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[Jaxilon]

Whoa, that's a lot of little squares.

I would test out some ideas if it was me.

I think my first experiment would be to paint horizontal lines and then vertical lines after that had dried raising the brush up every now and then. Obviously using a slightly darker color for the vertical lines....you could do it the other way around and go vertical first if you cared. Then I would go back through and touch up with some lighting or whatever. Not sure this will work because I am a noob with watercolor but that would be my first guess other than doing each shingle at a time.

You could cut yourself a little square of stiff sponge and make a brush for stamping each shingle. Then just be real careful to line them up.

You could attempt painting each square, haha.

Find a piece of screen, maybe remove some strands and lay it on the paper and brush over that with a mostly dry brush and see what happens.

Ok, those are my 2 minutes worth of experimental ideas. Let us know what you decide.

PS. If all that failed I would probably do the shingles with ink and then fill in the squares with the paint.