My advice for the yellow is:
1) use a little (watered down/wetbrush) brown ink in joins and creases. Just there, not all over the yellow.
2) Then add some pretty obvious highlights with a mix of the yellow you are using and white.
Pure white will make it look like it is shiney, don't want that. Also the white will make the yellow be more opaque.
Highlight using your favorite method. I'd advice against drybrushing at this late a stage, don't want your leather jacket etc drybrushed in light yellow.
Thin paints.
I'm addicted to licking my brushes all the time myself, water tends to make the paint too runny. (I'll probably get ill from contamination at some point though)
Chtulu is probably referring to the paint on the helmet having an obvious texture.
This is a likely result of using a thick paint or applying many layers to make it 'cover' (?). Thick paints just dry too fast.
Some colours will show the underlying coat if applied thinly, mostly reds, yellows and other bright colours. It helps to either use a white undercoat for those areas or use a darker like-colour that will function as the shadows. scabred for red and snakebitebrown for yellow for instance.
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I like your mini conversion of the guns.
along with battle damage, maybe apply some tinbitz/rust? Maybe not, badmoons are notoriously rich and would be likely to have lotz of gretchin polishing their loot.
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My stuff:
I'm happy about how a base conversion for one of my devastator/longfangs turned out. The model that is standing on a small rock, I nremoved the rock and cut up several spare pieces from a plastic kit to produce a darkangel devastator half burried in the snow. The Longfang is now standing on his corpse instead of the pebble. (I had 3 of the pebble guys and having them all standing on that same rock would be silly)
[insert image here later]
edit2: cruddy zoom, maybe later, only got fuzzy photos.
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edit: Your remark about drybrushing threw me into explain-to-noob mode, obviously you are no noob. For one thing, no noob would paint so neatly or mod his models so subtly. My apologies if I made you feel lectured to.