Eventually you get to a point where the initial skin just doesn't allow you to go any further, especially when you start working on the face. But thanks to Pixologic, now you can just duplicate your SubTool, use ZRemesher to get a nice base mesh and just project all the detail you had on the previous version.
As for the clothing and accessories there are several ways to go. The one I rarely use is actually the Extract option from a mask on your mesh. I find it a lot more troublesome then the other options.
My first option is to use the base mesh's geometry. I just duplicate the SubTool, go to the lowest subdivision level and just Del Higher. That way I can isolate the part of the geometry that I want and just Delete Hidden, leaving me with a piece of nice geometry to sculpt, for instance, the gloves.
In some cases the geometry of the base mesh is just too far from what you need, and in those situations I just append a ZSphere and do some really quick manual retopology.
For the shoe laces I just use one of the amazing free brushes available at Badking
Here you can see a turnaround of the character after 4 hours of sculpting. By now I already have the main elements of the silhouette without the backpack
At this point I return to 2D sketching, painting over my BPR renders of the model views. I still hadn't worked on the back-pack robot design and I also wanted to try different hair styles with the helmet on.
When I'm designing the backpack I mostly worry about the overall silhouette, but also about its functionality.
I usually start with a simple line drawing and then add a base value to distinguish the different materials. Then I create a layer for light, a layer for shadows and a layer for specular. Then all you have to do is try to match the light you have on your BPR renders.
Here you can see me trying 3 different helmets and on the profiles. I also try 2 different hairstyles. I decide to go with the longer hair and the first mask
Back to sculpting
Since I know what I want for the backpack robot, I go back to ZBrush. For this type of hard-surface sculpting I usually start out with a sculpt sketch of the main forms, working with DynaMesh to fuse different basic shapes together – like the cylinder at the base of the backpack where the arms retract to.
It's pretty common for me to go back and forth between working in DynaMesh mode and ZRemeshing it to go back to a more controllable, multi subdivision levels base mesh. It's so fast to switch between the 2, and you can keep all the details you want and clean up what you don't want.
In some situations I still find it easier to jump into 3ds Max and poly model.
In this image you can see the complete model turn around and some illustrations on the different modes these characters can incorporate
Detailing and finishing the sculpt
There's a great expression that goes something like this: ‘You need 10% of the time to do 90% of the work, and then you need 90% more time to do the remaining 10% of the work.' I think this is very accurate. The main sculpting of the overall silhouette is really fast. Detailing and doing the final adjustments is where you'll spend most of your time, especially in characters with lots of accessories or clothing.
At this stage, the background story is of great help. It is what guides the choices you make for what accessories to add, how much wear, how many wrinkles, what he needs to survive: how can I tell my character's story visually?
Here you can see I add a wrinkles and pores pass to the character, as well as a series of details like bolts and straps