lot!

Quote:
Originally Posted by Arkitexas View Post
If more available, USG Structo-Base gypsum plaster can substitute for Red-Top. Structo-Base is higher strength but costs more than Red-Top. ... In any case, Structo-Lite it isn't recommended with smooth plaster finish coats.
Exactly. But finding a by-the-bag supplier for good old-school sanded base-coat plaster of any kind seems an impossible task in 2011. The suppliers are saying they have not stocked plaster for the last ten years. Price of materials is not really a problem since the area is only 55 sq ft.

I was hoping to have the project completed before Christmas for my wife, who has been very very patient while I solved the many many problems I created for myself along the way. If no sanded plaster is available, I may have to fall back on the Structo-Lite even though it's not recommended.

I did find a document from the Perlite Institute that said:

Quote:
Where perlite-gypsum plaster with smooth troweled finish is to be applied over expanded metal or wire lath, it is recommended that perlite fines be added to the finish coat mix.
Don't know whether to take this advice with a grain of ...hmmm, perlite

Last edited by diy888; 12-13-2011 at 08:32 AM.
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Old 12-14-2011, 07:45 PM   #4
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can sand be added to perlite plaster like Structo-Lite


I am pretty sure that I saw at the Lowes in Plymouthmeeting PA bags of basecoat and top coat plaster. It was in the end isle with all the masonary products
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