Stucco News

Question and answers on Stucco and Plastering-- November, 2014

   Pre-wetting CMU prior to scratch coat


I have a new project where the stucco installation foreman told my field QA inspector, he has never heard of "pre-wetting CMU prior to scratch coat."

We have always held the belief that SSD (Wetting the blocks ) is the best state for CMU during scratch coat application.  However, I have had a hard time pointing to a written standard that agrees with my opinion of "best practice."  

We constantly badger installers to keep the plaster moist for 3-days after texture to gain optimum hydration but I've never had an installer balk on pre-wetting.

Any good reference sources I could cite?

I can give you a reference-you should always wet the block, CMU, before the scratch
coat, except in cold weather.

The idea of wetting the block is to slow down the suction, or the rate the block absorbs
water. On a high suction surface, like blocks, the idea is the mortar is absorbed into the
pores and not the water absorbed out of the mortar.

Of course this will provide a better bond and a stronger product. It is a good idea to knock the dust
off the block anyway.

There is no advantage to wetting the block in cold temperatures, say less than 40 degrees.
The cold temperature slows down the water absorption, and helps set the mortar up faster.

It may be a common practice in Florida not to wet the block first when they put three coats
on the block the same day. I don't know so I can't say this first hand. I can see when more
wet mortar is put on immediately it would control the suction. I don't know. Maybe someone
from Florida can write to clarify this.