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Latly, I really like to use a pen and a marker for quick sketch, so this is prosses. At first I used 10% maeker for just ideas in the flame. It is very rough sketch, but I make sure what is going on each part. and then I used 0.3 pen that is very thin "Hi-Tec". I don't want to use a ruler too much, so I can say this was 30% ruler and 70% free hand drawing. When I make a copy, 10% marker lines were just gone, so do'nt need to afraid to use 10% maeker.
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After make a copy I use marker with French Grey that is my favorite color. I think it looks much better than cool color. Anyway I used 10% to 60% markers and SAKURA MICRON. Use SAKURA MICRON for dark area makes a drawing looks good.
I thought I need practice about prpduct design drawing technics.