You have to totally rethink your finishing style with these guys. The Nylon matrix is very porous, so not only does the outside get dirty, in beTWEEN does too!
The first thing I did was seal with clear acrylic medium (Michaels in the fine art section). If you don't have that - or he's already getting a bit grubby, use white acrylic paint. Don't thin it much at all and just dab/stroke to get it into the nylon. You don't have to take him apart to do this, but it helps if you feel up to it. Otherwise just make sure there's not extra acrylic between the joints when you're working and just pop them loose if they do stick. This will seal him and make him easier to clean. Once you've got two coats on him he should be ready to blush!