I have never tried pastels, but I have white washed 3 shermans in different forms. Now for me I build the sherman,paint (with details) wash, dry brush, and seal it then take on the task of white washing. By all means it is not pretty, I look at it this way, If I was a tank crew member, and say 2 other people on this forum were crew members with me, we would be B S ing, chewing tobacco, dipping snuff, smoking, keeping warm, we would want to get the job done and done with speed, every person that would be white washing would be doing it in his own way. So in white washing a model, I take this in account, showing different area's done differently. If anyone is interested I will tell you about the styles I use.
My 2 pennies worth..........And the main thing....HAVE FUN!!!!!