Thygo

Hi @Thygo,
Welcome to the forums. 
This only works in Affinity Designer. The brush in Affinity Photo is destructive If you're using photo you can create a mask and paint with black/white/gray. 
Selections and Masking
Making Selections Refining Selections Pixel Selections from Layers Quick Masks Mask Layers Exposure Merging Clipping vs Masking Vector Masking Using Adjustment Layers on Masks - YouTube / Vimeo Thanks,
Gabe. 

Select the eraser and then select a brush and erase away  what you are effectively doing is creating a mask, so its a nondestructive process and you can turn the mask layer off.