Applying a canvas brings enexpected behaviour

[cinematik]

there's a very direct way to do this:

1) make a new layer on top of your paint layers (you should merge thick paint on canvas before doing this, though)

2) fill it will white paint with a paint bucket tool

3) in the layers panel, set layer mode to "multiply" and "bump blend mode" to "replace"

4) merge the underlying thick layers if you haven't done it already, and merge the white (which is invisible due to way "multiply" layer mode works) layer down last. you can play with multiply layer's opacity too if you need to trim just a bit of the paint depth.

hi, nekomata
good explanation, thnks

Though multiply on white should not change a color seems it does.
Probably it's hard to notice difference here but it's present. On the right side the image with white layer on top and the resulting image is slightly lighter.