When doing German smear you need to power wash the brick and clean it so you wont get bleed through changing the color of the mortar. It is possible to clean the brick without a power washer but you'll need the correct chemicals and to clean the brick by hand very meticulously.
The brick needs to be wet when applying the mortar as well. Clear out the room and cover everything to clean your brick. be sure to not let excess mortar land on anything that isn't brick if it sets you wont have a easy time getting it off if ever.
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